AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Conservation of momentum: This is called conservation of momentum.
The rule that total momentum before an event equals total momentum after it, in a closed system, is called conservation of momentum.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 2 of 3
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The rule that total momentum before an event equals total momentum after it, in a closed system, is called conservation of momentum. It applies whether the objects bounce apart, stick together or blow apart. An explosion is the clearest test: a firework or a recoiling rifle starts at rest, so the total momentum before is zero, and the total afterwards must also be zero. That can only happen if the two parts move in opposite directions with momenta of equal size, which is why a light bullet leaving at high speed is matched by a heavy rifle recoiling slowly. Conservation of momentum holds even when kinetic energy is not conserved. In a collision where the objects stick together, kinetic energy is transferred to thermal and sound stores, but the total momentum is unchanged.
What earns the marks
- Naming the principle as conservation of momentum.
- Stating that it applies in a closed system, where no external resultant force acts.
- Setting the total momentum before an explosion from rest equal to zero.
- Showing that the two parts must have equal and opposite momenta afterwards.
Common mistakes and how to put them right
Saying momentum is conserved because energy is conserved.
Forgetting to use the combined mass when two objects move off together.
Ignoring direction, so a rebounding object's momentum is added rather than subtracted.
Claiming an explosion breaks conservation of momentum because nothing was moving at the start.
Exam tips
- Name the principle in words before you use it; the phrase itself is often a marking point.
- For explosions, write 'total momentum before = 0' as your first line.
- Keep kinetic energy out of the argument unless the question specifically asks about it.
Try the lesson
The start of the Conservation of momentum: This is called conservation of momentum. lesson, made from checked teaching notes for this specification point.
Have a go first
Two ice skaters stand still and push apart. Which statement describes the total momentum of the two skaters immediately after the push?
Listen
The rule that total momentum before an event equals total momentum after it, in a closed system, is called conservation of momentum. It applies whether the objects bounce apart, stick together or blow apart. An explosion is the clearest test: a firework or a recoiling rifle starts at rest, so the total momentum before is zero, and the total afterwards must also be zero. That can only happen if the two parts move in opposite directions with momenta of equal size, which is why a light bullet leaving at high speed is matched by a heavy rifle recoiling slowly. Conservation of momentum holds even when kinetic energy is not conserved. In a collision where the objects stick together, kinetic energy is transferred to thermal and sound stores, but the total momentum is unchanged. A strong answer earns marks for these points. naming the principle as conservation of momentum. stating that it applies in a closed system, where no external resultant force acts. setting the total momentum before an explosion from rest equal to zero. showing that the two parts must have equal and opposite momenta afterwards. Exam tip: Name the principle in words before you use it; the phrase itself is often a marking point.
Key words
Say what each word means before you turn the card.
conservation of momentum
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closed system
Try answering aloud or in your head before turning the card.
See every definition
- conservation of momentum
- The rule that total momentum before an event equals total momentum after it, in a closed system.
- closed system
- A system in which no external resultant force acts.
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