6.2 Conservation of Momentumpp. 215 - 221

Mr. Richter

Agenda

Warm-Up Check HW Return Lab Notebooks Intro to Conservation of Momentum Notes:

N3L and Conservation of Momentum Conservation of Momentum Recoil

Lab Wednesday

Objectives: We Will Be Able To… Describe the interaction between two objects in terms

of the change in momentum of each object. Compare the total momentum of two objects before

and after they interact. State the law of conservation of momentum. Predict the final velocities of objects after collisions,

given the initial velocities.

Warm-Up:

A man and an elephant standing on skateboards push off of each other.

Which, if either, pushes with more force? Which, if either, pushes for a longer amount of time?

Newton’s Third Law and Conservation of Momentum

N3L and Conservation of Momentum

When two objects collide they push off of each other. According to Newton’s Third Law, they push with equal

and opposite forces. Also, they must push for the same amount of time.

One cannot push the other without being pushed as well.

This means that their impulses (FΔt) are equal and opposite, which means their change in momentum must be equal and opposite as well.

N3L and Conservation of Momentum

Algebraically:

The Law of Conservation of Momentum

In words: the total momentum of all objects interacting with one another remains constant, regardless of the nature of the forces between the objects.

Momentum is conserved whenever two or more objects collide (more on specific types of collisions in the next section)

This is only true in the absence of friction.

Momentum Conservation ExampleBefore the Collision After the Collision

Conservation of Momentum: Problem Solving

A 20 kg Object A traveling to the right at 5.0 m/s collides with a stationary 15 kg Object B initially at rest. After the collision, object A continues moving to the right at 4.0 m/s. What is Object B’s velocity after the collision?

1.3 m/s to the right

RecoilWhen objects push off of each other.

Recoil

Recoil scenarios are special types of collisions when two objects push off of each other when both are initially at rest.

Some examples (all are assumed frictionless): Our warm up, with the man and the elephant. Shooting a rifle Two ice skaters pushing away

Recoil

The conservation of momentum equation changes when both initial velocities are zero.

The momentum of one object is equal and opposite the momentum of the other object in recoil scenarios.

Recoil Problem Solving

An 80.0 kg man and a 400 kg elephant stand next to each other on skateboards. If they pushed off of each other, the man would move to the left at 12.0 m/s. What would the elephant’s velocity be?

2.40 m/s to the right

Wrap-Up: Did we meet our objectives?

Describe the interaction between two objects in terms of the change in momentum of each object.

Compare the total momentum of two objects before and after they interact.

State the law of conservation of momentum. Predict the final velocities of objects after collisions,

given the initial velocities.

Homework

Due Thursday p. 219 #1, 2, and 4 p. 221 #1-3