 A 100% inelastic collision is one of the two extreme cases that can happen when two objects collide. The other extreme case is the 100% elastic collision to which I will get in another video. Now back to inelastic collision, what happens in an inelastic collision is that two objects are initially moving separately and after they collide, they stick together. Now what's interesting in the inelastic collision, the kinetic energy is actually not conserved. What happens? Some of the kinetic energy is used up to deform the objects. Think of a car collision, the car doesn't look as pretty as it looked at the beginning after it went into a collision. There is some energy needed to deform the metal, that energy is taken away from the amount of kinetic energy. So in the end we have kinetic energy final is the kinetic energy initial plus the work done during the collision. So it could be the work done to deform the material or some other forms of energy, maybe some heat is produced, maybe some sound is released. So that's why kinetic energy is not conserved in an inelastic collision. However, linear momentum is conserved. So we have momentum final is equal to momentum initial, which means that initially if I had a mass one here and a mass two here, I had a momentum of mass one times velocity one. Don't forget this is a vector plus mass two velocity two as a vector, initial initial. And on the other side we have mass one plus mass two together traveling both at a common refinal. This is kind of the key for that it's an inelastic collision is if they're moving together after a collision.