Momentum and impulse
Impulse is the product of a force and the time it acts for, and it equals the change in an object's momentum.
Practise this conceptMomentum is the product of an object's mass and velocity — a heavy truck rolling slowly and a light car speeding along can have the same momentum, and both would take the same push to stop.
Impulse is what changes that momentum: it's the product of a force and how long that force acts for, and it always equals the resulting change in momentum. This is why airbags and crumple zones work — they don't reduce the impulse needed to stop you (that's fixed by your mass and speed), they stretch out the time over which it's delivered, which means a smaller force is enough to produce it.
A gentle push held for a long time can produce exactly the same impulse, and therefore the same change in momentum, as a hard shove applied for an instant — force and time trade off against each other as long as their product stays the same.
Key ideas
Requires: Newton's second law
Unlocks: Conservation of momentum
Formulas
| Symbol | Name | Unit |
|---|---|---|
| momentum | kg*m/s (kilogram metre per second) | |
| mass | kg (kilogram) | |
| velocity | m/s (metre per second) |
Common misconceptions
- Momentum and force are the same physical quantity.
- A larger force always produces a larger impulse, no matter how long it acts for.
- Impulse and momentum are measured in the same units as force.