Momentum
Why is a slow lorry so much harder to stop than a fast football?
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Explain
Big, fast things are hard to stop.
Small, slow things are easy to stop.
Momentum tells you how hard something is to stop. It depends on mass and speed together.
A heavy lorry going slowly can have more momentum than a light ball going fast.
Momentum has a direction too. It points the way the object is moving.
p = m × v. Its unit is the kilogram metre per second (kg m/s), and it is a vector.
Unlike kinetic energy, momentum depends on speed, not speed squared.
Worked example: a 1500 kg car moves at 4 m/s, and a 0.4 kg ball moves at 25 m/s. Their momenta are 6000 kg m/s and 10 kg m/s.
- More mass, more momentum
- More speed, more momentum
- Harder to stop
Want the full story?
Newton really wrote his second law in terms of momentum: force equals the rate of change of momentum. Momentum matters even for light, which has no mass, because light still carries momentum.
The Formula
momentum = mass × velocity
- the momentum, measured in kilogram metres per second (kg m/s)
- the mass, measured in kilograms (kg)
- the velocity, measured in metres per second (m/s)
Try the numbers
Momentum: 100 kg m/s
Try it at home
You need two balls of different sizes, a toy car.
You need: two balls of different sizes, a toy car.
Stop a rolling ball with your hand.
- Roll a small ball slowly at a toy car.
- Roll a heavier ball at the same speed.
- Compare how far the toy car moves.
What's happening: the heavier ball has more momentum, so it gives the car a bigger push.
Physics is life
Your body
A running friend is hard to stop.
Daily life
Trains take a long way to stop.
Nature
Elephants are hard to stop.
Space
Rockets gain momentum.
Quick check
Question 1 of 3
Words to know
Momentum: Mass multiplied by velocity. It tells you how hard something is to stop.