What is a Force?
A Force is a push or a pull on an object.
Forces are vectors because they have
magnitude and direction.
Force is equitable to energy in later
Make a list of all the forces you can think
of…. Think back to the elementary
playground – your first physics lab.
The 4 Forces in Nature:
There are four basic forces in
1. Gravity- the attractive force between all
matter in the universe.
2. Electromagnetic – the forces of
attraction in atoms and magnets.
3. The Strong nuclear force that keeps the
4. The Weak nuclear force that causes
Dynamics and Statics
Dynamics is the study of the effects of
forces on matter in motion. When two
objects interact, forces are the cause of
the interaction. This includes a baseball
being hit by a bat and a rocket ship
Statics is the study of forces acting on
objects in equilibrium (stationary).
Hanging signs are examples here. Both
are major engineering areas of study.
Newton's First Law of
Motion – The Law of
The name for this tendency is INERTIA - the
tendency for an object to remain in the same
state of motion (or rest).
An object will continue in its state of rest or
uniform motion in a straight line unless an
outside unbalanced force acts upon it. When
forces are balanced, there is no acceleration.
All situations on Earth’s surface are called
inertial reference frames. A non-inertial
reference frame is one in which an object is in
freefall or accelerating and may not obey the law
Newton’s Second Law of
Motion: Force ∝ Acceleration
One of the most important formulas in all of
physics is ΣF = ma . This says that the
summation of the forces on an object is
directly proportional to the acceleration of
the object. It says if you push hard on a
basketball it goes fast and when you push
easy, it goes slow….. You already knew that !
Force is a vector quantity and is the net force
of the sum of all acting forces in all
directions. The acceleration will vary directly
with the net applied force and inversely with
Newton’s Third Law of
Motion: Forces in Pairs.
Forces in Pairs states that every balanced
force in nature is accompanied by an equal
and opposite force. Forces always are in
pairs between pairs of objects. The action
and reaction forces act on different bodies.
Examples are a rocket blasting off, a hang
glider flying in air and you sitting in your
desk. You push down on the chair and it
pushes back up on you.
Units of Force: The Newton
A force is measured in terms of the
acceleration it gives a standard mass.
The SI unit of force in physics is named
after the father of physics; Newton. The
Newton (N) is defined as the force that
will cause a 1.00 kg mass to accelerate at
1.00 meter per second squared.
Proof: F = m a
=(1.00 kg)(1.00 m/s2
= 1 kg x m/s2
= 1 N
Weight and Mass:
Mass is a measure of the inertia of a body. It
depends upon the amount of matter contained
within an object. The relationship between
mass and inertia is direct. The unit of
measurement for mass is the Kilogram (kg).
Formula: m = F/a. Mass is CONSTANT.
Weight is the gravitational force exerted
object. Weight is a vector force and a negative
symbol shows force in a downward direction.
A medium apple weighs about one Newton.
Formula: W (weight) or Fg
= m x g. Weight
changes according to where you are and the
acceleration of gravity changes (outer space).
Ways to Measure Mass
a. Triple beam balance (or electronic
version) will compare the weights of two
objects. One weight is placed on the pan
and then the masses on the beams are
adjusted to equal the force on both sides
of a pivot point.
b. A second method to measure mass
is that a known force is applied to an
object on a frictionless surface and then
the acceleration is measured. The
formula used is : mass = Force divided
by acceleration. m = F/a
Free Body Diagrams
pictures of how
forces act upon
an object or
W is the weight
and Fn is the
T is tension.
The Normal Force is the
Earth’s response to Weight.
Newton’s 3 law
can be shown in
the Normal force
of the Earth
Friction is a force that opposes the motion of
two objects that are touching each other. It
does this by creating temporary
electromagnetic forces between the contact
points of the two surfaces.
Friction acts in a direction parallel to the
surfaces in contact and opposing the motion.
The force exerted by a two surfaces touching is
called a contact force. When a contact force
acts perpendicular to the common surface of
contact it is called a Normal Force (meaning
perpendicular). Friction is independent of the
areas of the surfaces in contact but is directly
proportional to the mass.
Friction Opposes Motion
Note that the
vectors of force /
Types of Frictional Force
1. Static friction is the force of friction
that resists the start of motion. This is
always greater than the sliding frictional
2. Sliding friction (also called Kinetic
Friction) is the force that resists the
existing or continuing motion.
3. Air resistance is a special case of
sliding friction. This is due to air
molecules colliding with a moving object.
Ff air ∝v2
. The air friction force is a square
function of the velocity and in reality a
Friction in Life
the starting of
Relationship between Static
and Kinetic Friction.
What Causes Friction?
a side view of
Steps to Solve Force Problems
a. Read and label the problem.
b. Draw an accurate diagram (free
body diagram). Show all forces exerting
force on the object.
c. Choose a convenient x-y coordinate
system for resolution of vectors.
d. Determine knowns and unknowns
to assist in choosing equations.
e. Solve it roughly to get an idea of the
range of reasonable answers.
f. Solve with equations.
g. Keep track of units throughout the
equation and check the answer.
Special Case Problem: The
Please note the special case with the
skier going downhill (Inclined plane) .
When the axis is chosen for vector
resolution it is slanted to a normal
Cartesian system. This means that the
vertical and horizontal components in the
vector switch trig functions: Fx (parallel to surface)
mg sin θ and Fy (perpendicular
= mg cos θ. This switch allows us to
identify the force causing the sliding
down force as the vertical sine.
The Inclined Plane
force is no