Momentum is one of the most reliable ideas in physics: in a closed system, no matter how objects collide, bounce, or stick together, the total momentum before an interaction always equals the total momentum after. That single rule — conservation of momentum — is what lets you predict the outcome of a collision without knowing anything about the forces involved during the impact itself.
What momentum measures
Momentum (p = mv) combines how much mass is moving with how fast it's moving. A heavy truck crawling along a parking lot and a lightweight bicycle flying down a hill can have similar momentum, even though their speeds are wildly different — mass and velocity trade off against each other. Because velocity is a vector, momentum is too: two objects of equal mass and speed moving toward each other have momentum that points in opposite directions, and their momenta can partially or fully cancel when combined.
Why total momentum doesn't change
Newton's third law says every force has an equal and opposite reaction force. During a collision, whatever force object A exerts on object B, object B exerts the exact opposite force back on A, for exactly the same amount of time. Those forces change each object's momentum by equal and opposite amounts, so the total momentum of the system — the sum of both objects' momenta — never changes, regardless of how complicated or violent the collision is. This is true whether the collision is elastic (bouncy, kinetic energy conserved) or inelastic (objects deform or stick together, kinetic energy is lost to heat and sound).
Perfectly inelastic collisions
The simplest collision to analyze is a perfectly inelastic one, where the two objects stick together afterward and move as a single combined mass. Because they move together, they share one common velocity — found by dividing the total momentum before the collision by the total mass of both objects. This is exactly what car-crash analysts, ballistics investigators, and physics students use to reconstruct what happened in an impact from before-and-after measurements, without needing to know the messy details of the forces during the collision itself.