It turns out that having a net force is not enough to cause a change in the motion of an object. A net force must actually be present for some instant of time. A huge force acting for zero seconds accomplishes nothing. In fact, a small force acting for a long time can be as effective as a huge force acting for a short time.

Take the case of stopping a car. You can abruptly stop a car by stomping on the brakes or you can stop it gradually by lightly applying the brakes. In either case the car gets stopped. When we multiply the average force by the time it acted for, we find that in all three cases shown above the answer is the same. The product of average net force and change in time is called Impulse. The formula for it actually comes from a little manipulation of Newton's Second Law. This can be seen in the formula manipulation shown below.

The product of average net force and time interval is measured in Newton•Seconds [Ns]. Impulse is often symbolized with the capital letter "J".

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