Momentum: GCSE Physics
p = mv, conservation of momentum, force as rate of change of momentum and safety
The short version
Momentum is a vector. Closed systems conserve momentum. F = Δp / Δt explains crumple zones and air bags.
Key idea
p in kg m/s; m in kg; v in m/s. Vector: pick a positive direction
Grade 2–3: Heavy and fast means a lot of momentum - harder to stop.
Grade 5–6: I can calculate p = mv and I know it has a direction.
Grade 9: I assign signs along a line and I never mix up p with KE or with weight.
Go a bit deeper
Vector: opposite directions get opposite signs. Include both objects
Grade 2–3: When two things push, the total "amount of going" stays the same if you count both.
Grade 5–6: I can do a stick-together trolley question with one direction positive.
Grade 9: I refuse to conserve momentum for one object on its own when it hits the Earth, and I can do an explosion from rest.
Worked example
Worked example
Grade 2–3: Heavy and fast means a lot of momentum - harder to stop. Grade 5–6: I can calculate p = mv and I know it has a direction. Grade 9: I assign signs along a line and I never mix up p with KE or with weight.
Lessons in this topic
- Momentum
- Conservation of momentum
- Force and changing momentum
- Safety: crumple zones and air bags
- Elastic and inelastic in outline
In the app each lesson is video (when we have a real YouTube id) then intro, learn and summary steps.
What the examiner wants
Calculate p = mv, use conservation in collisions (HT), and link stopping time to force for safety features.
Try these questions
Cover the answers and have a go first. Worked solutions are underneath.
Momentum p =
The unit of momentum is
In a closed system, collisions conserve
A 4 kg object at 5 m/s has momentum
Common mistakes
- Do not use km/h in p = mv without converting to m/s. Momentum is not a force.
- Do not "conserve" speed or KE by default. Do not forget the stationary object still has mass in the after line if they stick.
- Stopping does not mean Δp = 0. Δp is the change: after minus before. Using speed without reversing a bounce undercounts Δp.
- Do not say the air bag "reduces momentum to zero so there is no force". There is still a force; it is smaller because time is larger.
Answers and working
Frequently asked questions
Is GCSE Momentum on Combined Science and Triple?
Momentum is Triple-only Physics. Combined Science students can skip it.
Which exam board is this Momentum page for?
The ideas are board-agnostic for AQA, Edexcel, OCR, WJEC and CCEA. Paper lengths and question numbers differ — check your specification.
Is Momentum Foundation or Higher?
The core ideas appear on both tiers. Higher papers add extra application and some HT-only detail, marked in lessons with a Higher badge.
Learn, try, get feedback, fix the mistake
Open the stepped lesson, then the quiz with hints. Required-practical and calculation questions use the same method you will need in the paper.
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