# Motion, forces and Newton's laws

> Scalars and vectors, motion graphs, the equations of motion, Newton's three laws, free-body diagrams and projectile motion.

Canonical: https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.kinematics-dynamics
Last updated: 2026-08-01

Flagship subject: topics and specification statements are authored to the WJEC specification. No question in it has been through human review yet.

- Subject: [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- Specification reference: Unit 1.1–1.3
- Difficulty: 2/5
- Assessment objectives: AO1, AO2
- Status: authored · checked · 2026-08-01

## Specification coverage

| Reference | Statement | AOs |
| --- | --- | --- |
| Unit 1.1(a) | distinguish scalars from vectors and resolve a vector into perpendicular components | AO1 |
| Unit 1.1(b) | read displacement-time and velocity-time graphs: gradient is velocity, area is displacement | AO1, AO2 |
| Unit 1.1(c) | apply the constant-acceleration equations to motion in one dimension | AO2 |
| Unit 1.1(d) | state Newton first, second (as rate of change of momentum) and third laws and draw free-body diagrams | AO1, AO2 |
| Unit 1.1(e) | define displacement velocity acceleration and distinguish average from instantaneous values | AO1 |
| Unit 1.2(a) | resolve motion in two dimensions including projectile motion with constant acceleration | AO2 |
| Unit 1.3(a) | analyse forces on inclined planes and with drag including terminal velocity | AO2 |
| Unit 1.3(b) | interpret force-time and momentum-time graphs including impulse as area | AO2, AO3 |

## Summary

Scalars and vectors, motion graphs, the equations of motion, Newton's three laws, free-body diagrams and projectile motion.

## What earns marks

- The gradient of a displacement–time graph is velocity; the area under a velocity–time graph is displacement.
- Newton's second law in full: force equals rate of change of momentum, F = Δp/Δt, reducing to F = ma at constant mass.
- Newton's third law pairs are equal, opposite, of the same type and act on different bodies.
- For projectiles, resolve into independent horizontal (constant velocity) and vertical (constant acceleration) components.

## Where marks are lost

- Treating weight and the normal contact force as a Newton's third law pair.
- Forgetting that a body at terminal velocity has zero resultant force, not zero force.
- Mixing up distance and displacement for motion that reverses.

## Misconceptions

### A book resting on a table: its weight and the table's normal contact force are a Newton's third law pair.

Example: The student calls the two forces 'equal and opposite reactions', missing that both act on the book.

A third-law pair is the same type of force acting on different bodies (book pushes table, table pushes book). Weight (Earth pulls book) and the normal force (table pushes book) are two different forces on the same body, so they are not a pair.

Instead: Name the two bodies each force acts on. Weight pairs with the book's gravitational pull on the Earth; the normal force pairs with the book's push down on the table.

### At terminal velocity the forces on a falling object become zero, so it keeps falling at constant speed.

Example: The student writes that drag 'disappears' at terminal velocity, when in fact drag has grown to equal weight.

At terminal velocity the resultant force is zero - weight and drag balance - but each individual force still acts. Zero resultant force means no further acceleration, not that the forces have vanished.

Instead: State that weight and drag are equal and opposite, so the resultant force is zero and the velocity is constant.

## Related

- [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- [Motion, forces and Newton's laws lesson](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.kinematics-dynamics)