A collision occurs when two objects bump into each other. In physics, collisions are grouped into two main types based on what happens to energy.

What is an Elastic Collision?

An elastic collision is a perfect bounce. In this type of event, both the total momentum and the total kinetic energy remain exactly the same before and after the crash. Momentum is simply the mass of an object multiplied by its velocity. Kinetic energy is the energy an object has because it is moving. In the real world, no collision is perfectly elastic, but collisions between subatomic particles or billiard balls come very close.

What is an Inelastic Collision?

An inelastic collision happens when objects crash and change their shape or generate heat. While the total momentum is still conserved—meaning it stays the same—the total kinetic energy is not. Some of that energy is transformed into sound, heat, or permanent deformation of the objects. If two objects stick together after hitting each other, it is called a perfectly inelastic collision. Think of a lump of clay hitting a wall and sticking to it; all that movement energy is lost as the clay deforms.

Real-World Examples

• Elastic: Atoms hitting each other in a gas often behave like elastic objects because they don't stick together.
• Inelastic: A car crash is a classic inelastic collision. The metal bends, the car makes a loud crashing sound, and heat is produced, meaning kinetic energy is lost.
• Perfectly Inelastic: A bullet fired into a block of wood that stays embedded inside is a perfectly inelastic collision because the two objects move as one single mass afterward.

Key Differences

The main way to tell them apart is to look at what happens to the energy. If the objects bounce away perfectly without losing speed to heat or sound, it is elastic. If they dent, break, or stick together, the kinetic energy has been used up in other ways, making it an inelastic collision. Scientists use these rules to predict how objects will move after they hit each other.