Group Work

1. A hockey puck of mass 0.25 kg slides eastward across the ice at 25 m/s.a. What is its momentum p1 (magnitude and direction)?

b. The puck collides with a hockey stick that was lying motionless on the ice while its owner fights. The puck rebounds in the exact opposite direction of its approach, moving at a speed of 10 m/s. Now what is its momentum p2 (magnitude and direction)?

c. What was the momentum change p = p2 – p1 of the puck from before to after the collision (magnitude and direction)?

Exam ½ Retake

• should be available for you Thursday

• Finish the Equations of Motion exercise

Newton’s Third Law

actually stems from conservation of momentum

What’s the Point?

• Where do forces come from?

• Nothing changes its motion on its own!

• Conservation of momentum is one if the biggest ideas in physics.

Objectives

• Given the force exerted by one object on another, determine the reaction force.

• Use the conservation of momentum to analyze the motion of interacting objects.

Poll Question

If a 0.25-g insect collides with a 1250-kg compact car, which experiences the greatest (magnitude of) force in the collision?

A. The insect.

B. The car.

C. It’s a tie.

D. Insufficient information to answer.

Newton’s Third Law

• To every action there is an equal and opposite reaction.

• If object A exerts force F on object B, object B exerts force –F on object A, along the same line of interaction.

• FAB = –FBA

Bug + Windshield

Small car: 1250 kg

From the same force, the bug accelerates a lot more!

Large insect: 0.00025 kg

Poll Question

A. Oh, sorry, you’re right.B. It won’t, trust me.C. The cart’s pull isn’t the only force on you.D. It has to work. Newton must be wrong.

Your educated mule argues that there is no point in pulling a cart, because the cart will pull back on him as hard as he pulls on it. What should you tell him?

Interaction Forces

All forces are interaction forces!– gravity– wind– jumping– everything!

• This means: whenever something accelerates, something else accelerates in the opposite direction! Whoa!

Poll Question

If a 0.25-g insect collides with a 1250-kg compact car, which experiences the greatest (magnitude of) impulse in the collision?

A. The insect.

B. The car.

C. It’s a tie.

D. Insufficient information to answer.

All-Class Work

2. Show that when two otherwise isolated objects interact, their total change in momentum is zero.

p1 + p2 = 0or

p1 = –p2

Hint: When force F1 is applied to the first object for time t, what is its momentum change? What happens to the second object during this time?

Poll Question

When a bug hits a car windshield, whose momentum changes the most? (Assume there are no external forces.)

A. The bug’s.

B. The car’s.

C. It’s a tie.

D. Need more information to know.

Conservation of Momentum

Conservation of Momentum

The total momentum of an isolated system never changes.

Conservation of Momentum

• Newton’s first law: – no outside force– no change in v– thus no change in p

• So an isolated object’s momentum never changes.

Conservation of Momentum

• Newton’s third law: – interacting objects apply equal and opposite

impulses to each other– they experience equal and opposite

momentum changes

• So their total momentum remains the same.

Group Work

3. Continue the hockey scenario from problem 1. (puck p = 8.75 kg m/s W)

a. What was the momentum change of the hockey stick during the collision? (Momentum is conserved.)

b. If the hockey stick has a mass of 1.0 kg. What is its velocity (magnitude and direction) after the collision?

Elastic Collisions

• Objects bounce apart after collision– same relative speeds as before

• Total momentum is conserved– Some momentum is transferred from one

object to another

• Kinetic energy (more on that later) is also conserved

Totally Inelastic Collisions

• Objects cling together after collision– same final velocity

• Total momentum is conserved in the coupled mass

Inelastic Collisions

• Objects bounce apart after collision– relative speed less than initial

• Total momentum is conserved

• Kinetic energy less than initial

Group Work

4. If, instead of bouncing apart, the puck and stick clung together when they collided (totally inelastic collision), what would their velocity (magnitude and direction) be after the collision?

Hint: What is their momentum after the collision?

Group Work

5. Don’t calculate, just think and answer: If the puck were initially moving eastward, but had rebounded off the stick so that it moved northward after the collision, which direction would the stick have been moving after the collision?

Stick and puck have opposite momentum changes.

Reading for Next Time

• Work and Energy

• Power

• Important ideas– Work and energy are scalars– Kinetic and potential energy