Chapter 10

Section 4 – pg 393Newton’s Third Law

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

Pg 393

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)

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

Pg 394

Do Action-Reaction Forces Cancel?

• The action and reaction forces do not cancel out because they are acting on different objects.

Pg 395

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

Pg 396

• 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

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

Chapter 10 Section 4Homework- Pg 399

1A. State Newton’s third law of motion

1B. According to Newton’s third law of motion, how are action and

reaction forces related?

1C. A boy catches a ball while standing on roller skates. What

happens to the boy’s velocity and the ball’s velocity?

2A. What is momentum?

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

2C. Why is it important for drivers to allow more distance between their

cars when they travel at faster speeds?

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

3B. The total momentum of two marbles before a collision is 0.06

No outside forces act on the marbles. What is the total momentum of the

marbles after the collision?

4. What is the momentum of a 920-kg car moving at a speed of 25 m/s?

Remember: Momentum = Mass x Velocity

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 = 1,000 350 x 2 = 700