Physics 9702 · syllabus 3
Cambridge A Level Physics 9702 — Dynamics topical past papers
Forces and changes in motion through Newton's laws, linear momentum and its conservation, drag and terminal velocity, and elastic and inelastic collisions.
- Questions
- 475
- Years
- 2009–2026
- With mark scheme
- 100%
- Questions 2022–2026
- 202
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What Dynamics covers
Dynamics explains why motion changes. Mass is the property of an object that resists a change in its motion, and Newton's second law in its general form says that the resultant force equals the rate of change of momentum; F = ma is the special case for constant mass. Weight is the gravitational force on a mass, W = mg, and Newton's third law pairs forces of the same type acting on different objects, which is why a book's weight and the table's push on it are not a third-law pair.
Real motion meets resistance. Drag grows with speed, so a falling object accelerates less and less until drag balances its weight and it reaches terminal velocity. You describe this qualitatively and sketch the velocity–time graph; no coefficients of friction or viscosity are needed. The same reasoning explains why a car whose driving force is constant settles at a top speed.
Momentum is conserved in every interaction when no external resultant force acts, in one dimension or two, but kinetic energy is conserved only in elastic collisions, where the relative speed of approach equals the relative speed of separation. This is AS content, examined on Papers 1 and 2, and it underpins circular motion, the kinetic theory of gases and photon momentum later in the course.
In the syllabus
The headings this topic has in the Cambridge International 2025–2027 syllabus, as the board prints them.
- 3 Dynamics
- 3.1 Momentum and Newton's laws of motion
- 3.2 Non-uniform motion
- 3.3 Linear momentum and its conservation
What you should be able to do
- Apply F = ma, recognising that acceleration is always in the direction of the resultant force
- Use F = Δp/Δt to find the average force during an impact from the change in momentum
- Identify Newton's third-law pairs and explain why they never cancel on a single object
- Describe the motion of an object falling with air resistance and explain why it reaches terminal velocity
- Solve collision and explosion problems using conservation of momentum in one and two dimensions
- Decide whether a collision is elastic by comparing kinetic energies or relative speeds
Key ideas
- Momentum p = mv, a vector, in kg m s⁻¹ (equivalently N s)
- Resultant force F = Δp/Δt; for constant mass this becomes F = ma, with 1 N = 1 kg m s⁻²
- Weight W = mg, with g = 9.81 m s⁻² near the Earth's surface
- Conservation of momentum: total momentum before = total momentum after, when no external resultant force acts
- Perfectly elastic collision: kinetic energy is conserved, and relative speed of approach = relative speed of separation
How Dynamics has been examined
From 2022 to 2026, Dynamics was on Paper 2 · AS Level Structured Questions in 11 of 11 exam series, worth about 20% of that paper’s marks.
| Paper | Questions | Share of marks | Series |
|---|---|---|---|
| Paper 1 · Multiple Choice | 162 | 13% | 11 of 11 |
| Paper 2 · AS Level Structured Questions | 38 | 20% | 11 of 11 |
| Paper 4 · A Level Structured Questions | 2 | 1% | 1 of 11 |
Momentum’s own count, over the questions in its bank for these years: a series is one exam session, every zone included, and a question filed under two chapters counts in both. Median marks per question: 1.
Filed in Momentum under: Dynamics.
A worked example
A trolley of mass 2.0 kg moving at 3.0 m s⁻¹ collides head-on with a stationary trolley of mass 1.0 kg. Afterwards the 2.0 kg trolley continues in the same direction at 1.0 m s⁻¹. Find the velocity of the 1.0 kg trolley and show that the collision is elastic.
- Total momentum before = 2.0 × 3.0 + 1.0 × 0 = 6.0 kg m s⁻¹
- Conservation of momentum: 6.0 = 2.0 × 1.0 + 1.0 × v, so v = 4.0 m s⁻¹ in the original direction
- Kinetic energy before = ½ × 2.0 × 3.0² = 9.0 J
- Kinetic energy after = ½ × 2.0 × 1.0² + ½ × 1.0 × 4.0² = 1.0 + 8.0 = 9.0 J
- Kinetic energy is unchanged, and the relative speeds of approach and separation are both 3.0 m s⁻¹, so the collision is elastic
Answer: 4.0 m s⁻¹ in the original direction; kinetic energy is 9.0 J before and after, so the collision is elastic
Written by Momentum for this page, with its own numbers; it is not taken from a past paper.
Mistakes that cost marks
- Ignoring the sign of a velocity that reverses — a 0.50 kg ball hitting a wall at 6.0 m s⁻¹ and rebounding at 4.0 m s⁻¹ changes momentum by 5.0 N s, not 1.0 N s
- Naming weight and the normal contact force as a third-law pair — they act on the same object and are different types of force; the partner of the Earth's pull on you is your pull on the Earth
- Assuming kinetic energy is conserved because momentum is — in an inelastic collision some kinetic energy becomes internal energy and sound, while momentum is still conserved
- Adding momenta in two dimensions as if they were scalars — momentum is conserved separately along each of two perpendicular directions
- Saying the resultant force is zero at the top of a vertical throw — the velocity is momentarily zero, but the weight still acts, so the acceleration is g
Exam technique
- Draw a before-and-after sketch with masses, velocities and a positive direction marked before writing the momentum equation
- When stating Newton's laws or the principle of conservation of momentum, include the condition — no external resultant force — because a statement without it is incomplete
- For 'show that the collision is elastic', calculate the total kinetic energy before and after, or the relative speeds, and state the comparison in words
- On a terminal-velocity graph, start the gradient at g and let it fall smoothly to zero, with the curve levelling off at the terminal velocity
Practising in Momentum
- Every question in a chapter as one list, from every paper and year in the bank, newest first or shuffled.
- Each question turns over to its own part of the mark scheme, where the board published one.
- Narrow by paper, year, session, zone and marks, and star the questions worth keeping.
- Pick chapters, a number of questions and a time, and the exam builder draws a mock to sit and mark.
Past-paper questions and mark schemes are the copyright of Cambridge International. Momentum shows them to signed-in learners for revision and does not publish them on these pages.