Track momentum in collisions
Momentum is mass × velocity and is conserved in a closed system. Safety features reduce force by increasing collision time or reducing the change in momentum experienced by a person.
GCSE Physics resources
AQA GCSE Forces
Momentum, Conservation and Safety
Momentum is mass × velocity and is conserved in a closed system. Safety features reduce force by increasing collision time or reducing the change in momentum experienced by a person.
Start here
The revision snapshot
Momentum is mass × velocity and is conserved in a closed system. Safety features reduce force by increasing collision time or reducing the change in momentum experienced by a person.
By the end, you should be able to:
- Track momentum in collisions.
- Apply the forces model, evidence or method to an unfamiliar exam context.
- Recognise and correct this wrong turn: Forgetting direction/signs.
Before you revise
Diagnostic question
Choose an answer from memory. Your result tells you what to watch for in the guide.
Core revision guide
Learn the model, then use it
Momentum, Conservation and Safety questions become manageable when the central model, evidence and exam method are kept together. Read the explanation, test the misconception and then apply the idea without looking back.
Build the physics picture
Momentum has direction, so choose a positive direction before adding values in a collision. In a closed system, total momentum before equals total momentum after. Airbags, seatbelts and crumple zones increase stopping time, reducing the average force for the same momentum change.
Relationships used in this guide
Momentum: p = mv (kg m/s, kg, m/s). Force and momentum change: F = Δp ÷ t (N, kg m/s, s). Choose each relationship from the physical change described, convert quantities into compatible units and keep the unit beside the final answer.
Misconception clinic
Replace the tempting answer
Read each belief, say what is wrong with it, then compare your correction.
Tempting idea: Forgetting direction/signs.
Use instead: Momentum is mass × velocity and is conserved in a closed system. Safety features reduce force by increasing collision time or reducing the change in momentum experienced by a person.
Tempting idea: Using total mass before a collision incorrectly.
Use instead: Momentum has direction, so choose a positive direction before adding values in a collision. In a closed system, total momentum before equals total momentum after. Airbags, seatbelts and crumple zones increase stopping time, reducing the average force for the same momentum change.
Tempting idea: Saying airbags reduce momentum change to zero.
Use instead: Choose a positive direction, write total momentum before and after, then explain the safety mechanism using time and force.
Retrieval practice
Quick checks
Explore the relationship
Momentum and safety model
Change momentum or collision time, then calculate average force using F = Δp ÷ Δt.
What to notice: use the readout to describe how one variable changes when the other is controlled.
Worked examples
See the method being built
These examples expose the difference between a secure track momentum in collisions method and the common error “Forgetting direction/signs.”.
Example 1 · Calculate momentum
A 1200 kg car travels at 15 m/s. Find momentum.
- Use p = mv.
- Multiply 1200 × 15.
- Give kg m/s.
Show the answer
p = 18 000 kg m/s.
Example 2 · Collision conservation
A 2.0 kg trolley at 3.0 m/s hits a stationary 1.0 kg trolley and sticks. Find final velocity.
- Initial p = 2.0 × 3.0 = 6.0.
- Combined mass = 3.0 kg.
- v = 6.0 ÷ 3.0.
Show the answer
Final velocity = 2.0 m/s in the original direction.
Example 3 · Safety explanation
Explain how an airbag reduces injury.
- Identify same momentum change.
- State collision time increases.
- Link to average force.
Show the answer
The airbag increases stopping time, so the same change in momentum happens with a smaller average force.
Exam precision
Exam technique: Track momentum in collisions
Do this: Choose a positive direction, write total momentum before and after, then explain the safety mechanism using time and force.
Independent practice
Exam-style questions
Write an answer before opening the marking guidance. Then edit the exact phrase or step that would gain the next mark.
1. State the unit of momentum.
[1 mark]Show marking guidance and model answer
Marking guidance: Award one mark for the precise statement shown in the model answer.
Model answer: kg m/s.
2. Calculate momentum of a 0.50 kg ball at 8.0 m/s.
[3 marks]Show marking guidance and model answer
Marking guidance: Award one mark for the correct relationship, one for a valid substitution and one for the final answer with its unit.
Model answer: p = 0.50 × 8.0 = 4.0 kg m/s.
3. State the conservation rule for a closed system.
[1 mark]Show marking guidance and model answer
Marking guidance: Award one mark for the precise statement shown in the model answer.
Model answer: Total momentum before equals total momentum after.
4. Explain why a crumple zone reduces force.
[3 marks]Show marking guidance and model answer
Marking guidance: Award up to 3 marks for distinct physics points joined into a clear cause-and-effect chain.
Model answer: It increases collision time for the same change in momentum.
5. A 5 kg object has momentum 20 kg m/s. Find velocity.
[3 marks]Show marking guidance and model answer
Marking guidance: Award one mark for the correct relationship, one for a valid substitution and one for the final answer with its unit.
Model answer: v = p/m = 20 ÷ 5 = 4.0 m/s.
6. Why is direction needed in a collision calculation?
[2 marks]Show marking guidance and model answer
Marking guidance: 2 marks are available for relevant, linked physics points that match the model answer.
Model answer: Momentum is a vector, so opposite directions have opposite signs.
Questions pupils ask
Momentum, Conservation and Safety FAQs
What is the main idea in Momentum, Conservation and Safety?
Momentum is mass × velocity and is conserved in a closed system. Safety features reduce force by increasing collision time or reducing the change in momentum experienced by a person.
What mistake should I avoid in Momentum, Conservation and Safety?
Forgetting direction/signs. Momentum has direction, so choose a positive direction before adding values in a collision. In a closed system, total momentum before equals total momentum after. Airbags, seatbelts and crumple zones increase stopping time, reducing the average force for the same momentum change.
Useful next steps
Connect this guide to your next task
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