In physics, impulse is the change in momentum of an object over a period of time. Momentum is a measure of an object's resistance to changes in its velocity. It is equal to the mass of the object times its velocity. The relationship between impulse and momentum is that impulse causes changes in momentum. For example, if you hit a tennis ball with a racket, the force of the impact causes the ball to change its velocity (momentum). The bigger the force, the greater the change in momentum will be.