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 quick 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…
- Explain track momentum in collisions using precise physics language.
- Apply the relevant equations, diagrams or practical method to an exam-style situation.
- Use the mini quizzes and practice answers to identify and correct a misconception.
Core learning content
Learn the idea before you practise it
Start with this short teaching sequence for Momentum, Conservation and Safety. Read the model, use the visual, then test your recall before moving into examples and exam practice.
Build the 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.
Use the right relationship
Choose a relationship because of the physical change in the question, not because it is familiar. Weight: W = mg (N, kg, N/kg). Newton’s second law: F = ma (N, kg, m/s²). Momentum: p = mv (kg m/s). Keep the unit next to each value while you substitute.
What a strong answer does
Choose a positive direction, write total momentum before and after, then explain the safety mechanism using time and force. Before writing, underline the command word and identify the information that matters. Keep a calculation as equation, substitution and answer with unit; keep an explanation as a linked chain using because, so or therefore. Finish by checking that the final sentence answers the exact context in the question and that the number, direction or conclusion agrees with the evidence given.
Retrieval practice
Quick checks
Answer without looking back first. The feedback explains the model, so a wrong answer still helps you learn.
Mini quiz 1
Check the core idea
Mini quiz 2
Apply the model
Mini quiz 3
Use exam precision
Try it
Momentum and safety model
Why does increasing collision time reduce force?
What to notice: the readout and written explanation remain available if you do not use the controls.
Worked examples
See the method being built
Cover the answer, attempt the method and then compare your physics language as well as the number.
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.
Misconception clinic
Common wrong turns
Each of these is tempting because it sounds almost right. Replace it with the precise physics before you meet it in a question.
- Forgetting direction/signs.
- Using total mass before a collision incorrectly.
- Saying airbags reduce momentum change to zero.
Independent practice
Exam-style questions
Use the working space first. Reveal the guidance only after you have committed to an answer, then improve the exact part that would gain the next mark.
1. State the unit of momentum.
[1 mark]Show answer and marking guidance
kg m/s.
2. Calculate momentum of a 0.50 kg ball at 8.0 m/s.
[3 marks]Show answer and marking guidance
p = 0.50 × 8.0 = 4.0 kg m/s.
3. State the conservation rule for a closed system.
[1 mark]Show answer and marking guidance
Total momentum before equals total momentum after.
4. Explain why a crumple zone reduces force.
[3 marks]Show answer and marking guidance
It increases collision time for the same change in momentum.
5. A 5 kg object has momentum 20 kg m/s. Find velocity.
[2 marks]Show answer and marking guidance
v = p/m = 20 ÷ 5 = 4.0 m/s.
6. Why is direction needed in a collision calculation?
[2 marks]Show answer and marking guidance
Momentum is a vector, so opposite directions have opposite signs.
Questions pupils ask
GCSE Physics FAQs
What is the key idea in Momentum, Conservation and Safety?
Forces questions often hide a simple model inside a diagram. Draw the forces, find the resultant, then connect the resultant to the motion or deformation.
How should I practise Momentum, Conservation and Safety?
Read the snapshot, attempt the worked examples without looking, complete the mini quizzes and then use the practice questions to check your wording and method.
Next step
Keep your revision moving
Physics sticks when you move between explanation, retrieval and application.