1. Section 4 – pg 393 Newton’s Third Law
Chapter 10
Section 4 – pg 393
Newton’s Third Law

2. Newton’s Third Law of Motion
Pg 393
Newton’s Third Law of Motion
If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction
For every action there is an equal but opposite reaction

3. Action-Reaction Pairs
Pg 394
Action-Reaction Pairs
When you jump, you push on the ground with your feet (action force)
The ground pushes back with equal force (reaction force)
A kayaker speeds up by exerting an action force on water with a paddle (action force)
The water pushes back on the paddle with a equal force (reaction force)

4. Pg 394
Detecting Motion
Motion in action-reaction pairs is not always noticeable.
When you push off the earth during jumping, the earth inertia is too great for the force you exert to cause any noticeable motion

5. Do Action-Reaction Forces Cancel?
Pg 395
Do Action-Reaction Forces Cancel?
The action and reaction forces do not cancel out because they are acting on different objects.

6. Momentum All moving objects have a “quantity of motion” (momentum)
Pg 396
Momentum
All moving objects have a “quantity of motion” (momentum)
Momentum: a characteristic of a moving object that depends on both the mass and the velocity of the object
Momentum = Mass x Velocity
Described by its direction as well as quantity
The momentum of an object is in
the same direction as its velocity

7. The more momentum a moving object has, the harder it is to change its velocity
A car has a lot of momentum because it has a lot of mass
A bullet has a lot of momentum because it has a high velocity
Trains have more mass than cars, so they win
A bullet traveling through air at about 1.5 times the speed of sound can be seen in this image. When a moving object breaks the sound barrier, a shock wave and often turbulence are created. In this image, the curved line at the nose of the bullet is a shock wave. Other shock waves can be seen alongside the bullet, and a turbulent wake trails behind it.
Pg 397

8. Conservation of Momentum
The total amount of momentum objects have is conserved when they collide.
Momentum may be transferred from one object to another as the velocity of the objects change, but none is lost
The law of conservation of momentum states that, in the absence of outside forces, the total momentum of objects that interact does not change
Friction is an example of an outside force
Pg 397

10. Chapter 10 Section 4 Homework- Pg 399

11. 1A. State Newton’s third law of motion

12. 1B. According to Newton’s third law of motion, how are action and reaction forces related?

13. 1C. A boy catches a ball while standing on roller skates
1C. A boy catches a ball while standing on roller skates. What happens to the boy’s velocity and the ball’s velocity?

14. 2A. What is momentum?

15. 2B. What is the momentum of a parked car?

16. 2C. Why is it important for drivers to allow more distance between their cars when they travel at faster speeds?

17. 3A. What is meant by “conservation of momentum”?

18. 3B. The total momentum of two marbles before a collision is 0
3B. The total momentum of two marbles before a collision is No outside forces act on the marbles. What is the total momentum of the marbles after the collision?

19. 4. What is the momentum of a 920-kg car moving at a speed of 25 m/s?
Remember: Momentum = Mass x Velocity

20. 5. Which has more momentum: a 250 kg dolphin swimming at 4 m/s, or a 350 kg manatee swimming at 2 m/s?
250 x 4 = 1, x 2 = 700