Conservation of Momentum
Learn Conservation of Momentum for GCSE Physics with this free worksheet and full mark scheme — Higher-tier exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) physics topic: in a closed system the total momentum before an event is equal to the total momentum after it.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
This is a Higher tier only topic, so there's no Foundation paper — only the Higher worksheet and mark scheme below.
Topic overview
In a closed system, the total momentum before an event is equal to the total momentum after the event. This is called conservation of momentum. This is a Triple-only and Higher-only topic.
A closed system means no external forces act. The principle applies to collisions, where objects come together, and to explosions, where objects push apart.
Direction is essential. Momentum in opposite directions has opposite signs, so a total momentum can be zero even when objects are moving. In an explosion the total momentum before is zero, so the total after must also be zero — which is why the fragments move apart with equal and opposite momenta.
Revision notes
The principle
In a closed system, total momentum before an event equals total momentum after.
A closed system is one where no external forces act. This applies to both collisions and explosions.
Applying it to collisions
Calculate the total momentum before, remembering to use signs for direction.
Set that equal to the total momentum after, then solve for the unknown. Objects that stick together after colliding have a combined mass.
Applying it to explosions
Before an explosion the objects are stationary, so the total momentum is zero.
The total after must therefore also be zero, meaning the fragments move apart with momenta that are equal in size and opposite in direction.
Key points
- Total momentum before equals total momentum after.
- This applies in a closed system.
- A closed system has no external forces.
- It applies to collisions and explosions.
- Use signs for opposite directions.
- Explosions start with zero total momentum.
Worked examples
Example 1
State the principle of conservation of momentum. [2 marks]
Model answer
In a closed system, the total momentum before an event (1 mark) is equal to the total momentum after the event (1 mark).
Example 2
A 2 kg trolley moving at 3 m/s collides with a stationary 1 kg trolley and they stick together. Calculate their combined velocity. [4 marks]
Model answer
Momentum before = 2 × 3 = 6 kg m/s, plus zero for the stationary trolley (1 mark). Total mass after = 2 + 1 = 3 kg (1 mark). Momentum is conserved, so 6 = 3 × v (1 mark). v = 6 ÷ 3 = 2 m/s (1 mark).
Example 3
Explain why the fragments of an exploding object move apart in opposite directions. [3 marks]
Model answer
Before the explosion the object is stationary, so the total momentum is zero (1 mark). Momentum is conserved, so the total momentum afterwards must also be zero (1 mark). The fragments must therefore have equal and opposite momenta, so they move in opposite directions (1 mark).
Common mistakes
Ignoring the direction signs.
Momentum is a vector; opposite directions must take opposite signs.
Forgetting to add the masses when objects stick together.
The combined mass is the sum of both.
Saying momentum is conserved when external forces act.
It is conserved only in a closed system.
Saying the total momentum after an explosion is not zero.
If it was zero before, it must be zero after.
Exam tips
- Assign directions and signs before calculating.
- Write momentum before = momentum after explicitly.
- Add masses when objects stick together.
- Remember this is Triple-only AND Higher-only.
Key terms
- Conservation of momentum
- Total momentum before equals total momentum after.
- Closed system
- A system with no external forces acting.
- Collision
- An event where objects come together.
- Explosion
- An event where objects push apart from rest.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.