Elastic and Inelastic Collisions: Difference and Principles

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• 0:01 Collisions and Momentum
• 0:53 Elastic Collisions
• 2:20 Inelastic Collisions
• 3:48 Lesson Summary

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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

When objects come in contact with each other, a collision occurs. In this lesson, you'll learn about the two types of collisions as well as how momentum is conserved in each.

Collisions and Momentum

If you're a sports fan, you've seen plenty of collisions. These collisions occur between players but also between a baseball and a bat, a tennis ball and a racket, and a football and the kicker's foot. All of these are collisions because two or more objects came in contact with each other.

During a collision, the objects that come in contact with each other are considered a system. And as we learned in another lesson, this system follows the law of conservation of momentum, which states that the momentum of a system does not change when there are no external forces acting on it.

The momentum of the overall system doesn't change, but that doesn't mean that the momenta of the individual components of the system don't. Quite the opposite is true - the different parts that make up the system do have a change in momentum, but they are equal in magnitude and opposite in direction, so they cancel each other out.

Elastic Collisions

There are two types of collisions to be familiar with. The first is an elastic collision. This is when objects collide without permanent deformation and without generating heat. Think of a band of elastic - you can stretch it and pull it all you want, but once you let go, it returns to its original shape and size. The same is true for elastic collisions - the objects come out of the collision unharmed, like they never collided at all.

You can see this type of collision when you play a game of pool. Hitting the cue ball sends it across the table into another ball, and as it collides with the second ball, it sets it in motion. The momentum of the cue ball is transferred to the second ball, but neither ball is permanently deformed. Each ball looks the same as it did before the collision; they've just moved to different locations on the table.

Objects that collide in an elastic collision transfer momentum if they are traveling in the same direction, but when they are traveling at opposite directions, they will bounce when they meet. You can try this at home by dropping a bouncing ball on the floor. As it hits the floor, it bounces back up into the air with the same momentum as before.

Perfectly elastic collisions are a bit ideal. In most cases, some heat is generated when the billiard balls collide or the dropped ball hits the floor. But since the objects retain their shape after the collision, we can use this term to describe this type of collision in the real world.

Inelastic Collisions

The other type of collision that may occur is an inelastic collision. This is when the objects are deformed or stick together. Momentum is still conserved in these situations, but this time the objects do not escape the collision unharmed. And unlike elastic collisions, inelastic collisions do give off quite a bit of heat when the two objects come together.

Inelastic collisions happen all the time between cars on the road. During a head-on collision, two cars come together from opposite directions and both cars have a change in momentum because they come to rest. However, because the cars were traveling in opposite directions, the momentum for the system is the same both before and after the collision. After the collision, the momentum of the system is zero because neither car is moving. But it is also zero before the collision because the opposite directions of travel cancel each other out. It's like one car is traveling at 25 km/h while the other car is traveling -25 km/h.

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