1. Chapter 2 – Lesson 4

2. Newton’s Third Law of Motion
When one object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first object.
ANOTHER WAY TO SAY IT….
For every action there is an equal and opposite reaction.

3. Think about it… How is the gymnast able to flip over?
When the gymnast the vault, the vault
against the gymnast.
The of the force arrows are the
but the direction is
PUSHES
PUSHES BACK
LENGTH
SAME
OPPOSITE

4. Force Pair The forces two objects apply to each other.
Force pairs will always act in
The girls feet act on
The boat acts on
OPPOSITE
DIRECTIONS
THE BOAT
THE GIRLS FEET

5. Do Action-Reaction Forces Cancel?
If 2 equal act in opposite directions they are balanced and cancel each other out. There is no movement
Action – Reaction forces cancel out because they are acting on
DO NOT
2 DIFFERENT OBJECTS

6. FORCE PAIR
If one of the pairs is much more you will only see the object
Example – When you
push down on the Earth,
you won’t see the Earth
move, only you jumping
in the air
MASSIVE
LESS MASSIVE
MOVE

7. ACTION AND REACTION FORCE PAIR called the ACTION FORCE
In a one force is called the and the other force is
called the
For every there is a
that is in strength, but in direction.
Action – Reaction forces don’t cancel because they
FORCE PAIR
ACTION FORCE
REACTION FORCE
ACTION FORCE
REACTION FORCE
EQUAL
OPPOSITE
ACT ON DIFFERENT OBJECTS

8. How do Action-Reaction forces work when you are…
JUMPING
When you jump, you push down on the ground.
The ground then pushes up on you. It is this upward force that pushes you into the air.

9. How do Action-Reaction forces work when…
A rocket launches
*When the rocket fuel is ignited, a hot gas is produced. As the gas molecules collide with the inside engine walls, the walls exert a force that pushes them out of the bottom of the engine.
The action force is -
THE DOWNWARD PUSH
The reaction force is -
THE UPWARD PUSH ON THE ROCKET ENGINE BY GAS MOLECULES

10. Why doesn’t the bowling ball move backward if the pin is pushing on it equally?
The bowling ball has more
MOMENTUM

11. Momentum The measure of how hard it is to stop a moving object.
Momentum = x
P = x
Momentum is a vector quantity
Units:
Mass = Velocity = Momentum =
MASS
VELOCITY m v kg
m/s
kg * m/s

12. Momentum More = more More = more More = needed to an object.
Which object would have more momentum?
MASS
MOMENTUM
VELOCITY
MOMENTUM
MOMENTUM
GREATER FORCE
STOP

13. Practice Problem 1
What is the momentum of a bird with a mass of kg flying at 15 m/s?

14. Practice Problem 2
A golf ball travels at 16m/s, while a baseball moves at 7 m/s. The mass of the golf ball is kg and the mass of the baseball is kg. Which has greater momentum?

15. Law of Conservation of Momentum
The total momentum of a group of objects stays the same unless outside forces act on the objects.
The cue ball has because it has mass and velocity.
When it hits the other balls , the cue balls
decrease.
The other balls start moving (and have mass) which mean they now have
MOMENTUM
VELOCITY
& MOMENTUM
MOMENTUM

16. Stopping an object with momentum
To stop an object we need to apply a force over a certain period of time. We call this impulse
Impulse – the force acting on an object in a specific amount of time.
IMPULSE = F x t
Force (F) = N
Change in time ( t) = s
Impulse (I) = N s

17. Using Newton’s 2nd Law to determine how hard it is to stop a moving object.
Newton’s 2nd Law tells us that F=ma
We could also look at this as:
And if we were to manipulate this formula by multiplying both sides by time it would end up looking like:
Impulse = change in momentum

18. Why does an egg break or not break?
An egg dropped on a tile floor breaks, but an egg dropped on a pillow does not. Why?
FΔt= mΔv
In both cases, m and Δv are the same.
If Δt goes up, what happens to the force?
Right! Force goes down. When dropped on a pillow, the egg starts to slow down as soon as it touches it. A pillow increases the time the egg takes to stops.

19. Practice Problem
A net force of 100 N is applied to a 20kg cart that is already moving at 3m/s. The final speed of the cart was 8m/s. For how long was the force applied?
Identify the variables
Mass = 20kg
Δvelocity = 8m/s-3m/s = 5m/s
Time = ?
Force = 100N

20. Practice Problem
A .057 kg tennis ball falls on a tile floor. The ball changes velocity from -1.2 m/s to +1.2 m/s in 0.02 s. What is the average force on the ball?
Identify the variables:
Mass = kg
Δvelocity = +1.2 – (-1.2) = 2.4 m/s
Time = 0.02 s
using FΔt= mΔv
F x (0.02 s) = (0.057 kg)(2.4 m/s)
F= 6.8 N

21. Collisions Objects collide in two different ways: Elastic collision –
Inelastic collision –
The amount of momentum involved before the collision will
When colliding objects bounce off each other
When objects collide and stick together.
always be the same after the collision.

22. Collisions with 2 moving objects
When two objects are moving in the and a occurs, the momentum of the slower object and the momentum of the faster object
SAME
DIRECTION
COLLISION
SPEEDS UP
SLOWS DOWN
BEFORE COLLISION
4 m/s
2 m/s
AFTER COLLISION

23. Collisions with 1 moving object
When object is moving and into a nonmoving object, all the is to the object.
ONE
COLLIDES
MOMENTUM
TRANSFERRED
NONMOVING
BEFORE COLLISION
4 m/s
0 m/s
AFTER COLLISION

24. Collisions with connected objects
When one object is moving and , but to a nonmoving object, the gets evenly split between the
COLLIDES
CONNECTS
MOMENTUM
TWO MOVING OBJECTS
BEFORE COLLISION
4 m/s
0 m/s
AFTER COLLISION