Momentum and energy are useful concepts because these quantities are conserved, that is, they do not change with time. Likewise, you cannot create momentum out of nothing, nor can you get rid of it - you can only transfer it from one body to another. For instance, a traveling car has a lot of momentum. If the car strikes an unlucky pedestrian, then the car's momentum is partly transferred to the pedestrian and partly transferred to other things it comes in contact with, including the earth. If some of the kinetic energy of the objects is transformed into heat or other forms of energy, then the collision is inelastic .
Most collisions encountered in everyday life are inelastic.
Notice that kinetic energy is only conserved in the special case of elastic collisions. In general, kinetic energy is not conserved. However, momentum is always conserved, regardless of whether the collision is elastic or inelastic. In this lab, we will study the collision of gliders on an air track. First, we will study an inelastic collision, in which the two gliders stick together after the collision.