Often real-world collisions involve objects sticking together. When a projectile like a bullet or a baseball reaches its target, it is often embedded in the target or caught by the target. When two people or two vehicles collide, they often become entangled (accidentally or on purpose).

During collisions, external forces are often neglected and it is assumed that momentum remains constant to a good approximation. A totally inelastic collision greatly simplifies the equations of momentum conservation for a collision, since in the final state only one object remains. For a two-body totally inelastic collision, the equation of momentum conservation becomes:

or, in terms of the masses of the initial objects:

These equations are easily generalized to more objects.