Basic SI Units
Derived SI Units
Kg / Cubic Metre (kg/m3)
Metre per second (m/s)
• Matter - anything that occupies
- consists of atoms and molecules.
• Atom - smallest particle in an element
has the properties of the
• Molecules - the combination of two or
NATURE OF MATTER
• Nucleus - each carrying a positive charge are called protons.
In addition to the protons the nucleus usually contains electrically neutral particles
called neutrons. Neutrons have the same mass as protons whereas electrons are very
• Neutron - simply adds to the weight of the nucleus.
- has no charge.
• Proton - positively charged.
- has mass same as the neutrons.
• Electron -orbits define the size or volume occupied by the atom
- negatively charged.
- total number of negatively charged electrons matches
number of positively charged protons.
• The sub-atomic components of atoms.
0 ( zero )
In the nucleus
+1 ( positive )
In the nucleus
1 / 1850
-1 ( negative )
Arranged in energy levels
or shells around the
• Atomic Number – no. of protons in the atom’s nucleus.
eg. Hydrogen has 1 proton – atomic no. is 1.
Carbon has 6 protons – atomic no. is 6.
• Mass Number – total no. of protons and neutrons in nucleus.
eg. Lithium has 3 protons & 4 neutrons – mass no. 7
Relative Atomic Mass
The mass of atom in relation to the mass of a reference atom.
Element chosen on which to relate mass of all atoms is Carbon.
Carbon (12) - 6 protons + 6 neutrons
Hydrogen - 1 proton + 0 neutron = mass no. 1
*so relative atomic mass for Hydrogen is one twelfth of Carbon.
are atoms that have same atomic number but different mass numbers.
Atom gains / loses one or more neutrons from nucleus will become
Are present in most elements and may also be man made.
• From left to right across a period =
protons/electrons count increases by one.
• Atomic number = increases from left to
1. Vertical column
2. Elements have same number of outer shell
/ valency electrons.
Noble gases - group 8.
Transition metals - middle of the table.
Pure metals - group 1, except Hydrogen.
• Atoms bond together to form a molecule.
Consists of single type of atom.
Made up of two or more atoms.
Structure of Atoms
Electrons are arranged in energy levels and shells around the nucleus and
with increasing distance from the nucleus .
• Valence electrons :
- how many electrons an atom has in its outer shell.
- determines the chemical behavior.
- The more valence electrons, the more want for the atom to
get more electrons.
1. bonding capacity of an atom; usually equals the number of unpaired
electrons in the atoms outermost shell.
2. No. of bonds it can make with a monovalent atom such as Hydrogen.
A group of two or more similar or dissimilar atoms bonded together .
Metallic bonds - metal + metal (eg: Alloy)
- do not produce molecules
- weakest bonds of all
Covalent bonds - non metal + non metal (eg : water)
- produce molecules, strongest bond.
- metal + non-metal (eg: sodium chloride)
• Defined as :
‘ A substance made up of different elements that are chemically bonded and
are so united that the whole has properties of its own, which are unlike those
of its constituents.’
• All other molecules made up of two or more atoms are known as Chemical
States of matter
• All matter exists in one of three physical states.
• Physical state refers to the condition of a compound and has no
affect on a compound's chemical structure.
• Solid. A solid has definite mass, volume and shape.
• Liquid. A liquid has definite mass and volume but takes the shape
of its container.
• Gas. A gas has definite mass but takes the volume and shape of its
• increase in the temperature will increase the energy of its
Changes between states
• Solid to Liquid – Fusion
Heat energy required to complete process of converting unit
mass of substance from solid to liquid state without change of
temperature is called the Latent Heat of Fusion.
• Liquid to Gas – Vaporisation
Heat energy required to vaporise a unit mass of liquid without
temperature rise is called Latent Heat of Vaporisation.
• Gas to Liquid – Condensation
Heat energy expelled is called the Latent Heat of Condensation.
• Liquid to Solid – Solidification
Loss of molecular energy is the Latent Heat of Solidification.
Some solid substance when heated do not melt, but form a
Solid to vapour, without passing through liquid state.
Eg. Dry ice
Liquid change to vapour.
Some liquid have low boiling point, so easily change from liquid
Eg. Methylated spirits.
statics ; forces
centre of gravity
stress and strain
properties of matter
pressure and buoyancy in liquids
Newton’s law of motions
First Law of Motion
A body at rest will remain at rest unless given an external force, or a
body which is moving will keep on moving unless given an external
force. ( Inertia )
Second Law of Motion
A force proportional to the rate of change of its velocity is produced
whenever a body ( or mass ) is accelerated.
F = ma
Third Law of Motion
For every action, there is an equal and opposite direction.
• If a Force is applied to a body it will cause that body to move in the
direction of the applied force.
• force has both magnitude (size) and direction.
• Forces cannot be directly observed, only their effects can be seen.
Compressive - force applied on an object to squeeze it.
Tensile - pair of forces try to extend an object.
Shear - Pair of forces tries to cause one face of material to
slide relative to an adjacent face.
• Statics is used to describe study of bodies at rest when forces are
• External force – force applied to an object from outside its
• Internal force – force induced in the object to react against
externally applied force. ( Reaction )
VECTOR AND SCALAR
• Scalar – just a magnitude, there is no direction associated.
eg. Time , mass, volume.
• Vector – require both magnitude and direction to be fully defined.
eg. Force, velocity, acceleration.
• 2 or more forces act at same line, produce resultant force.
• If at straight line, subtract or addition.
• Forces do not act in a straight line – use the ‘parallelogram law’.
Find the resultant of two forces of 4.0 N and 5.0 N acting at an angle of 45
degree to each other.
Using a scale of 1.0 cm = 1.0 N, draw parallelogram ABCD with AB = 5.0 cm,
AC = 4.0 N and angle CAB = 45 degree, see figure 8.
Worked example: three forces acting on a mass.
First resolve each force into its vertical and horizontal components.
MOMENTS AND COUPLES
• If a body free to rotate about some point ;
Applied force will cause rotation
Force required dependent on how far from hinge force is applied.
iii. Turning effect = magnitude and distance.
Moment = force x distance.
In SI units, Newtonsmetres = Newton x metres
The Principle of Moments
‘If a body is at rest under the action of several forces, the sum of the
clockwise moments about any axis is equal to the sum of the anti
clockwise moments about the same axis.’
Type 1 – beam balances where arms are of equal length.
Type 2 – lever arrangement can best be seen in design of a
Type 3 – large effort moves through small distance to overcome small
load, which moves through a large distance.
• IN EQUILIBRIUM
• COUPLE – WHEN TWO EQUAL BUT OPPOSITE
DIRECTION, CAUSES ROTATION
For example, suppose it is necessary to calculate the resultant moment of
a pivot acting on a bell crank lever, refer to diagram below.
CENTRE OF GRAVITY
• Gravity is a force which is always present and is a pulling force in
the direction of the center of the earth.
• This force acts on every body through an imaginary point called the
center of gravity (C of G).
• A point where all the weight of a body appears to be
concentrated. (total weight can be considered to act through that
datum position )
(total weight can be considered to act through that datum
There must be a datum point, such as where when moment in clockwise
direction will be balanced by moment in anti-clockwise direction, if given
the same amount of force.
Stability / balancing
• The lower the C of G, the stable an object is.
• The wider the base, the more stable an object
is – C of G towards the base.
• The location of the center of gravity in the
human body varies slightly, depending on
• a male with a muscular upper body and
small hips, the center of gravity is higher
than in a female with narrow shoulders
and wide hips
• in an infant with a large head in relation
to its body, it is higher than in an adult
• When force applied to
C of G, the body will not rotate.
• But if the force is applied
offset of the C of G, the body
will rotate, or torque will
Expressed as an algebraic formula,
W1 X1+ W2 X2 + W3 X3 = (W1 + W2 + W3) x G
Where G is the position of the centroid, with respect to the datum.
Why do we need to know c of g of an
To ensure the aircraft is safe to fly, the center-of-gravity must fall within
specified limits established by the manufacturer.
C of G range – C of G limits are specified longitudinal (forward and aft) and/or
lateral (left and right) limits within which the aircraft's center of gravity must
be located during flight.
To evenly load the aircraft – equipments, passengers, baggage, cargo, fuel,
So that C of G range will not be exceeded – prevent aircraft unstable during
Also affects C of G in flight – fuel usage, passengers’ movement, etc.
Similar to aircraft, force applied will be acted through the C of G,
resulting in torque.
Aircraft rotate about its C of G.
If force is exerted on a body, there will be mechanical pressure acting on
the body which is called the stress.
A body with having twice the size of other body subjected to a force, it will
be stronger and less likely to fail due to applied the applied force.
So, stress is said :
*units : Newton metre-2 , Nm-2
Components will fail due to over-stressed, not over-loaded.
Eg. A tennis ball sealed from atmospheric pressure. So, as long as the
external forces acting on it does not exceed the internal forces, the ball will
maintain its shape.
Forces applied to the body will cause distortion of the body and change to the
material’s cross-sectional area ;
eg. Tensile Forces will cause elongation .
Compressive Force will cause reduction in dimension.
Most material have elastic properties ( it will to return to its original shape after
the force is removed ) - provided forces does not exceed limit of elasticity.
There are 5 types of stress in mechanical bodies :
force that tends to pull an object apart
• resistance to an external force that
tries to push an object together.
• Torsional stress is applied to a material when it is twisted.
• Torsion is actually a combination of both tension and compression
• In flight, the force of lift tries to bend an aircraft's wing upward.
• combines tension and compression is the shear stress, which tries to slide
an object apart.
If the outside force is great enough to cause the object to change its
shape or size, the object is not only under stress, but is also strained.
If a length of elastic is pulled, it stretches. If the pull is increases, it
stretches more; if the pull is reduced, it contracts.
Hooke’s law states that the amount of stretch (elongation) is proportional
to the applied force.
How stress varies with stress when a steel wire is stretched until it breaks.
Hooke’s Law states that, ‘the amount of stretch (elongation) is proportional to
the applied force.’
Strain - the degree of distortion then has to be the actual distortion divided
by the original length (in other words, elongation per unit length).
Strain = change in dimension / original dimension ( No units )
*for shearing and torsional stresses, they are expressed as ‘shearing
action’ – when one layer of materials move relative to another in
direction of applied force.
Shear strain – straight motion.
Torsion strain – rotational motion.
• when the applied load causes
one 'layer' of material to move
relative to the adjacent layers.
• form of shear stress resulting
from a twisting action.
Twist will be proportional to the
Properties of matter
Is the spreading of a substance of its accord.
due to molecular action, e.g. a smell, whether pleasant or not, travels
quickly from its source to your nostrils where it is detected.
occurs in liquids and gases but not in solids.
This suggests that the surface of a liquid behaves as if it is covered with an
elastic skin that is trying to shrink.
The surface tension can be reduced if the liquid is ‘contaminated’, adding a
detergent to the water will cause our needle to sink.
In a liquid, the molecules still partially bond together and prevents liquid
from spreading nag expanding out.
Force of attraction between molecules of
Force of attraction between
molecules of same substances.
If a glass tube of small bore is dipped into water , the water rises u the
tube a few centimetres.
The adhesion between the glass and the water exceeds the cohesion of
the water molecules, the meniscus curves up , and the surface tension
causes the water to rise.
Mechanical properties of
A strong material requires a strong force to break it
A stiff material resists forces which try to change it’s shape or size.
The ability to recover to its original shape and size after the force deforming it has been
Materials that can be rolled into sheets, drawn into wires or worked into other useful
shapes, without breaking are ductile.
A material that is fragile and breaks easily .
Pressure and buoyancy in
The equivalent term associated with fluids is pressure:
pressure = force / area
p = F/A.
Pressure is the internal reaction or resistance to that external force.
Pascal’s Law : “pressure acts equally and in all directions throughout that fluid.”
pressure can be transmitted to some other point in order to
generate another force.
Units of pressure
SI system pressure :
Pascal = force per unit area ( Nm-2)
Atmospheric pressure :
Milibars ( mb ) or pounds per square inch ( psi )
Sea level standard atmpresssure :
1013-2 mb or 14.69 psi ( at 0C)
Archimedes’ Principle states that when an object is submerged in a liquid, the object
displaces a volume of liquid equal to its volume and is supported by a force equal to the
weight of the liquid displaced.
THE BUOYANCY OF A SUBMERGED BODY =
WEIGHT OF DISPLACED LIQUID – WEIGHT OF THE BODY
1. The body will float--if the buoyancy is positive
2.The body will sink--if the buoyancy is negative
3.The body will be stuck--if the buoyancy is neutral
• Is the uniform motion in a straight line.
• Motion is the change of position of a body with reference to
A person sitting in a moving car and passes a
The person is considered to be at a state of
reference to the car.
The car is considered to be in motion in
to the building.
Speed tells us how quickly an object is moving at any given point in time.
Scalar quantity – does not take into account the direction of the object
Average speed – dividing the distance travelled by the time taken.
rate of change of displacement or position
where v represents speed.
• An aircraft flies at 80 km/ hr for 15 minutes and at 120 km/hr for a
further 15 minutes. How far has it travelled and what is the
average speed ?
Includes direction, distance in straight line and time.
Vector quantity – magnitude + distance.
An aircraft moves 200miles South West
from A to B in one hour.
*the velocity is 200mph South West.
Is the rate of change of velocity over time.
If the rate of change is constant, acceleration is described as being
Acceleration - If the velocity of an object increases over time.
Deceleration / retardation – If the velocity decreases over time.
If a car is travelling at initial constant velocity ‘u’ and then accelerated with
uniform acceleration ‘a’ to final velocity ‘v’, therefore :
*Unit : m / s 2
u + at
Equation of linear motion
Equations of linear motion
V = u + at
s = ut + ½at2
v2= u2 + 2as
Free falling objects
• if a stationary object is released and free falls under attraction of the force
of gravity, it will accelerate at 9.81 m/s 2
• So the ‘a’ will be 9.81 m/s 2 ,
but in this case it is given the symbol ‘g’.
• If the object is thrown upwards, its g will be – 9.81m/s2
Initial velocity of objects freefalling from rest is zero, so the equations for free fall
• Rotational motion means motion involving curved paths and therefore
change of direction.
• Only cases of constant acceleration are considered here.
• They are equivalent to those linear equations of motion :
Continuous force applied to a body moving in circular path to keep it in
that particular circular path, preventing it from travelling in straight line (
tangential to circle it is rotating in ) – due to the inertia of the object.
Basically a force acting inwards toward the centre of the circle.
Is directly proportional to the mass of the object in circular motion.
Is inversely proportional to the radius of the circle in which the object
Force = ma, so we can say that the object has an inwards acceleration
called Centripetal Acceleration which is ;
v2 , so
*where v is linear velocity and r is radius of circular path.
• Is the equal but opposite reaction to the Centripetal Force.
( Newton’s 3rd Law )
Tensile Force at the other end of the string acts outwards of the circle.
Relationship between Angular and Linear Motion :
Providing that we know the value of the rotating
body’s radius ( r ), we can use the relationship
between the radius and the radian to convert angular
values to linear values and vice versa.
Linear acceleration :
Some masses move from one point to another, then back to the original
point, and continue to do this repetitively.
The time during which the mass moved away from, and then returned to
its original position is known as the time period, and the motion is known
as periodic motion.
Example : pendulum
When the mass then displaced from its rest position, it will accelerate back towards
its rest position.
On reaching it however, it will not stop, because its inertia carries it on to an equal
but opposite displacement.
the time period can be measured from a any position, through to the next time that
position is reached, with the motion in the original direction.
Cycle–for one complete to and fro movement.
Periodic Time – time taken to complete 1 cycle.
Frequency – the number of cycle occurring in 1 seconds. ( Hertz – HZ )
Amplitude – maximum displacement of a body from its middle or rest position.
Periodic time, T can be calculated by ;
T = 2 √ ( L / g )
L – length of the pendulum
g – magnitude of acceleration due to gravity = 9.81 m/s2
For time period ( T ) and frequency ( f ) ;
Spring – mass systems
If the mass at the spring is displaced and the force is released, the spring
force will cause the mass to return to its original position.
It will behave like pendulum, so it continue to move up and down before it
Springs obey Hooke’s Law.
The resulting motion, up and down, resulting in :
Force ( F )
if mg = kE, then the spring constant k = mg
Also, extension E = mg
Frequency (f) = ( 1 / 2 ) √ ( k/m )
F is force.
m is suspended mass.
g is the acceleration due to gravity.
E is the spring extension.
k is the spring constant.
Simple theory of vibration,
harmonics and resonance. but strict law.
Analysis of oscillating systems will show that they often obey simple
Acceleration is proportional to the displacement from the neutral position, and in the
opposite sense to the direction of the velocity.
Referred to as Simple Harmonic Motion, when acceleration is directed towards fixed
point in its path and is proportional to its displacement from that point.
Vibration Theory is based on the detailed analysis of vibrations and is
essentially mathematical, relying heavily on trigonometry and calculus,
involving sinusoidal functions and differential equations.
• Damped vibrations :
Simple pendulum / spring mass will vibrate at constant frequency and
amplitude, once it is started.
But vibrations will die away due to other motions such as friction, air
When force subjected to force vibration, it will vibrate along with natural
frequency of the object.
If the natural frequency matches the resonant frequency with forced
vibration causes the amplitude to increase dramatically.
Natural frequency tries to damped out the amplitude but cannot damp its
own natural frequency.
So the two amplitudes combine to produce resonance.
Velocity ratio, mechanical
advantage and efficiency.
Machine – utilise some form of motion to convert an applied force into a
useful work output
The input forces is often amplified many times by the machine so that we
can overcome a heavy load with little effort.
Eg ; levers, pulleys, gears, screws.
Ratio of distance the effort is required to move in comparison with the
distance the load moves in the same time.
Velocity Ratio = Distance moved by effort
Distance moved by load
• Describes the ratio of load moved with the effort required to move it.
Mechanical Advantage = Load
*A rusty car jack will have a low mechanical advantage because
the effort would be used to overcome the friction of a
corroded screw thread.
• Describes the ratio of the useful work done by a machine to the total work put into
Efficiency = Work Output x 100% or Mechanical Advantage x100%
• Expressed as percentage and is always less than 100%.
*Friction and slippage can detract from efficiency of a machine.
• Used to gain mechanical advantage.
• Most basic form : seesaw that has weight at each end.
( weight on one end tends to rotate it clockwise, weight on the other end tends
to rotate it anti-clockwise )
First Class Lever
Lever has fulcrum between load and effort.
Less effort required to lift the load.
Second Class Lever
• Has fulcrum at one end of the lever and effort is applied to the opposite
Third Class Lever
• Force is applied between fulcrum and load.
• Used to move the load a greater distance than effort applied.
• Disadvantages : much greater effort required to produce moment.
Pulley wheel has circumferential groove to accept a rope.
Effort is applied by pulling on a rope.
Tension created in the rope and movement are transmitted through
arrangement to the load.
Single fixed pulley is a convenient means of lifting a light load.
So, for instance, if one end of the rope is attached to a fixed object, pulling on
the other end will apply a doubled force to any object attached to the axle.
• Is the quantity of matter that it contains.
• Constant regardless of its location.
• Basic SI unit : kilogram (kg)
• Imperial : pounds (lbs)
• Force with which gravity attracts a body.
• Varies with distance between body and centre of the earth, so if farther
than centre of the earth, the less it weighs.
• So, it is said that an object in deep space does not has no weight, but do
• Gravitational acceleration is considered as 9.81m/s2
• Is a vector quantity that has magnitude, direction and a point of
• Which changes a body’s state of rest or of uniform motion.
• Newton’s First Law
• A body at rest will stay at rest unless given an external force, or a moving
body will continue on moving unless given and external force.
• Is the resistance to movement or changes.
• Work is done when a force move.
Work done = force x distance moved in the direction of the force.
• Unit : Newton metre (Nm) or the joule
1 joule = the work done when a force of 1 Newton is applied through
a distance of 1 metre
• The amount of work done in specific time.
Power = work done
force x distance
• Si unit : Watt ( W ) – 1 Joule/ second
• Is the rate of work done when 1 Joule is achieved in one second.
*One horsepower is the equivalent of 746 Watts
Brake Horse Power
• To rate the engine power.
• The engine is made to do work on a device known as dynamometer or ‘brake’ –
loads the engine output.
Shaft Horse Power
• Measure output shaft of a turboprop engine, since the power produced at shaft
is what will be delivered to propeller.
Is the capacity to do work.
Si unit : Joules
Can appear as several form ; mechanical, chemical, heat, electrical, and
But only deal with Mechanical Energy, and appears as potential and
Energy cannot be created or destroyed, it can only be changed from one
form to another.
Energy will not be converted into 100% work, they will always appear in a
less useful form such as wasted heat.
But it may be converted into 100% energy.
Potential Energy (PE)
Energy is possesses by virtue of its position or state.
Example : A mass raised to a height above the ground has potential energy
since its weight is capable of doing work as it descends to the ground
under the influence of gravity.
Work = force x distance
*since PE is for mass at elevated position,so distance is the height above
Potential Energy = mass x gravity x height
Energy it possesses by virtue of its motion.
Arises from the work done on it.
When body set in motion by a force doing work it acquires kinetic energy,
which will work against any forces that try to resist it.
Conservation of Energy
“ Whenever energy is converted from one form to another,
none of it is lost.”
The sum of the energy can always be accounted for in the other forms of
energy that may have converted into.
After an object hits ground on a fall, the energy has converted into heat
energy and will be dissipated into the air.
So a comparison between work out and work in is obviously a measure of
the system efficiency.
Efficiency = work output
*usually expressed as percentage ( less than 100 % ).
Defined as energy between two bodies because of difference in
If two bodies at different temperature, are bought into contact, their
temperature become equal.
Energy that flows from a hot place to a cooler place.
Heat energy can be transferred by ; conduction, convection and radiation.
Product of its mass and velocity.
momentum = mass x velocity
SI unit ; kgm/s
Impulse of a force
• If a body was subjected to a sudden blow, shock load or impact, it will be
possible to measure change in momentum.
• Forces which have the short time duration are called the Impulsive forces.
• Change of momentum due to impulsive force is called the Impulse.
• Impact duration small – impulsive force large.
Impact duration large – impulsive force small.
Product of force and time or change in momentum.
For example, if we rest the hammer, on top of the head of a nail, neither
the hammer nor the nail has any momentum.
However, if we bring the hammer down from a height and strike the nail
sharply on its head, both the hammer and nail will move after the impact.
In a short time they will come to rest with the nail having penetrated
whatever it is under it.
Impulse = change in momentum = mass x change in velocity.
= force ( N ) x time ( seconds )
CONSERVATION OF MOMENTUM
“When two or more masses act on each
other, the total momentum of the masses
remains constant, provided no external
forces, such as friction, act.”
CONSERVATION OF MOMENTUM
Mass remains at rest or continue to
move at constant velocity, unless acted
on by an external force.
Mass remains at rest or continue to
move at constant velocity, unless acted
on by an external force.
F = ma
Rate of change of momentum is
proportional to the applied force.
For every action, there is a n equal and
If mass A exerts force on B, then B exerts
an equal but opposite force on A.
Changes in Momentum
Change of Momentum = final momentum - initial momentum
= ( mv)
Rate of change of momentum = change in momentum
*The rate of change of momentum is proportional to magnitude of force
When two bodies, one of low mass and the other of high mass, are acted
upon by the same force for the same time, the low body mass will build up
higher velocity than the heavy mass.
At (a) – mass A overtakes mass B.
At (b) – mass B will be accelerated by impulsive force delivered by
and mass A will be decelerated by an impulsive force delivered by B
At (c) – after the impact, mass A and B will have new velocities Va
Momentum before impact equals to momentum after impact.
A moving snooker balls, each ball has its own momentum before collision. After collision,
the sum of the two balls’ momentum will be the same as the sum prior to the collision
even though their velocities may have changed.
sum of momentums of balls =
prior to impact
mu1 + mu2
sum of momentum after impact
mv1 + mv2
Moment of inertia
considers the effect of mass on bodies whose moment is rotational.
Moment of inertia is a function of mass and radius.
Consider the two cylinders, of equal mass, but different dimensions,
capable of being rotated.
The LH cylinder is easier to rotate than RH cylinder.
is a rotor having freedom of motion in one or more planes at right angles
to the plane of rotation.
used in several of an aircraft’s instruments, which are vital to the safety of
the aircraft in bad weather.
For example wheels, engines, propellers, electric motors and many other
components must run with perfect smoothness.
With the rotor spinning, the gyroscope will possess two fundamental
Gyroscopic rigidity or inertia
maintains the axis of rotation constant in space.
if a gyroscope is spinning in free space and is not acted upon by any outside
influence or force, it will remain fixed in one position.
The degree to which the rotor offers resistance depends on 3 things :
Mass of the rotor – greater the mass, greater resistance to change in
direction of plane of rotation.
Angular speed of the rotor – higher the speed, greater the resistance.
Radius of gyration of the rotor – bigger the radius, greater the rigidity.
angular change of direction of the plane of rotation of a gyroscope, as a
result of an external force.
The rate of this change can be used to give indications such as the turning
rate of an aircraft.
The rotor will rotate about axis AA.
Apply a force so that it acts on the rim of the rotor at 900.
Move this force around the rim of the rotor so that it moves through
900and in the same direction as the rotor spins.
Precession will move the rotor in the direction that will result in the axes
of applied force and of rotation coinciding.
For a constant gyroscopic speed, the rate of precession is proportional to
the applied force.
The opposite also applies, so for a given force the rate of precession is
inversely proportional to rotor speed.
Determining Precession Direction
If a mass is mounted on a rotating shaft, and the centroid is offset from
the axis of rotation, mass will exert centrifugal force on the shaft.
Even if the eccentricity is small, force may be considerable at high speed.
So, it will cause the shaft to bend.
If large stresses produced, will cause damage to bearing.
Addition by vibrations from supports and surroundings will cause
Some eccentricity is due to manufacturing imperfections or design, so it is
Balancing is needed to eliminate effect of centrifugal force.
Eg ; weights put on car wheels to balance them, make it easier to drive at
Force that resists any sliding movement between two contacting surfaces.
Can be taken as advantage, for brakes on vehicle to try to walk on smooth
Frictional force depends on nature between two surfaces.
Acts in any directions but always acts in opposing motion.
angle of the plane (θ) is increased, the body remains stationary, until at
some particular value of θ, it begins to move down the plane.
• At this maximum value, the force opposing motion
Fmax= mg sin θ ,
• and the normal reaction between the body and the plane
R = mg cosθ.
F/ R = mg sin θ / mg cosθ= tan θ
• ratio F/R (tan θ) is termed the Coefficient of Friction ( μ ) - < 1.
Coefficient of Static Friction
Static friction is friction between two solid objects that are not moving
relative to each other.
The static friction force must be overcome by an applied force before an
object can move.
The maximum possible friction force between two surfaces before sliding
begins is the product of the coefficient of static friction.
sometimes referred to as limiting friction.
Coefficient of Static Friction (μ) =
Frictional Force , F = μN
Friction Force ( F )
Normal Reaction ( N )
Coefficient of Dynamic Friction
Dynamic friction is when two objects are moving relative to each other
and rub together (like a sled on the ground).
The amount of force required to keep the object moving is called the
coefficient of dynamic friction.
usually less than the coefficient of static friction for the same materials
There are several types of friction:
Dry friction resists relative lateral motion of two solid surfaces in contact.
Dry friction is subdivided into static friction between non-moving surfaces,
and kinetic friction between moving surfaces.
Fluid friction describes the friction between layers within a viscous fluid
that are moving relative to each other.
Lubricated friction is a case of fluid friction where a fluid separates two
Skin friction is a component of drag, the force resisting the motion of a
solid body through a fluid.
Internal friction is the force resisting motion between the elements
making up a solid material while it undergoes deformation.
Fluid – term used for liquid and gases.
Specific gravity and density
• Density – mass per unit volume.
• Varies with :
Temperature ( for solids and liquid )
Temperature and pressure ( gas )
Density ( ρ ) = mass
• Eg : liquid that fills a certain container has a mass of 756 kg. The container
is 1.6 m long, 1.0 m wide and 0.75 of a metre deep. The liquid density
should be ?
Standard conditions for the measurement of gas density is established at
00C and a pressure of 1013.25 milli-bars
(Standard atmospheric pressure).
Temperature change will not change the mass of the substance, but as
temperature change, the substance tend to expand or contract, altering
It is necessary to compare density of one substance with the other to
achieve a standard which all other substance can be compared.
For solid and liquid, compare with water at 40C.
For gas, compare with air.
Relative Density = mass of any volume of a substance
mass of equal volume of water / air
Eg ; if a hydraulic fluid has relative density of 0.8, then 1L of the liquid
wieghs 0.8 times as much as 1L of water.
Used to measure the relative density of liquids.
Glass float contained within cylindrical glass body.
Weight at the bottom, scale at the top.
When liquid drawn into the body, float displays relative density on
Immersion in pure water will give out reading of 1.000.
App. in aviation : to measure battery electrolyte and fuel.
Measure of the resistance of a fluid which is being deformed by either
shear stress or tensile stress.
Also known as "thickness" or "internal friction“.
Eg : water has ‘thin’ velocity, while honey has ‘thick’ velocity.
so there is friction between two liquid surfaces even when they consist of
the same liquid.
This internal friction opposes the motion of one layer over another and,
when it is great, it makes the flow of the liquid very slow.
Viscosity of a liquid rapidly decreases as its temperature rises.
Viscosity of different liquids can be compared in different ways.
Eg ; if we allow fluids of different viscosity run out of container, the
higher the viscosity, the longer time taken to empty the container.
It is important to know about viscosity as aircraft uses fluids such as oil
in the engine for lubrication.
• Resistance present on a thin, flat plate which is edgewise on to a fluid flow.
• Near the surface, the fluid is slowed up due to roughness of the skin, and
fluid can also be considered as stationary at the surface.
• Effected by skin smoothness.
• The rougher the skin, the higher the friction.
Eddies or turbulent airflow
• swirling of a fluid and the reverse current created when the fluid flows past
• The moving fluid creates a space devoid of downstream-flowing fluid on the
downstream side of the object.
• Eg. If you put plate at right angles to flow, turbulence will be created behind
the plate and a very high resistance .
Effect of streamlining
• flowing steadily over a smooth surface, narrow layers of it follow smooth
paths that are known as streamlines.
• This smooth flow is also known as laminar flow.
• If laminar flow encounter obstructions, the streamline will break and
become irregular or turbulent.
If fluid flows slowly along pipe,
the flow is streamline.
If flow is very fast and exceeds
a certain critical speed,
the flow will become turbulent.
The Compressibility of Fluids
All fluids are compressible, so that their density will change with pressure.
i. assume as incompressible – provided under steady flow conditions, and
changes of density small.
i. easily compressed – except when changes of pressure and
Static and Dynamic Pressure
Pressure acting on x x1 is due to the weight of the fluid acting downwards.
W = mg ( g = gravitational force )
mass = volume x density
= height x cross-sectional area x density
so, downwards force = h . ρ . g . A acting on A
and pressure will be = h ρ g . A
Act depth h, within a stationary fluid of density ρ.
Reduces density accompanied by reduced pressure.
Eg, moving air is essential in flight.
Dynamic presssure = ½ ρv2 where ρ = density, v = velocity.
What is the pressure at 10.5 m deep of liquid in a pool having the density of
What is the depth of an object from the surface if pressure exerted on it is
100psi in a 5.3kg/m3 liquid ?
• a principle that explains the relationship between potential and kinetic
energy in a fluid.
• In a fluid the potential energy is that caused by the pressure of the fluid,
while the kinetic energy is that caused by the fluid’s movement.
• As a fluid enters a venturi tube, it is travelling at a known velocity and
• When the fluid enters the restriction it must speed up, or increase its
kinetic energy. However, when the kinetic energy increases, the potential
energy decreases and therefore the pressure decreases.
Laws Of Thermodynamics
Energy cannot be created or destroyed. It can only be converted from one to
another. When the energy converts from one form into other forms the total
quantity of energy remains the same.
Heat can only transfer from a high temperature region to a lower temperature
region. It cannot naturally transfer the other way.
The transfer of energy from matter becomes increasingly difficult as its
temperature approaches absolute zero. It is considered impossible at absolute
• Heat is a form of energy that causes molecular agitation within a material.
• is a measure of the kinetic energy of molecules.
• Temperature scale;
Freezing – 00C
Boiling – 1000C
• Farenheit Scale - when you increased the temperature of a gas by one
degree Celsius, it expands by 1/273 of its original volume.
• So if the temperature was decreased to 273 degrees below zero, the
volume of the gas would also decrease to zero, and there would be no
more molecular activity– absolute zero.
• On the Celsius scale absolute zero is - 2730C. On the Fahrenheit scale it is
• Conversion ;
– °C x 9/5 + 32 = °F ( from Deg. Centigrade to Farenheit )
– (°F - 32) x 5/9 = °C ( from Farenheit to Deg. Centigrade )
• is the exchange of thermal energy from a hot body to a cold body.
• When a hot body and a cold body have contact, heat will flow from the
hot body to the cold body until they both reach thermal equilibrium (they
are at the same temperature).
• For example ;
When an aircraft’s brakes are applied, the kinetic energy of the moving
aircraft is changed into heat energy by the rubbing action of the brake friction
material against the brake discs.
• SI system :
British thermal unit (Btu)
Heat & Work Conversion Factors
0.000 947 Btu
specific heat - amount of heat per unit mass required to raise the
temperature by one degree Celsius.
Relationship between heat and temperature change :
Q = cmΔT
Q is the heat added in 0C
C the specific heat capacity in J / kg0C
M is the mass in kg
ΔT is Tfinal– Tinitialin 0C
Different materials require differing amounts of heat energy to change
The heat energy required to change the temperature of 1 kg of material by
1 K is known as the specific heat capacity (c) of the material.
Due to the high specific heat of water, oceans and large lakes serve as
Land surfaces have a much lower specific heat, and the temperature can
vary significantly throughout the day.
three methods by which heat is transferred from one location to another
or from one substance to another, which are :
When body having high heat energy in contact with body having low heat
Eg; When hot object in contact with cold object, energy of molecules from
hot object will be transferred to molecules of cold object until they have
the same amount of energy ( stabilize ).
Various metals have different rates of conduction.
Liquids are poor conductors of heat in comparison with metals.
.Gases are even worse conductors of heat than liquids.
Insulators are materials that reduce or prevent heat conduction.
process by which heat is transferred by the movement of a heated fluid.
Transfer of heat by convection is often hastened by the use of a ventilating
fan to move the air surrounding a hot object.
Eg ; when heat is absorbed by a free-moving fluid, the fluid closest to the
heat source expands and its density decreases.
is the only form of energy transfer that does not require the presence of
refers to the continual emission of energy from the surface of all bodies.
This energy is known as radiant energy of which sunlight is a form.
This is why you feel warm standing in front of a window whilst it is very
Expansion and Contraction
All materials expand and contract with a change in temperature.
gases which expand the greatest amount .
Solids and liquids expand much less than gases
Expansion – considered as change in length, change in area or change in
Different materials expand at different rates.
Expansion of Solids
• Expansion is proportional to the increase in temperature to the original
dimension and depends on the actual material used.
L2 - L1
L2and L1 are final and initial lengths,
θ2and θ1are final and initial temperatures
αis a material constant (coefficient of linear expansion).
Expansion of Fluids
Fluids expand more than solids.
For gases, as volume and temperature changes are usually accompanied
by pressure changes.
The law of Thermodynamics
Thermodynamics is the study of the way that one does work with heat.
Energy conservation limits the amount of work we can get out of a certain
amount of heat.
first law of thermodynamics
• States that energy is conserved.
• The change in internal energy of a system is equal to the heat added to
the system minus the work done by the system.
ΔU = Q – W
(Δis the mathematical symbol for a change in a quantity)
is the change in internal energy,
is the heat added to the system
is the work done by the system.
• ΔQ is positive if it is put into the system, negative if it is taken out of the
• ΔW is positive if the system does work on its surroundings and is negative
if work is done on the system.
• The internal energy is the sum of the kinetic and potential energy of the
atom and molecules that make up the system.
second law of thermodynamics
general principle which places constraints upon the direction of heat
transfer and the attainable efficiencies of heat engines.
States that heat transfer will occur naturally of its own accord down the
Heat will naturally flow from a hot region to a cool region but not the
other way around.
the basic sense of the principle :
Heat will not flow spontaneously from a cold object to a hot
You cannot create a heat engine which extracts heat and
converts it all to
There is a thermal bottleneck which constrains devices
stored energy to heat and then use the heat
to accomplish work.
Ideal gas - one in which all collisions between atoms or molecules are
perfectly elastic and in which there are no intermolecular attractive forces.
a collection of perfectly hard spheres which collide but which otherwise
do not interact with each other.
all the internal energy is in the form of kinetic energy and any change in
internal energy is accompanied by a change in temperature.
If temperature is constant ;
If pressure is constant ;
Isothermal and Adiabatic Processes
Isothermal process - in which the temperature in a system remains constant.
Adiabatic process - one where no heat is added to, or taken away from the
heat engine is a system that performs the conversion of heat or thermal
energy to mechanical work.
Heat Engine Processes
a useful process is the adiabatic process where no heat enters or leaves
The first law of thermodynamics with Q=0, i.e. heat = zero shows that all
the change in internal energy is in the form of work done.
internal energy is proportional to temperature, there is no change in the
internal energy of the gas during an isothermal process.
All the heat added to the system is used to do work.
• consisting of two isothermal processes and two adiabatic processes.
• can be thought of as the most efficient heat engine cycle allowed by
• the Carnot efficiency sets the limiting value on the fraction of the heat
which can be so used.
• In order to approach the Carnot efficiency, the processes involved in the
heat engine cycle must be reversible and involve no change in energy
available to do work.
• This means that the Carnot cycle is an idealisation, since no real engine
processes are reversible and all real physical processes involve some
increase in energy available to do work .
Heat Flow to Hotter Region
internal energy will not spontaneously flow from a hot region to a cold
But if external force is given, heat can flow from cold region to hot region.
Usually this is done with the aid of a phase change, i.e., a refrigerant liquid
is forced to evaporate and extract energy from the cold area. . Then it is
compressed and forced to condense in the hot area, dumping its heat of
vaporisation into the hot area.
Speed Of Light
is one form of transmission of Electro-magnetic energy.
travels at high speed (about 3 x 108metres per second) and in straight
Can be ‘bent’ or reflected.
Laws Of Reflection and Refraction
• Light can also be reflected.
• Observation and measurement will show that ;
a. the incident and reflected rays lie in the same plane.
b. the angle of incidence equals the angle of reflection.
Plain and Curved Mirrors
When you look in a mirror, you see a reflection, usually termed an image.
For example, if an object is viewed from two different angle, the reflected
– appear to come from which corresponds to the image.
– lies on the same normal to the mirror as the object.
– appears the same distance behind the mirror as the object is in front.
– Appears as the same size.
For mirror that is not plain, it may be curved, spherical and parabolic.
Incidence equals reflection - still holds, but the curved surface allows the
rays to be focused or dispersed.
FP is known as the focal length.
Note the rays actually pass through F, and a
real image can be formed.
FP is still the focal length, but the image is
The size of the image depends on the position of the object.
image may be smaller or larger.
If the object is near to the mirror, the image will be far and larger from the
mirror but in opposite plane.
• If the object is far from the mirror, the image will be nearer and small,
between the position of object and mirror in opposite plane.
magnification = image height
For spherical mirror, magnification = image distance
Concave mirrors (e.g. shaving mirrors) give a magnified, erect (right way
up) image, if viewed from close-to.
Convex mirrors (e.g. driving mirrors) give a smaller, erect image, but with a
wide field of view.
Parabolic reflectors can focus a wide parallel beam. By placing the bulb at
the focus, they can produce a strong beam of light. (Conversely, they can
focus microwave signals when used as an aerial).
A submerged object is often seen at reduced depth.
This is because the ray has been ‘bent’ at the water / air boundary.
Since the angle of incidence and refraction is not the same ;
sine i = μ ( a constant )
depends on the 2 mediums involved.
μ = speed of light in medium 1
speed of light in medium 2
Ray (1) has been refracted across the
boundary, but ray (2) has been internally
reflected at the boundary.
critical angle of incidence when the ray in the
denser medium does not emerge, but travels
along the boundary.
sine C = 1 / μ exists
Refraction is the basic principle which
explains the workings of prisms and lenses.
Total Internal Refraction
• Refraction at a denser medium, a beam of light is bent towards the normal
and, vice versa.
APB – ray reflected away.
AP’D – angle of incidence increased, angle of refraction becomes
AP”C – Angle of incidence further increased , angle of refraction is >900 ,
remaining in the boundary.
Total internal refraction – where none of lights passing through the
Convex and Concave Lenses
The light rays then meet the surface of the lens at an angle to the normal,
and are then refracted.
Images can be real or virtual, erect or inverted, and larger or smaller.
The nature of the image will depend on the type of lens, and the position
of the object in relation to the focal length of the lens, (the focal length is
a function of the curvature of the lens surfaces).
Depends upon the total internal reflection of light rays.
Light can be trapped by total internal reflection inside a bent glass rod and
piped along a curved path as in the diagram below. a single, very thin
glass fiber behaves in the same way.
They are small and so, once light is introduced into the fiber with an angle
within the confines of the numerical aperture of the fiber, it will continue
to reflect almost losslessly off the walls of the fiber and thus can travel
long distances in the fiber.
If a bundle of parallel fibers is used to construct an optical transmission
line, images can be transferred from one point to another.
Fibre Optic Imaging
Principle : light striking at one end will be transmitted to the other end of
If the arrangement of fibres in the bundle is kept constant then the
transmitted light forms a mosaic image of the light which struck the end of
For example, wave is created at a lake or pond due to disturbance, such as
a rock thrown into the water.
The water wave has a crest and a trough and travels from one location to
One crest is often followed by a second crest which is often followed by a
third crest, and so on.
waves may be circular waves which originate from the point where the
disturbances occur; such circular waves travel across the surface of the
water in all directions.
Another example, if a slinky is stretched out from end to end, a wave can
be introduced into the slinky by either vibrating the first coil up and down
vertically or back and forth horizontally.
As the wave moves along the slinky, each individual coil is seen to move
out of place and then return to its original position.
The wave does not stop when it reaches the end of the slinky; rather it
seems to bounce off the end and head back from where it started.
Categories of Waves
• Waves come in many shapes and forms.
• Some share basic characteristic properties and behaviours, some waves
can be distinguished from others based on some very observable (and
some non-observable) characteristics.
• It is common to categorisewaves based on these distinguishing
Direction of movement
To categorize wave
Ability to be
Direction Of Movement
Wave moves in a direction perpendicular to the direction which the wave
If pulse is applied to the left end by vibrating it up and down, the energy
will be transported from left to right and particle will be displaced
upwards and downwards.
Wave in which particles of the medium move in a direction parallelto the
direction which the wave moves.
If force is applied to the left end by vibrating it left and right, the energy
will be transported from left to right and particle will be displaced
rightwards and leftwards.
Ability to transmit through vacuum
• wave which is capable of transmitting its energy through a vacuum.
• Electromagnetic waves are produced by the vibration of electrons within
atoms on the Sun's surface.
• These waves subsequently travel through the vacuum of outer space,
subsequently reaching Earth.
• Eg ; Light Wave.
• Wave which is not capable of transmitting its energy through a vacuum.
• Require a medium in order to transport their energy from one location to
• Eg ; Sound Wave.
Anatomy of Waves
dashed line - equilibrium or rest position of the string.
(if there were no disturbance moving through it )
Points A and F - crests of this wave (point on the medium which exhibits
the maximum amount of positive or upwards displacement from the rest
Points D and I - troughs of this wave (point on the medium which exhibits
the maximum amount of negative or downwards displacement from the
rest position )
Amplitude of a wave - maximum amount of displacement of a particle on
the medium from its rest position. ( from rest to crest )
a. Simply the length of one complete wave cycle.
b. Wave has a repeating pattern. And the length of one such
repetition (known as a wave cycle) is the wavelength.
c. Can be measured as the distance from crest to crest or from
trough to trough. ( B – G, E - J, D – I )
is a wave in which the particles of the medium are displaced in a direction
parallel to the direction of energy transport.
Wavelength - determined by measuring the distance between any two
corresponding points on adjacent waves. ( measure distance from a
compression to the next compression or from a rarefaction to the next
rarefaction; A – C , B – D )
Frequency and Period of a Wave
a. refers to how often the particles of the medium vibrate when a
wave passes through the medium.
b. number of complete vibration cycles of a medium per a given
amount of time and it as the units of cycles per second
or Hertz (Hz)
where 1 Hz is equivalent to 1 cycle/second.
c. . A detector could be used to detect the frequency of these
pressure oscillations over a given period of time.
d. unit : Hertz, Hz ( cycle / sec )
• Period ;
a. the time which it takes to do something.
b. the time for a particle on a medium to make one complete
c. When an event occurs repeatedly, then we say that the event is periodic.
d. measured in units of time such as seconds, hours, days or
• phenomenon which occurs when two waves meet while travelling along
the same medium.
Causes the medium to take on a shape which results from the net effect of
the two individual waves upon the particles of the medium.
Eg ; if 2 crests of wave having amplitude of +1 move into each other, the
resulting sine crest +2 is created at the moment when the 2 waves
Sometimes called Constructive Interference.
Occurs at any location along the medium where the two interfering waves
have a displacement in the same direction.
If both waves have an upward displacement; consequently, the medium
has an upward displacement which is greater than the displacement of the
two interfering pulses.
type of interference which occurs at any location along the medium where
the two interfering waves have a displacement in the opposite direction.
If a sine crest with an amplitude of +1 unit meets a sine trough with an
amplitude of -1 unit, destructive interference occurs.
The two pulses cancel each other for the duration of the overlap.
Once the two pulses pass through each other, there is still a crest and a
trough heading in the same direction which they were heading before
Principle of Superposition
“When two waves interfere, the resulting displacement of the medium at any
location is the algebraic sum of the displacements of the individual waves at that
To determine the shape of the resultant wave caused by the interference of
two separate waves.
Standing Wave pattern :
An interference phenomena.
When the vibration frequency of the source causes
reflected waves from one end of the medium to interfere
with incident waves from the source in such a manner
that specific points along the medium appear to be
Only created within the medium at specific frequencies of
vibration; these frequencies are known as harmonic
frequencies, or merely harmonics.
From the presence of two waves (sometimes more) of the
same frequency with different directions of travel within
Anti-nodes - point A on the medium moves from a positive to a negative
displacement over time.
Nodes - point B on the medium is a point which never moves.
stationary wave - is a wave that remains in a constant position.
often applied to a resonant mode of an extended vibrating object.
created by constructive interference of two waves which travel in opposite
directions in the medium, but the visual effect is that of an entire system
moving in simple harmonic motion.
modes of vibration associated with resonance in extended objects like
strings and air columns have characteristic patterns called standing waves.
arise from the combination of reflection and interference such that the
reflected waves interfere constructively with the incident waves.
An important part of the condition for this constructive interference for
stretched strings is the fact that the waves change phase upon reflection
from a fixed end.
• Sound wave - pressure disturbance which travels through a medium by
means of particle interaction.
• As one particle becomes disturbed, it exerts a force on the next adjacent
particle, thus disturbing that particle from rest and transporting the
energy through the medium.
• speed of a sound wave refers to how fast the disturbance is passed from
particle to particle.
• frequency refers to the number of vibrations which an individual particle
makes per unit of time.
• speedrefers to the distance which the disturbance travels per unit of time.
Speed of Sound
Is determined to be 331 ½ m / s at 00C – 1087 ft /s , 741 mph or 644 kts.
liquids are better transmitters of sound.
Eg. sound waves travel approx. 4 times faster in water than in air and
speed of sound in solids is even greater, sound travels through steel is 15
times faster than it travels in air.
speed ( sound wave ) = frequency x wavelength
Alteration in wavelength effects the frequency, but not the wave speed.
Doubling of wavelength results in halving the frequency, but wave speed
doe not change.
The speed of sound wave depends on the properties of the medium
through which it moves.
Primarily affected by temperature, the lower the temperature, the lower
the speed of sound.
speed of sound = √ ( γ R T )
γ = ratio of specific heats of the gas
R = gas constant
T = gas temperature ( in Kelvin )
It determines the nature and formation of shock waves.
Mach No = True Airspeed of aircraft
Speed Of Sound
amount of energy which is transported past a given area of the medium per unit of
Often referred to as ‘loudness’ , the energy of the wave.
The greater the amplitude of vibrations of the particles of the medium, the greater the
rate at which energy is transported through it, and the more intense that the sound
Intensity is the energy/time/area; and since the energy/time ratio is equivalent to the
quantity power, intensity is simply the power/area.
Intensity = Energy
or Intensity = Power
Time x Area
Eg ; if amplitude of sound wave is doubled, intensity will increase fourfold.
Unit : Watts / Meter2 . But scale to measure intensity; Desibel (dB)
• Best described as position on musical scale.
The ears of humans (and other animals) are sensitive detectors capable of
detecting the fluctuations in air pressure which impinge upon the
The human ear is capable of detecting sound waves with a wide range of
frequencies, ranging between approximately 20 Hz to 20 000 Hz.
The sensations of these frequencies are commonly referred to as the
A high pitch sound corresponds to a high frequency and a low pitch sound
corresponds to a low frequency.
effect produced by a moving source of waves in which there is an
apparent upward shift in frequency for the observer and the source are
approaching and an apparent downward shift in frequency when the
observer and the source is receding.
Occur with all types of waves - most notably water waves, sound waves,
and light waves.
Doppler Effect observed because the distance between the source of sound and the
observer is changing.
If the source and the observer are approaching, then the distance is decreasing and if
the source and the observer are receding, then the distance is increasing.
If the source is moving towards the observer, the observer perceives sound waves
reaching him or her at a more frequent rate (high pitch); and if the source is moving
away from the observer, the observer perceives sound waves reaching him or her at
a less frequent rate (low pitch).