Overview
What you’ll learn
- Calculate momentum with p = m v, watching the signs for direction (Higher).
- Apply conservation of momentum to collisions and explosions (Higher).
- Use F t = Δ(m v) to link impact time and force (Higher).
- Explain crumple zones, air bags and seat belts in momentum terms (Higher).
- Solve recoil problems where one object pushes another (Higher).
Specification checklist (AQA 8463 · Edexcel 1PH0 · OCR Gateway J249)
Statements are paraphrased from the shared DfE subject content and grouped by tier. Foundation content is studied by every student; Higher statements are examined on Higher-tier papers only, and statements badged Physics only are examined in separate (triple) Physics, not Combined Science. QuizQuip is not affiliated with AQA, Pearson Edexcel or OCR.
Higher — Higher tier only
- 10.1Define momentum as mass × velocity (p = m v), state its unit (kg m/s) and treat it as a vector, so objects moving in opposite directions have momenta of opposite sign. (Higher)
- 10.2State and apply the law of conservation of momentum: in a closed system, the total momentum before an event equals the total momentum after. (Higher)
- 10.3Apply conservation of momentum to collisions (objects joining together) and to explosions and recoil problems. (Higher)
- 10.4Explain that when a force acts on an object for a time, the object’s momentum changes: force × time = change in momentum (F t = Δ(m v)). (Higher)
- 10.5Explain how vehicle safety features — crumple zones, air bags and seat belts — increase the time over which momentum changes, reducing the force on occupants. (Higher)
- 10.6Explain why rapid changes of momentum produce very large forces, using examples such as collisions and catches. (Higher)
Key points
- 1Momentum p = m v, in kg m/s — a vector: opposite directions get opposite signs. (Higher)
- 2Conservation of momentum: total momentum before = total momentum after in any closed system. (Higher)
- 3Collisions that stick together: m₁u₁ + m₂u₂ = (m₁ + m₂) v. (Higher)
- 4Explosions and recoil: the momenta after add to zero if the system started at rest. (Higher)
- 5F t = Δ(m v): the same momentum change needs a smaller force if it happens over a longer time. (Higher)
- 6Crumple zones, air bags and seat belts all stretch the collision time, cutting the force on occupants. (Higher)
Notes · 1 of 3
What momentum is
Mass in motion — with a sign that says which way.
Momentum measures “how much motion” an object carries: p = m v, in kilogram metres per second (kg m/s). Because velocity is a vector, momentum is too: a 60 kg runner at +4 m/s carries p = 60 × 4 = +240 kg m/s; the same runner at 4 m/s the other way carries −240 kg m/s. Signs matter — take a positive direction and stick to it through the whole calculation.
Notes · 2 of 3
Conservation of momentum
Before equals after — crashes and cannons obey the same ledger.
In a closed system (no external forces), the total momentum never changes. Objects may collide, stick, or fly apart — momentum just moves between them. Two cases cover the exam: collisions that stick together, where m₁u₁ + m₂u₂ = (m₁ + m₂)v; and explosions/recoil, where an initially stationary system splits into pieces whose momenta cancel — one piece flies one way and the rest recoils the other.
Notes · 3 of 3
Impact forces and vehicle safety
Same momentum change, longer time, smaller force — the whole of car safety in one equation.
Rearranging Newton’s second law gives F t = Δ(m v): the force needed for a momentum change shrinks as the time grows. A 1 500 kg car stopping from 20 m/s must lose 30 000 kg m/s — in 1.5 s that takes F = 30 000 ÷ 1.5 = 20 000 N, but in 0.10 s it would take 300 000 N. Crumple zones crush (stretching the stop over tenths of a second), air bags cushion the head (again stretching the time), and seat belts spread the force across the body while holding the occupant inside the slowing cabin. Cyclists’ helmets and cricketers “drawing the hands back” when catching are the same physics.
Revise
Key terms
The words examiners expect you to use precisely.
- Momentum
- Mass × velocity, p = m v, in kg m/s — a vector (Higher).
- Conservation of momentum
- Total momentum before = total momentum after, in a closed system (Higher).
- Closed system
- One with no external forces acting, so momentum is conserved.
- Recoil
- The backward motion of an object that has pushed a smaller piece away, conserving momentum.
- Change in momentum
- Δ(m v) — the final momentum minus the initial momentum.
- Impact time
- The duration of the collision; stretching it reduces the force (Higher).
Show 3 more key termsShow fewer key terms
- Crumple zone
- A car region designed to crush, extending the collision time (Higher).
- Air bag
- A cushion that extends the time a passenger takes to stop (Higher).
- Seat belt
- A restraint that spreads the stopping force and keeps the occupant in the cabin (Higher).
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Treating momentum as a scalar and adding opposite velocities as positives.
Momentum has direction.
Instead: Choose + and − directions first; objects moving opposite ways contribute opposite-sign momenta.
Mistake: Quoting momentum in newtons or joules.
It is mass × velocity.
Instead: Momentum is in kg m/s.
Show 8 more mistakesShow fewer mistakes
Mistake: Saying momentum is created in an explosion.
An explosion is a closed system too.
Instead: Total momentum stays zero (if it started at rest) — the pieces carry equal and opposite momenta.
Mistake: Forgetting the combined mass after objects stick together.
The joined object carries both masses.
Instead: Use (m₁ + m₂) v for stuck-together collisions.
Mistake: Saying air bags reduce the momentum change.
The passenger still stops from the same speed.
Instead: Air bags stretch the stopping time, so the same momentum change needs a smaller force.
Mistake: Mixing up F = m a and F t = Δ(m v) contexts.
One is instantaneous; the other spans the collision.
Instead: F t = Δ(m v) is Newton’s second law spread over time — use it when a collision duration is given.
Mistake: Ignoring the direction of recoil velocities.
Recoil is opposite to the ejected piece.
Instead: Assign a negative sign to the recoil direction and keep the arithmetic consistent.
Mistake: Saying seat belts hold you still during a crash.
You still stop with the car — but stretched over a longer time and distance.
Instead: Seat belts extend the stopping time and spread the force, and keep you out of the windscreen.
Mistake: Believing crumple zones make cars unsafe because they collapse.
The collapse is the safety feature.
Instead: Controlled crushing extends the impact time and absorbs energy, reducing the force on the cabin.
Mistake: Adding the momenta of both objects and forgetting to divide by the total mass.
That leaves you with momentum, not velocity.
Instead: v = total momentum ÷ total mass after the collision.
Check yourself
Quick quiz
24 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Calculate momentum
The momentum of a 60 kg runner at 4.0 m/s is
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Practise
Topical questions
12 exam-style questions with model answers and mark schemes.
A set of structured and calculation questions matched to this chapter, in three tiers, each with a model answer, mark scheme and examiner tip. Higher-tier and Physics-only questions are tagged.
Practise topical questionsExam practice
Practice paper
A 30-mark paper in the style of a GCSE paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 10 Practice Paper
30 marks · 40 min · 5 pages
10 multiple-choice questions and structured questions with answer lines. Chapter 10: momentum p = m v, conservation of momentum, collisions and recoil, F t = Δ(m v), impact forces and vehicle safety features.
Mark scheme
PROChapter 10 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
How is this chapter examined?
Paper 1
1 h 45 · 100 marks
Chapters 1–7: energy, electricity, particle model, atomic structure
Paper 2
1 h 45 · 100 marks
Chapters 8–18: forces, waves, magnetism and electromagnetism, space physics
Tiers
Foundation (grades 1–5) · Higher (grades 4–9)
Higher assumes all Foundation content; the equation sheet is printed
Want a full paper? Try the Paper 1-style (Foundation), Paper 1-style (Higher), Paper 2-style (Foundation), Paper 2-style (Higher), Required practicals practice sets.
Questions
Frequently asked questions
Is momentum Higher tier only?
Yes — momentum and its conservation are Higher-tier material on all three English boards. Foundation students learn forces and acceleration but not momentum calculations.
Why do signs matter so much in momentum questions?
Momentum is a vector. Pick a positive direction at the start; any object moving the other way contributes negative momentum. Most lost marks come from missing signs.
Do air bags reduce the momentum change?
No — you stop from the same speed either way. What changes is the time, and therefore the force: F = Δ(m v) ÷ t.
Is kinetic energy conserved in collisions too?
Usually not. Momentum is always conserved in a closed system, but sticking collisions transfer some kinetic energy to thermal and sound stores.
Is this chapter examined on both papers?
It belongs to the Forces topic (AQA Paper 2 and equivalents) but only on Higher-tier papers.
Does QuizQuip have any connection to the exam boards?
No. AQA, Pearson Edexcel and OCR are named only to identify the specifications this content supports. QuizQuip is not affiliated with, endorsed by, or connected to AQA, Pearson Edexcel or OCR, and no past-paper or specimen material is reproduced.
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