Fiducia TogetherMomentum
Mass times velocity, and why it is conserved in every collision.
Please note: This page explains a word or a process in education. It is general information about the system, the law and the guidance in England — not advice about your child, your school or your exam. If a decision is going against you, get advice from one of the free services we link to before you agree to anything or miss a deadline. If you are revising, check your own exam board's specification: boards word things differently and yours is the one that marks you.
What it is
Momentum is mass × velocity, measured in kg m/s. It is a vector, so direction matters and is usually shown by sign.
Where it applies
Higher tier GCSE physics, and A Level.
How it works
In a closed system total momentum before a collision equals total momentum after. That single statement solves most collision questions.
Force equals rate of change of momentum, which explains crumple zones and airbags: increasing the time over which momentum changes reduces the force.
How to use it in your work
Assign a positive direction and stick to it. Most errors here are sign errors.
For safety-feature questions, always say it increases the TIME taken for the momentum change, so the force is smaller. That phrasing is the mark scheme.
Where to get help
Your teacher, your board's specification for the depth required at your tier, and Bitesize for the diagrams. The subject song runs the topics in order.
Revise this with a song
These are ours. We write revision songs because a lot of people remember a tune when they cannot remember a page, and because revising out loud in the kitchen beats staring at a highlighter.
- The Physics Song The GCSE physics specification, set to music.
Where to read more
Last reviewed 2026-08-28 by Fiducia Together · Next review due 2027-08-28
Please note: This page explains a word or a process in education. It is general information about the system, the law and the guidance in England — not advice about your child, your school or your exam. If a decision is going against you, get advice from one of the free services we link to before you agree to anything or miss a deadline. If you are revising, check your own exam board's specification: boards word things differently and yours is the one that marks you.