Two identical cars with mass m = 1,325 kg are traveling in opposite directions at speeds of 54 m/s. They collide head on. After the collision, both cars recoil and move at 43 m/s, in opposite directions. According to the law of conservation of linear momentum, what type of collision occurred and how much kinetic energy was lost? A) The collision was inelastic and 1.4 * 106 joules of energy were lost. B) The collision was inelastic and 0.7 * 106 J joules of energy were lost. C) The collision was perfectly elastic and 1.4 * 106 joules of energy were lost. D) The collision was perfectly elastic and 0.7 * 106 joules of energy were lost.

In a collision the kinetic energy is sometimes the same before the collision as it is afterwards - such a collision is called an elastic collision. Perfectly elastic collisions are rare - usually some of the kinetic energy is converted to sound or heat. Even a trampolinist colliding with a trampoline bed loses energy as the bed stretches and heats up. A perfectly inelastic collision is one where all the kinetic energy is converted into other forms - a piece of soft putty falling onto the floor is an almost perfectly inelastic collision, the kinetic energy of the moving putty is converted into heat and a little sound as the putty is deformed on impact. Most real collisions are usually somewhere between the two extremes.