See the following site explaining Conservation of Momentum and Energy (for perfectly elastic collision) & Conservation of Momentum only (for inelastic or a bit of/partially elastic): Given any two masses moving in a certain same direction initially, where one collides into the other(the faster one crashes onto the slower one), can you write a program that shows the calculation of final velocity using conservation of momentum(total momentum before = total momentum after))? Then try calculating the total kinetic energy of the cars BEFORE and total kinetic energy AFTER collision(by which the energy will have been reduced due to things like heat)..
https://physics.stackexchange.com/questions/92051/how-can-momentum-but-not-energy-be-conserved-in-an-inelastic-collision
