A heavy lorry crawling and a light bullet flying can carry the same punch. Momentum captures that combination of mass and velocity, and in any collision or explosion, when nothing pushes from outside, the total momentum is exactly the same before and after.
Momentum is mass times velocity: p = mv, measured in kg m/s. It is a vector, so direction matters. In a closed system with no external resultant force, the total momentum before an interaction equals the total momentum after it.
Set the mass and velocity of each trolley, then press Play to watch them move and collide. Toggle between a collision where they stick together and one where they bounce apart. Notice that in both cases, the total momentum before the impact is exactly the same as the total momentum after it.
Because momentum is a vector, you must include direction. A trolley moving left has a negative velocity if right is positive, so its momentum is negative. Adding momenta without checking their signs is the single most common error examiners see in Paper 4.
A 1000 kg car moving at 20 m/s collides with a stationary 1500 kg car. They lock together and move off as one. Find their common velocity.