Momentum is mass in motion
Momentum measures how much motion an object has, combining how heavy it is with how fast it moves.
Its unit is the kilogram-metre per second (kg·m/s). Unlike kinetic energy, momentum grows in step with both mass and speed — no squaring — and it points in the direction of travel, which makes it a vector.
Why momentum is conserved
In any collision or push with no outside force, the total momentum before equals the total after. That one rule explains recoil, rocket thrust, and every break on a pool table.
| Object | Momentum |
|---|---|
| Walking person (70 kg, 1.4 m/s) | ~98 kg·m/s |
| Car (1200 kg, 14 m/s) | ~16,800 kg·m/s |
| Rifle bullet (4 g, 900 m/s) | ~3.6 kg·m/s |
Momentum vs kinetic energy
Both describe a moving object, but they are not the same. Momentum is conserved in every collision; kinetic energy can be lost to heat and deformation. A crash conserves momentum while destroying kinetic energy — which is exactly why crumple zones work.
Frequently asked questions
What is the momentum formula?
p = m v, where m is mass in kilograms and v is velocity in metres per second. The result is in kilogram-metres per second (kg·m/s). A 1200 kg car at 14 m/s has a momentum of 16,800 kg·m/s.
What are the units of momentum?
Kilogram-metres per second (kg·m/s) in SI. There is no separate named unit, unlike the joule for energy. Momentum also has a direction, so it is a vector quantity.
What is conservation of momentum?
In any collision or interaction with no outside force, the total momentum before equals the total after. It is one of the deepest rules in physics and explains recoil, rocket thrust and every break on a pool table.
What is the difference between momentum and kinetic energy?
Momentum (mv) grows in step with speed and is always conserved in collisions; kinetic energy (½mv²) grows with the square of speed and can be lost to heat and deformation. A crash conserves momentum while destroying kinetic energy.
Is momentum a vector?
Yes. Momentum has a direction as well as a size, so two equal momenta pointing opposite ways cancel. That is why a gun and its bullet gain equal and opposite momentum, leaving the total unchanged.