Momentum and Conservation of Momentum — AQA GCSE Physics (Higher Tier)
Momentum is a property of moving objects, and it is conserved in collisions and explosions.
Momentum
Momentum is a vector quantity (it has direction):
$$p = m \times v$$
- p = momentum (kg m/s)
- m = mass (kg)
- v = velocity (m/s)
The greater the mass or velocity, the greater the momentum.
Conservation of momentum
In a closed system (no external forces), the total momentum before an event equals the total momentum after:
total momentum before = total momentum after
This applies to collisions and explosions. For example, when two objects collide, the total momentum is the same before and after (taking direction into account).
Worked example
A 2 kg trolley moving at 3 m/s collides with a stationary 1 kg trolley and they move off together.
- Momentum before = (2 × 3) + (1 × 0) = 6 kg m/s.
- Momentum after = (2 + 1) × v = 6, so v = 2 m/s.
Direction matters
Because momentum is a vector, you must include direction (e.g. using + and − for opposite directions). In an explosion, objects move apart so their momenta are equal and opposite, summing to zero (if they started at rest).
Exam tips
- Learn p = mv (kg m/s); momentum is a vector.
- Total momentum is conserved in a closed system (before = after).
- Include direction (use + and −) in calculations.
- Apply conservation of momentum to collisions and explosions.