Definition · Plain-language
Momentum
Momentum is a measure of the quantity of motion an object has, equal to its mass multiplied by its velocity.
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Mass in motion
Momentum captures how much motion an object carries, combining how heavy it is with how fast it is going. It is calculated as mass multiplied by velocity, so a lorry moving slowly can have more momentum than a fast bicycle, and the same object has more momentum the faster it travels. Because it depends on velocity, momentum is a vector: it has a direction as well as a size, pointing the same way as the motion. Its SI unit is the kilogram metre per second.
Conservation of momentum
The most important fact about momentum is that it is conserved. In a closed system with no external force, the total momentum stays constant: whatever momentum is lost by one object is gained by another. When two objects collide, their combined momentum before the collision equals their combined momentum afterwards. This principle explains recoil — a gun kicks back as the bullet flies forward — and is the reason rockets work, throwing exhaust one way to drive the rocket the other.
Momentum, force and impulse
Force and momentum are closely linked: a force is what changes an object’s momentum, and the longer a force acts, the greater the change. This product of force and time is called impulse, and it equals the change in momentum. The idea explains why crumple zones, airbags and crash mats make impacts safer: by extending the time over which momentum is brought to zero, they reduce the force felt. A small force acting for a long time can change momentum as much as a large force acting briefly.
Key facts
At a glance
- Definition: the quantity of motion, equal to mass times velocity
- Formula: p = m × v
- Quantity type: a vector — has magnitude and direction
- SI unit: kilogram metres per second (kg·m/s)
- Conservation: total momentum is constant with no external force
- Impulse: force × time equals the change in momentum
Common misconceptions
What people often get wrong
Often heard: Momentum is just another word for speed.
Actually: Momentum depends on both mass and velocity, not speed alone. A slow but heavy object can have far more momentum than a fast but light one, and momentum has a direction whereas speed does not.
Often heard: Momentum can simply disappear when things stop.
Actually: In a closed system momentum is conserved. When something stops, its momentum is transferred to whatever stopped it — the ground, a wall or another object — not destroyed.
Often heard: Momentum and kinetic energy are the same quantity.
Actually: They are different. Momentum is mass times velocity and is a vector; kinetic energy is half mass times velocity squared and is a scalar. Both describe motion, but they behave differently in collisions.
Going deeper
