“Momentum is conserved in any collision if the effect of any external forces present is negliable relative to the effect of the collision.”
To explain it a different way; imagine playing pool. If you have good form, and hold the Cue-stick properly, and follow through on a straight-shot, then the cue-ball goes straight, and it collides with, say, the eight-ball.
If a straight imaginary line can be drawn down the length of the stick, and through the very center of the cue ball and through the very center of the eight-ball… then the collision of stick and cue-ball, and the collision of cue-ball and eight ball will result in all of the from the cue-stick travelling first ball into the second, and the eight-ball will travel in that same straight line (assuming all other forces in the game, like the felt, the angle of the table-top, are all negligible.)
Another example is the famous “Newton’s Cradle”:
Imagining two objects colliding, the results are the same, if a little more complicated. You see, Momentum is the result of multiplying mass and velocity (which includes a direction, a vector).
So when two objects, each with mass and force, collide, the momentums of both objects can change, but the sum of the momentum’s remain the same.
