What is an inelastic collision?

A collision between objects that permanently changes the objects’ shape and/or generates heat.

In brief:

A collision between objects that permanently changes the objects’ shape and/or generates heat. In fancy physics-y language (borrowed from Paul Hewitt)—”A collision that results in lasting deformation and/or the generation of heat.”

A practical definition that covers the most common situation encountered in an introductory physics course:

“When two objects collide and stick together.”

For example:

  1. You throw a piece of clay at the wall, to which the clay sticks.
  2. Two automobiles crash into each other, and the front ends of each crumple up.
  3. A moving train car bumps into a train car that was stationary. After the collision the two train cars move on together, connected.
  4. You bounce a basketball on the ground for a while and notice that it gets warmer and warmer as you bounce it.

What’s happening in each of these examples?

  1. The two objects that collide are the clay and the wall. They stick together, so you have an inelastic collision.

    When two objects stick together, they form a new “shape,” so the collision between the clay and the wall does result in a permanent change of shape for both objects, despite the fact that you may still be able to distinguish the clay from the wall. Before you had two separate objects, and now you have one.

  2. Lasting deformation—the shape of each car is permanently changed.

  3. The cars stick together. Even though each appears to retain its initial shape, together the cars form one new object.

  4. Though the basketball and the ground do not stick together, the generation of heat tells us that we have an inelastic collision.

    Note:

    A basketball bouncing off the ground approximates an elastic collision because the ball and ground do not experience any lasting change of shape. In reality, however, it is not a perfectly elastic collision because the ball (and the ground!) heat up. Therefore, we can call it an inelastic collision.