Momentum is a property of moving objects — AQA GCSE Physics
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Momentum is a property of moving objects explained
Momentum measures how hard a moving object is to stop.
Read the full explanation
It is the product of mass and velocity, p = m v, with mass in kilograms, velocity in metres per second and momentum in kilograms metre per second, kg m/s. A 1200 kg car at 15 m/s has a momentum of 18 000 kg m/s. Because velocity is a vector, momentum is a vector too: it has a direction, and in a calculation with objects moving opposite ways one direction must be taken as positive and the other as negative. A slow lorry and a fast motorbike can have the same momentum, which is why mass and velocity must always be quoted together, never one on its own. Momentum is not kinetic energy: two objects can carry equal momentum yet very different energies, because energy depends on the square of the velocity.
Your focus
- Calculate the momentum of a moving object from its mass and velocity, giving the unit kg m/s.
- Rearrange p = m v to find the velocity of an object of known mass and momentum.
- Explain how a lorry and a motorbike can have equal momentum at very different speeds.
Momentum is a property of moving objects exam tips
Marking Points
- one mark for writing momentum = mass × velocity
- one mark for a correct substitution with mass in kg and velocity in m/s
- one mark for the answer with the unit kg m/s
- one mark for stating the direction, or using a sign, when momentum is treated as a vector
Examiner Tips
- 💡Write the equation before the numbers; p = m v is worth a mark on its own in many schemes.
- 💡Choose a positive direction at the top of your working and label it, then keep to it.
- 💡Check the unit carefully: kg m/s has no division sign anywhere in it.
Common Mistakes
- giving the unit as kg/m/s or kg m/s²
- leaving the mass in grams or the velocity in km/h before substituting
- treating momentum as a scalar and adding two oppositely directed momenta
- confusing momentum with kinetic energy, which uses the square of the velocity