NEWTON’S THIRD LAWSection 12.3

Newton’s Third Law

When an object exerts a force on a second object, the second object exerts an equal and opposite force on the first

Action/reaction forces

Examples of Action/Reaction Forces

Hammer and nail

Swimming

Do they always produce motion?

Do they cancel and produce a net force of 0?

Think About It…

Are brakes on a bicycle or a car larger?

Is it easier to catch a bowling ball or a tennis ball?

Without a glove, is it easier to catch a baseball thrown lightly or forcefully?

Momentum

Product of an object’s mass and velocity

Momentum (p) = m × v

Can vary based on your frame of reference

Calculating Momentum

What is the momentum of a 7-kilogram bowling ball that’s heading towards the pins at 6 m/s?

What are the units for momentum?

Calculating Momentum

What is the momentum of a 200,000-kilogram train traveling into the station at 18 m/s?

Is the momentum of a race car (9000 kg) going at 95 m/s higher or lower than the train?

Calculating Momentum

A man on a bicycle (total mass of 85 kg) rides down the street at 9 m/s.

A woman in a truck backs slowly out of her driveway at 2 m/s. The truck’s mass is 950 kg.

Which has a higher momentum?

Momentum is a vector!

Momentum can be positive or negative, depending on the direction of motion?

YOU NEED TO DRAW A PICTURE TO EXPLAIN THE DIRECTIONS!

What is the total momentum of the objects? Ptotal = m1v1 + m2v2

Two kids in two bumper cars travel towards one another. The first (mass of 80 kg) travels at 5 m/s while the other (mass of 75 kg) travels towards the first at 8 m/s.

What is the total momentum of the system?

A pitcher throws a baseball towards the plate at 42 m/s. The baseball has a mass of 0.25 kg. The batter swings his bat (which has a mass of 0.90 kg) towards the ball at 25 m/s. What is the total momentum?

Rolling Wagon

A child with a mass of 42 kg is rolling down a hill in a wagon. If the speed of the wagon is 7.5 m/s and the mass of the wagon is 30 kg, what is the total momentum of the system?

Conservation of Momentum

What does it mean for something to be “conserved”?

What happens when two bowling balls hit one another at the same speed but traveling in opposite directions?

If a moving object hits a non-moving one and they stick together, will the two objects be moving at a slower speed or a faster speed?

Conservation of Momentum

Momentum of a system is equal before and after a collision (as long as no outside forces, like friction, are acting on the objects of the system)

Conservation of Momentum

pbefore = pafter

mbeforevbefore = maftervafter

You might have to calculate the TOTAL momentum before/after the collision if there are more than two objects.

Types of Collisions

Elastic Collision Inelastic Collision

Example #1

In billiards, a cue ball is hit and begins rolling across the table at 3.5 m/s. It then strikes another ball. The cue ball stops dead. If the balls are of equal weight/mass, how fast does the other ball travel after the collision?

Example #2

A man pushes a 2 kg model train car along a track so that it reaches a velocity of 9 m/s. It then collides with a second 2 kg train car (not moving) and the two cars stick together and continue down the track. At what speed are the linked-together cars traveling now?

Example #3

A wheeled 300 kg cannon sits at rest with a 20 kg cannonball inside of it. When fired, the cannonball shoots east at 70 m/s. Which direction will the cannon travel in and at what velocity?