BHS -> Mr. Stanbrough -> Physics -> Mechanics -> Momentum -> this page

In everyday language, an impulse is something you have - "I just had an impulse, so I bought a new coat." In physics, an impulse is something else. (You suspected that, didn't you?) In physics, an impulse is something that you apply to an object, as in the analogy given before.

When you apply a force on an object, you also exert an impulse on it. When something exerts a force on you, it also exerts an impulse on you. Forces and impulses always go together.

More force means more impulse - in fact, impulse is directly proportional to the applied force. Double the force, double the impulse - triple the force, triple the impulse, etc.

Impulse, however, is not the same as force. Impulse also depends on how long the force is applied. More time, more impulse. Impulse is also directly proportional to the time for which the force is applied - twice as long means twice the impulse, three times as long means three times the impulse.

So, the impulse exerted on an object depends directly on both how much force is applied and for how long the force is applied. Impulse is the product of the force and the time.

Impulse = (force)(time) = Ft

Impulse is a vector quantity. It has the same direction as the applied force.

The fact that impulse depends on both force and time means that there is more than one way to apply a large impulse to an object - you can apply a very large force for a "reasonable" time, or apply a "regular" force for a very long time (or both!).

Impulse diagram
You can get the same impulse by applying a large force for a short time, or applying a small force for a long time.

Calculating Impulse:

Calculating the impulse exerted on an object by some force is generally pretty easy and uncomplicated - impulse is always the product of the force and the time the force is applied. Since it is always the product of a force and a time, impulse has units like "Newton seconds".


A force of 5 Newtons acts on a ball for 4 seconds. How much impulse was exerted on the ball?


diagram of the situationImpulse = (force)(time) = (5 Newtons)(4 seconds) = 20 Ns

last update November 10, 2007 by JL Stanbrough