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, Torque and angular 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.
Resources
Part of these UT courses
This concept is mapped as covering part of the following course(s) in the University of Tartu curriculum.
- The physical world viewLOFY.01.002
- Materials physics practical I — mechanics and heatLOFY.02.055
- Physics practical I — mechanics and heatLTFY.01.004
- Mechanics and heatLTFY.01.006