1) Newton's third law states that for every action, there is an equal and opposite reaction. Whenever one object exerts a force on a second object, the second object exerts an equal force back on the first. 2) Momentum is defined as the product of an object's mass and velocity. It represents the difficulty of stopping or changing the motion of an object. Objects with greater mass or velocity have greater momentum. 3) The change in an object's momentum is called impulse. Impulse depends on both the magnitude of the force applied and the duration of its application. A longer application of force results in a greater change in momentum.

1. Third Law

For every action there is an equal and opposite
reaction.
Some action/reaction pairs:
I push on the floor, the floor pushes back on me
with equal force (unless it breaks).
Tug of war (when at a standstill) – west group
pulls rope with a force equal in size but opposite
in direction to east group.

2. Simple Rule:

Action: Object A exerts a force on object B

Reaction: Object B exerts a force on object A
A B
F on A
due to B
F on B
due to A

3. 3rd
law example problem

A 5 kg and a 1 kg mass collide.

Which experiences the larger force?

Explain.

4. Rolling
How does a car accelerate?

The wheels are turned by the motor.
What would happen if there was no
friction between road and tire?

Wheel would spin but car wouldn’t
accelerate.
Friction between road and tire is also
responsible for force that keeps car
turning when changing direction.
F of tire
on road
F of road
on tire

5. Rolling

Video: Cars...ON ICE!

6. Action/Reaction different masses

What happens when A and B
(equal mass) collide?

Which one (if any) hits harder?

Now, what if A and C collide
where C is more massive than
A?

Which one hits harder?

Action/Reaction forces are a
pair – they are always and must
always be the same size.
A B
C A

7. Cannon
Action forces and reaction
forces must be the same
size.

Why does the cannonball
fly so fast?

Why doesn’t the cannon
move as much as the
ball?
Recall Newton’s 2nd
law:
acceleration = force/mass
F
m
a = F
m
a =

8. Rockets
If the rocket was in a
vacuum would it still
work?
Yes – Rockets are a
little like the cannon.
(Demo)

9. Wrapping up Newton’s Laws

An object will not accelerate (change direction
or speed) unless acted upon by an unbalanced
force.

The acceleration of an object is proportional to
the force on it and inversely proportional to
mass (a = F/m).

So what will happen to acceleration if you double
force with same mass?

If you double mass with same force?

If object A exerts a force on object B then
Object B exerts an equal in magnitude but
oppositely directed force on A.

10. Momentum

11. What is momentum?
Is it harder to stop something that is moving
quickly or something that is moving slowly?
Is it harder to stop something that has more mass
or something that has less mass?

How difficult something is to start or stop.

More than just inertia (mass)

includes how quickly something is already moving.

12. Momentum:

Momentum = mass * velocity
Momentum has a direction and that
direction matches the direction of motion.
Changing direction requires a change in
momentum.

13. 
So objects with wildly
different masses and
velocities can have
same momentum.
V
v

14. Example

A motorcycle of mass 200-kg travels with a
velocity of 50 m/s. What is its momentum?

A car of mass 1000-kg travels with a velocity of
10 m/s. What is its momentum?

15. Changes in Momentum

What causes a change in velocity?
An acceleration, but what causes an
acceleration?
A Force: The greater the force and the longer it acts
the greater the change in velocity.
m
v v
m
v
m
F

16. Impulse

Impulse is the change in momentum.
Consists of:

Force (produces the velocity change)

Time (how long the force was applied)

17. Impulse – Momentum:

Impulse = change in momentum
F*t = m2*v2 – m1*v1
OR
F*t = Δmv

18. Coming to a stop:

Braking: Low force over long time

Crash: High force over short time.

What is the momentum change for a truck braking to a
stop? For a truck hitting a wall?

Result is the same: same mass, same starting velocity and
final velocity is zero.
crunch!
v
F
v
v

19. Increasing Momentum

Larger force applied over longer time will
cause greatest increase.

Rocket: tremendous force applied for several
minutes to reach escape velocity.
Rifles: why is a rifle long?
Why wouldn’t you use a pistol to shoot
something far away?

20. Rifles:
Bang!
(hot gasses push)
Gasses escape...
Gasses escape...

21. Decreasing Momentum

Remember the truck: hitting a wall (fast stop)
results in much more force than a controlled
stop.

Increasing the time a collision takes reduces the
force experienced.

Can you think of any examples?
Airbags, crumple zones in cars.
Boxers who dodge backward as they are hit.
Bungee Jumping.
Examples are endless.

22. Decreasing Momentum

Slow change – by
moving backward
with the hit you
decrease the force by
increasing the
contact time.

Fast change – don’t
move the wrong way,
you increase the
force by decreasing
the contact time.

23. Demo: Fast vs. Slow changes

2 meter stick, 2 100 gram weights.

Weight 1 attached 90 cm from table

Weight 2 attached 10 cm from table
Both by thin threads.
What will happen when each is dropped (from
the same height)?

24. Bouncing

Which results in a
greater impulse:

bouncing

stopping

Look at momentum
change:
 bounce: m(v2-v1) =
m(v-(-v)) = 2mv

stop: 0 – m*(-v) = mv
v v
0
v
boing!

25. Demo: Happy and Sad balls.

One ball does not bounce.

Other ball will.

Which imparts more impulse? Why?

26. Conservation of Momentum
Momentum is conserved.

In a collision or other
interaction, momentum is
neither created nor
destroyed.
Recall the cannon example
from Newton’s Third law.

What was net momentum
before cannon fired?

What is net momentum
(ball + cannon) after it is
fired?

27. 
Momentum before =
0

Momentum after
must be 0
(momentum is
conserved).
 0 = mc*vc + mb*vb
 So vc and vb will be in
opposite directions to
make the sum zero.

Can we figure out
what proportion their
sizes will be?

28. Collisions

Imagine two 1-kg
carts.

Momentum before =
1kg*10m/s = 10
kg*m/s

momentum after
must be equal.

2 kg * v = 10 kg*m/s

v = 5 m/s

29. Wrapping up:

Momentum is conserved (I know, I know,
another conservation law – there are a
few of them).

Momentum is proportional to velocity and
mass of the object.

Impulse is a change in momentum. It is
proportional to the force and the contact
time.