There are circumstances where a large force acts for a very short time, so that neither the force nor the time for which it acts can easily be evaluated separately, e.g. a cricket bat hitting the ball, a shot being fired, a footballer kicking a ball, etc.

 

These are examples of impulsive forces and in such cases the impulse oft the force cannot be calculated using J = Ft. The change in momentum caused by the impulse, however, can be used to evaluate the impulse.