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
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- state the principle of conservation of momentum
- apply the principle of conservation of momentum to solve simple problems, including elastic and inelastic interactions between objects in both one and two dimensions (knowledge of the concept of coefficient of restitution is not required)
- recall that, for an elastic collision, total kinetic energy is conserved and the relative speed of approach is equal to the relative speed of separation
- understand that, while momentum of a system is always conserved in interactions between objects, some change in kinetic energy may take place
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.
Tips to avoid common mistakes in Linear momentum and its conservation
- › Recall that at terminal velocity, speed is constant, and therefore the kinetic energy remains constant, resulting in a horizontal line on the graph.
- › Define force using the general term 'rate of change of momentum' to ensure your answer applies to systems with varying mass.
- › Distinguish momentum (mv) from force (the rate of change of momentum) in all written explanations.
- › In velocity-time or displacement-time graphs involving terminal velocity, the initial gradient at release must equal g if air resistance is initially zero.
- › State clearly that at terminal velocity, Weight = Upthrust + Viscous Force, then solve for the unknown value.
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
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