A collision is a brief, intense interaction between two bodies. During the collision the internal contact forces are very large, while the external forces (weight, friction) give a negligible impulse: the total momentum is always conserved. What distinguishes the types of collision is what happens to the kinetic energy. This section classifies collisions, solves the elastic collision in one dimension, and shows the bubble diagram to see where the energy ends up.

Try it — interactive simulation

Spheres colliding in a box: the central arrow shows the total momentum, the panel the kinetic energy. With periodic edges the system is isolated and the total momentum stays constant; switch to inelastic collisions to see the kinetic energy drop while momentum is conserved. Click to add a ball.