Linear momentum and its conservation
Cambridge A-Level Physics (9702) · Unit 3: Dynamics · 7 flashcards
Linear momentum and its conservation is topic 3.3 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 3 — Dynamics , alongside Non-uniform motion. In one line: In a closed system, the total momentum remains constant if no external forces act on the system. Mathematically, this means the total momentum before an event (like a collision) equals the total momentum after the event.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 7 flashcards — 2 definitions, 4 key concepts and 1 calculation — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Principle of conservation of momentum
In a closed system, the total momentum remains constant if no external forces act on the system. Mathematically, this means the total momentum before an event (like a collision) equals the total momentum after the event.
What the Cambridge 9702 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- State the principle of momentum conservation.
- Use momentum conservation to solve simple problems on interactions between objects, both elastic and inelastic, in one dimension and in two dimensions. You do not need to know about the coefficient of restitution.
- Recall that an elastic collision leaves the total kinetic energy unchanged, and that the objects separate at the same relative speed as they approached.
- Understand that momentum is always conserved when objects in a system interact, but the kinetic energy may change.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
State the principle of conservation of momentum.
In a closed system, the total momentum remains constant if no external forces act on the system. Mathematically, this means the total momentum before an event (like a collision) equals the total momentum after the event.
Describe an elastic collision in terms of kinetic energy and relative speed.
In an elastic collision, total kinetic energy is conserved, meaning the total KE before the collision is equal to the total KE after the collision. Additionally, the relative speed of approach is equal to the relative speed of separation.
What is the difference between an elastic and an inelastic collision in terms of kinetic energy?
In an elastic collision, kinetic energy is conserved. In an inelastic collision, kinetic energy is NOT conserved; some kinetic energy is transformed into other forms of energy, such as heat or sound.
Two objects collide and stick together. Is this collision elastic or inelastic? Explain.
This is an inelastic collision. When objects stick together after a collision, kinetic energy is always lost (converted to other forms), so the total kinetic energy is not conserved.
A 2kg object moving at 3m/s collides head-on with a stationary 1kg object. If they stick together, what is their velocity after the collision?
Using conservation of momentum: (2kg)(3m/s) + (1kg)(0m/s) = (2kg + 1kg)v. Therefore, 6 = 3v, and v = 2 m/s. The combined object moves at 2 m/s in the original direction of the 2kg object.
Explain why momentum is always conserved in a closed system, even if kinetic energy is not.
Momentum conservation is a fundamental law based on Newton's laws of motion and the absence of external forces. Kinetic energy, however, can be transformed into other forms of energy during interactions, leading to a change in KE but not momentum.
Two objects of equal mass collide elastically head-on. Object A is initially moving and object B is stationary. What happens to their velocities after the collision?
In an elastic collision between objects of equal mass, the objects exchange velocities. Object A will come to rest, and Object B will move with the initial velocity of Object A.
Review the material
Read full revision notes on Linear momentum and its conservation — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 3 — Dynamics
Linear momentum and its conservation sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Linear momentum and its conservation deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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