Linear Momentum, Impulse & Energy Conservation

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  • 0:00 What Are Collisions?
  • 0:41 Impulse and Linear Momentum
  • 2:30 Energy in Collisions
  • 3:36 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

In this lesson, you'll learn about linear momentum, impulse, and energy conservation. These are all important components of collisions, which can be large, small, and everything in between!

What Are Collisions?

Car crashes that involve bent metal and destruction are exciting. And while they may be a major part of human life, in the context of the whole universe they aren't that big a deal. A collision occurs when one object hits or strikes another object. Collisions are happening constantly throughout the universe, like when asteroids hit each other or when molecules collide. What's neat is that collisions between asteroids and molecules are pretty similar to collisions between cars.

Let's talk about some of the key concepts involved in collisions to better understand what goes on during these interactions.

Impulse and Linear Momentum

Linear momentum and impulse are both important for understanding collisions. Linear momentum is a quantity that represents the motion of a body, and is defined as mass times velocity. Even without knowing the definition, you were probably already familiar with the concept of momentum, or mass in motion. For example, a football player running down a field is a body with a lot of momentum.

Because the momentum of an object is proportional to both its mass and velocity, moving objects that have a lot of mass have a lot of momentum, as do fast moving objects like race cars. If a moving object would be hard to stop, you can bet it has a lot of momentum. That momentum is linear as long as the object is moving forward, backward, left, or in any other straight line direction.

Momentum is always conserved in collisions. Momentum is never created or destroyed, it is only transferred from one object to another. This is called conservation of momentum. When two cars crash into each other, each individual car might experience a gain or loss in momentum, but within the overall collision the gains equal the losses, so there is no net change in momentum. Understanding conservation of momentum will allow you to calculate how the momentum of each object involved in a collision will change, as long as you know the momentum of the objects before the collision.

Collisions also involve impulse, which is the change in an object's momentum, either an increase or a decrease. An impulse is defined as a force applied over a period of time, or force times time. Whenever a force is applied to an object, that object will gain or lose momentum depending on both the magnitude of the force and over how much time that force is applied.

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