A perfectly inelastic collision happens when two objects hit each other and stick together, moving as one single object afterward.

The Key Feature: Sticking Together

In most crashes, objects bounce off each other. However, in a perfectly inelastic collision, the objects do not bounce. Instead, they combine into a single mass. Because they move together after the impact, they must share the same final velocity. This unique behavior makes them easy to identify in physics problems.

Conservation of Momentum

Even though the objects stick together, the total momentum—which is the measurement of an object's mass in motion—always stays the same before and after the crash. If you know the mass and speed of the objects before they hit, you can calculate how fast the combined mass will move afterward. The formula is: (m1 × v1) + (m2 × v2) = (m1 + m2) × V. Here, 'm' stands for mass, 'v' stands for velocity, and 'V' is the final speed of the joined objects.

Energy Loss

A perfectly inelastic collision is famous for losing kinetic energy. Kinetic energy is the energy an object has because it is moving. When the objects collide and stick, some of that energy is turned into heat, sound, or the energy used to deform the objects permanently. Because energy is "lost" to the surroundings, this type of collision is not elastic; it is the opposite.

Real-World Examples

• Mud hitting a wall: If you throw a ball of wet mud at a wall and it sticks without bouncing, it is a perfectly inelastic collision.
• A bullet in a block of wood: When a bullet hits a hanging wooden block and stays inside it, the two move together as one.
• Car wrecks: Sometimes, if two cars collide and their bumpers lock together or the cars crumple into one another, it acts like a perfectly inelastic collision.