The fist step in the process of modeling collisions between two moving objects is to define a new quantity—momentum. Momentum (p) is defined as mass times velocity. This means that the more massive an object is, the more momentum it has. That's why it's extremely difficult to bring a tractor-trailer to an abrupt stop. Likewise, a large velocity also results in a lot of momentum, so even if you're driving a little sports car, it's still difficult to stop at 100mph.
p = mv
where m = mass and v = velocity.
Because momentum is based on velocity, which constantly changes, it makes sense to talk about instantaneous momentum at specific points in time or change in momentum over a time interval.
Let's quickly ...