SIMPLIFIED: "10.2: Collisions"

Article Link: https://phys.libretexts.org/Bookshelves/University_Physics/Book%3A_Introductory_Physics_-_Building_Models_to_Describe_Our_World_(Martin_Neary_Rinaldo_and_Woodman)/10%3A_Linear_Momentum_and_the_Center_of_Mass/10.02%3A_Collisions 

Note: The above image was taken from the original article.

This article is all about collisions! There are essentially two types of collisions: elastic and inelastic. There are also explosions but the two main collisions you should know are elastic and inelastic. Here is the difference: in elastic collisions, the kinetic energy in the system is conserved after the collision, while in inelastic collisions, the kinetic energy in the system is NOT conserved.

In the real world, it is basically impossible to have a perfectly elastic collision because whenever two things collide with each other, no matter how little the impact is, some energy will be lost due to heat or friction. Therefore, when you do physics labs for elastic collisions, in reality, there is a tiny bit of energy lost. 

There is also something called a perfectly inelastic collision. This is when two objects collide and STICK together, moving together as one velocity. This means that when you set up the momentum equation, you need to assume that both masses have the exact same velocity after the collision. 

Ultimately, these definitions are helpful when solving momentum problems involving at least two masses. If the collision is elastic, you need to remember that the total energy in the system is conserved. 

Finally, there are explosions. This is a concept that is not as common in an advanced physics course such as AP Physics but it is still good to know. An explosion means a single object explodes into different pieces. Surprisingly, the total momentum of the system will still be conserved both before and after the explosion, but unlike elastic collisions, the total kinetic energy of the system will not be conserved because there is dissipated energy during the explosion due to heat.