What Is Impulse in Physics?

When you apply a force for a certain amount of time, you create an impulse. In fact, that's the definition of impulse — impulse equals the force applied multiplied by the time it was applied. Here's the equation:

Impulse = Ft

Note that this is a vector equation because the force has a direction; therefore the impulse does as well. Impulse can be an important quantity when you're solving physics problems because you can relate impulse to momentum, and you must work with momentum to solve most collision problems in physics.

Here's an example of impulse in action: You're playing pool, and you strike a pool ball with the cue. The cue may be in contact with the ball for only a millisecond, but there's an observable result — the ball is now in motion. That is a result of impulse.

What are the units of impulse? You have force multiplied by time, so the unit is the Newton-second.

Sample question

1. Suppose that you're playing pool and hit a pool ball for 10.0 milliseconds (a millisecond is 1/1,000 of a second) with a force of 20.0 N. What impulse did you impart to the pool ball?

   The correct answer is 0.200 N-s, in the direction of the force.

   1. Use the equation Impulse = Ft.

   2. Plug in the numbers:

      Impulse = Ft = (20.0 N)(1.00 x 10^–2 s) = 0.200 N-s, in the direction of the force.

Practice questions

1. You're disgusted with your computer and give it a whack. If your hand is in contact with the computer for 100.0 milliseconds with a force of 100.0 N, what impulse do you impart to the computer?

2. You're standing under the eaves of your house when a huge icicle breaks off and hits you, imparting a force of 300.0 N for 0.10 seconds. What was the impulse?

Following are answers to the practice questions:

1. 10.00 N-s

   1. Use the equation Impulse = Ft.

   2. Plug in the numbers:

      Impulse = Ft = (100.0 N)(0.10000 s) = 10 N-s

2. 30 N-s

   1. Use the equation Impulse = Ft.

   2. Plug in the numbers:

      Impulse = Ft = (300.0 N)(0.10 s) = 30 N-s