1. Momentum is always conserved in collisions!

2. Energy is always conserved. So why do we worry about conservation of kinetic energy in elastic collision? Can’t we always at least apply the conservation of energy during collision? The answer is yes and no.  We can apply it since it’s true, but it usually does us no good to apply it since it’s too difficult for us to keep track of the energy during a collision. For example, imagine closing your eyes while a friend of yours rolls one pool ball into another. You know when they collide.  How? Because you can hear the sound of the collision. Sound waves carry energy.  The balls also heat up slightly during a collision. It takes energy to heat up an object.  These forms of energy transfer are too subtle or complicated for us to keep track of, so, although the energy is conserved, it’s too difficult for us to apply.  Except in the case of elastic collisions.  In this case, not only is the total energy conserved, but so is the kinetic energy (energy of motion). This is simply because the conservation of momentum applies to the time interval from immediately before the collision to immediately after the collision.  During this time interval, nothing has had a chance to move, so the potential energies (energy of position) do not change.  Therefore, since the total energy does not change and the potential energies do not change, it follows that the kinetic energy cannot change!  this is important for us, since it gives us more information about the motion of the objects undergoing the collision.