4.28: Law of Conservation of Momentum

Difficulty Level: At Grade Created by: CK-12
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Practice Law of Conservation of Momentum

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These skaters are racing each other at Newton’s Skate Park. The first skater in line, the one on the left, is distracted by something he sees. He starts to slow down without realizing it. The skater behind him isn’t paying attention and keeps skating at the same speed.

Q: Can you guess what happens next?

A: Skater 2 runs into skater 1.

Conserving Momentum

When skater 2 runs into skater 1, he’s going faster than skater 1 so he has more momentum. Momentum is a property of a moving object that makes it hard to stop. It’s a product of the object’s mass and velocity. At the moment of the collision, skater 2 transfers some of his momentum to skater 1, who shoots forward when skater 2 runs into him. Whenever an action and reaction such as this occur, momentum is transferred from one object to the other. However, the combined momentum of the objects remains the same. In other words, momentum is conserved. This is the law of conservation of momentum.

Modeling Momentum

The Figure below shows how momentum is conserved in the two colliding skaters. The total momentum is the same after the collision as it was before. However, after the collision, skater 1 has more momentum and skater 2 has less momentum than before.

Q: What if two skaters have a head-on collision? Do you think momentum is conserved then?

A: As in all actions and reactions, momentum is also conserved in a head-on collision. You can see how at this URL:

Summary

• Whenever an action and reaction occur, momentum is transferred from one object to the other. However, total momentum is conserved. This is the law of conservation of momentum.

Vocabulary

• momentum: Property of a moving object that makes it hard to stop; equal to the object’s mass times its velocity.
• law of conservation of momentum: Law stating that, when an action and reaction occur, the combined momentum of the objects remains the same.

Practice

Watch the astropitch animation at the following URL. Experiment with different velocities. Then take the quiz and check your answers.

Review

1. State the law of conservation of momentum.
2. Fill in the missing velocity in the diagram of a vehicle collision seen in the Figure below so that momentum is conserved.

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Color Highlighted Text Notes

Vocabulary Language: English

TermDefinition
law of conservation of momentum Law stating that, when an action and reaction occur, the combined momentum of the objects remains the same.
momentum Property of a moving object that makes it hard to stop; equal to the object’s mass times its velocity.

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