These two laws are most often used when two objects collide. Momentum is conserved if there are no external forces, and it changes very little if the forces during the collision are much greater than the external forces. For example, if two cars collide, the two cars are the system. The forces between them are much larger than the forces on the wheels that come from outside the system, so momentum is conserved. Is energy conserved? While total energy is conserved, the kinetic energy before the collision is much greater than the energy afterward. Much of the energy goes into bending metal and breaking glass and plastic. Automobile crash reconstruction is a way of using conservation of momentum to figure out the speeds of one or both of the cars before the collision.

Kinetic energy is not always converted to other forms of energy in collisions. The toy “Newton’s Cradle” has a set of steel balls that swing on strings. In these collisions kinetic energy is almost totally conserved. The incoming ball stops when it hits a second ball, and the second ball moves away with the same speed, and thus kinetic energy, as the incoming ball had. Collisions of pool or billiard balls is another case where kinetic energy is conserved. This kind of collision is called “elastic” while the case of cars colliding is called “inelastic.”