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Analyzing Elastic & Inelastic Collisions

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  • 0:04 Analyzing Collisions
  • 1:42 Elastic vs. Inelastic…
  • 2:26 Identifying Collisions
  • 3:44 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 have the chance to explore the differences between elastic and inelastic collisions, and use that knowledge to solve problems. Lesson topics will include equations and theories related to momentum.

Analyzing Collisions

Crash! Two cars hit each other. Metal bends. Upholstery burns. The car chase ends, and the criminal is caught. Particularly in the movies, there's something exciting about a good car crash; but when all the dust has settled, someone needs to figure out what happened and why.

Car Crash
Car Crash

There are several tools we can use to understand car crashes. The most important one is conservation of momentum. In a nutshell, we define momentum as mass times velocity, or m*v, where momentum is always conserved. That means the total momentum before a collision happens is equal to the total momentum after a collision happens.

We can use this information about momentum to write an equation, and if we know all of the numbers but one, we can solve the equation and find what we're missing.

Conservation of Momentum
Conservation of Momentum

But what if we don't know what the rest of the numbers in the equation are? Well, in that case, we need to use another tool, such as the impulse momentum theorem. The impulse momentum theorem states that the impulse (force multiplied by time) applied to an object in a collision is equal to the change in momentum of that object. For instance, if we know the length of the cars involved in the crash and how much force was felt, that will help us a lot.

Impulse-Momentum Theorem
Impulse-Momentum Theorem

However, there's one more piece of information that can help us solve collision problems: what type of collision occurred?

Elastic vs. Inelastic Collisions

There are two types of collisions: elastic and inelastic. An elastic collision can be thought of as a bouncy collision, while an inelastic collision can be thought of as a sticky collision. For example, the 'cleaner' the bounce the more elastic the collision.

A fully elastic collision is one where kinetic energy is conserved, or the total kinetic energy before the collision is equal to the total kinetic energy after a collision. Billiard balls tend to have pretty clean elastic collisions.

A fully inelastic collision is one where kinetic energy isn't even close to being conserved. This happens because energy is absorbed by the surroundings. To be exact, it's one where the objects stick completely together and move forward as one object.

Identifying Collisions

But how do you know whether a collision is elastic or inelastic? The idea of bouncy and sticky collisions is a bit subjective. For instance, how bouncy does a collision have to be to be considered elastic? You can't really judge that with your eyes. Like everything in physics, we need numbers.

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