Forces and Motion

1. 09/21/12
Forces and Motion

2. Moments 09/21/12
A moment is a “turning force”, e.g. trying to open or close a
door or using a spanner. The size of the moment is given by:
Moment (in Nm) = force (in N) x distance from pivot (in m)
Calculate the following turning moments:
5 metres
100 Newtons
2 metres
200 Newtons

3. Balancing moments 09/21/12
2 metres 2 metres
200 Newtons 100 Newtons
Total ANTI-CLOCKWISE Total CLOCKWISE turning
turning moment = 200x2 = moment = 100x2 = 200Nm
400Nm
The anti-clockwise moment is bigger so the seesaw will
turn anti-clockwise

4. Balanced or unbalanced?
09/21/12

5. Stability 09/21/12
1. Centre of mass is within
the wheelbase – no problem!
2. Centre of mass is directly above
the edge of the wheelbase –car is on
the point of toppling
3. Car falls over

6. Centripetal force 09/21/12
Consider a ball of Pleistocene attached to some string:
The ball is kept in its path by the
tension in the string – an example of a
CENTRIPETAL FORCE. This force also
produces the change in velocity due to
the direction constantly changing.
This force is INCREASED if you increase the mass of the
object, its speed or decrease the radius of the circle.
Other examples
of centripetal
forces:
Orbits Electrons

7. Momentum 09/21/12
Any object that has both mass and
velocity has MOMENTUM. Momentum
(symbol “p”) is simply given by the formula: P
Momentum = Mass x Velocity
(in kgm/s) (in kg) (in m/s)
M V
What is the momentum of the following?
1) A 1kg football travelling at 10m/s
2) A 1000kg Ford Capri travelling at 30m/s
3) A 20g pen being thrown across the room at 5m/s
4) A 70kg bungi-jumper falling at 40m/s

8. Conservation of Momentum 09/21/12
In any collision or explosion momentum is conserved (provided that there
are no external forces have an effect). Example question:
Two cars are racing around Teville Gate. Car A collides with the back of
car B and the cars stick together. What speed do they move at after the
collision?
Speed = 50m/s Speed = 20m/s
Mass = 1000kg Mass = 800kg
Mass = 1800kg Speed = ??m/s
Momentum before = momentum after…
…so 1000 x 50 + 800 x 20 = 1800 x V…
…V = 36.7m/s

9. More questions… 09/21/12
1) A white snooker ball moving at 5m/s strikes a red ball and pots it.
Both balls have the same mass. If the white ball continued in the same
direction at 2m/s what was the velocity of the red ball?
2) A car of mass 1000kg heading up the M1 at 50m/s collides with a
stationary truck of mass 8000kg and sticks to it. What velocity does
the wreckage move forward at?
3) A defender running away from a goalkeeper at 5m/s is hit in the back
of his head by the goal kick. The ball stops dead and the player’s speed
increases to 5.5m/s. If the ball had a mass of 500g and the player had
a mass of 70kg how fast was the ball moving?
4) A gun has a recoil speed of 2m/s when firing. If the gun has a mass of
2kg and the bullet has a mass of 10g what speed does the bullet come
out at? (Hint: total momentum before is zero and the gun has a
negative velocity afterwards)

10. A past exam question…
09/21/12
June 2000
Two lorries are travelling in the same direction along a motorway.
Lorry A Lorry B
Mass = 20,000kg Mass = 30,000kg
Speed = 14m/s Speed = 20m/s
1) Calculate the momentum of Lorry A as it travels along the motorway.
2) Calculate the momentum of Lorry B as it travels along the motorway.

11. Newton’s Laws of Motion 09/21/12
These are my three laws of
motion (summarised):
1) If an unbalanced force acts on an
object that object will either accelerate
or change direction:
F
2) That force is given by F=ma
M A
3) When a force acts on an object
there is an equal force acting in
the opposite direction (“Action and
reaction are equal and opposite”)

12. Newton’s 2 nd
Law 09/21/12
Instead of F=ma Newton actually said that the force acting on
an object is that object’s rate of change of momentum. In
other words…
Force = Change in momentum (in kgm/s) ∆mv
(in N)
Time (in s)
F T
For example, David Beckham takes a free kick by kicking a stationary
football with a force of 40N. If the ball has a mass of 0.5kg and his
foot is in contact with the ball for 0.1s calculate:
1) The change in momentum of the ball,
2) The speed the ball moves away with

13. Example questions 09/21/12
1) A golfer strikes a golf ball with a force of 80N. If the ball
has a mass of 200g and the club is in contact with it for
0.2s calculate a) the change in momentum of the golf ball,
b) its speed.
2) A tennis player strikes a serve with a force of 30N. If the
ball has a mass of 250g and the racket is in contact with it
for 0.15s calculate the ball’s change in momentum and its
speed.
3) A footballer takes a goal kick by kicking a 0.4kg football
away at 10m/s. If his foot was in contact with the ball for
0.1 seconds calculate the force he applied to the ball.
4) A golfer strikes a 200g golf ball away at 50m/s. If he
applied a force of 50N calculate how long his club was in
contact with the ball for.

14. Energy loss in collisions
09/21/12
In the “Forces” module we looked at how to calculate an object’s kinetic
energy:
Kinetic energy = ½ x mass x velocity squared
in J in kg in m/s
We’ve also said that in a collision momentum is conserved (unless an
external force acts). The same cannot usually be said for kinetic energy…
For example, consider the following collision. How much kinetic energy is
lost?
Before
Speed = 50m/s Speed = 20m/s
Mass = 1000kg Mass = 800kg
After
Speed = 20m/s Speed = 30m/s
Mass = 1000kg Mass = 800kg

15. Energy loss in collisions
09/21/12
Consider a head-on collision where the cars stick together. How much
kinetic energy is lost in this example? Where does all the energy go?
Before
Speed = 50m/s Speed = 30m/s
After
Speed = 10m/s
In this example more kinetic energy was lost. We say
it was a “less elastic collision”. An “elastic collision” is
one where the kinetic energy is conserved.

16. The Structure of the Earth 09/21/12
A thin crust -
10-100km thick
A mantle – has the
properties of a solid
but it can also flow
A core – made of
molten nickel and iron.
Outer part is liquid
and inner part is solid
The average density of the Earth is much higher than
the crust, so the inner core must be very dense

17. Tectonic theory 09/21/12
People once thought that the oceans and the continents were formed by
shrinkage from when the Earth cooled down after being formed.
Alfred Wegener proposed something different. Consider Africa and South
America:
These continents look
like they “fit” together.
They also have similar
rock patterns and fossil
records. These two
pieces of evidence led
me to believe that
there was once a single
land mass. This is my
TECTONIC THEORY.

18. Tectonic theory 09/21/12
The Evidence:
1) Some continents look like they used to “fit” together
2) Similar rock patterns and fossil records
The Problems:
Wegener couldn't explain
how continental drift
happened so nobody
believed him
The Answer:
1) Scientists discovered 50 years later that the Earth generates massive
amounts of heat through radioactive decay in the core. This heat
generated convection currents in the mantle causing the crust to move
2) We also now know that the sea floor is spreading outwards from plate
boundaries
Conclusion – scientists now believe Wegener’s Tectonic Theory

19. Tectonic plates 09/21/12

20. Movements of the crust 09/21/12
When the lithosphere (“crust”) moves three things can happen:
1) Plates move past
each other, causing
earthquakes
3) Plates
move towards
2) Plates move away from each other – a
each other – a
“constructive plate margin”. The gap is
“destructive
filled with magma which cools to form
plate margin”.
basalt. This is called sea floor spreading.
The thinner
one slides
underneath
(“subduction”)
and partially
melts. This
causes
volcanoes and
earthquakes.

21. Evidence for sea floor spreading 09/21/12
Since the Earth was formed the north and south poles have
periodically “________ ____”. When tectonic plates move
apart and _____ fills the gap the iron particles in the magma
orientate themselves in line with the Earth’s ________ field.
This means that the rock formed on the sea floor contains a
“magnetic __________” of the changing field:
These magnetic patterns can be used to prove that sea floor
spreading does happen, and at a rate of about 2cm per _____.
Words: impression, magma, swapped over, magnetic, year