GCSE Physics resources

AQA GCSE Forces

Newton’s Laws, Resultant Force and Acceleration

A resultant force causes acceleration, rather than being required to maintain velocity. Use the sum of forces in F = ma.

AQA 8463AQA 8464 GCSE PhysicsCombined Science: Trilogy FoundationHigher

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The revision snapshot

A resultant force causes acceleration, rather than being required to maintain velocity. Use the sum of forces in F = ma.

By the end, you should be able to:

  • Relate resultant force to acceleration.
  • Identify Newton’s third-law forces on different bodies.
  • Explain terminal velocity using changing drag.

Before you revise

Diagnostic question

Choose an answer from memory. Your result tells you what to watch for in the guide.

A car moves right but has a resultant force left. What can happen?

Read the feedback for every option
  1. Force determines acceleration, not existing velocity.
  2. Acceleration opposite velocity decreases speed.
  3. Non-zero resultant causes acceleration.
  4. Mass remains in the system.

Core revision guide

Learn the model, then use it

A resultant force causes acceleration, rather than being required to maintain velocity. Use the sum of forces in F = ma.

First and second laws

Scope: 8463 and 8464 · Foundation and Higher

With zero resultant force an object remains at rest or continues at constant velocity. For fixed mass, acceleration is proportional to resultant force and inversely proportional to mass: Fresultant = ma. A 600 N driving force opposed by 200 N drag gives 400 N resultant, not 600 N. Acceleration points with the resultant, which may oppose velocity and slow the object.

Third law: identify two bodies

Scope: 8463 and 8464 · Foundation and Higher

If a swimmer pushes water backwards, water pushes the swimmer forwards with an equal force of the same type. The pair acts on different objects, so it does not cancel in the swimmer’s force equation. Weight and support acting on one resting book balance but are not a third-law pair.

Terminal velocity

Scope: Terminal-velocity graph/force interpretation: 8463 Physics only; inertial mass: both courses Higher

A falling object initially has weight greater than drag and accelerates down. Increasing speed increases drag, reducing resultant force and acceleration. When drag equals weight, resultant and acceleration are zero; the object continues falling at terminal velocity. It does not stop. At Higher, inertial mass is the ratio resultant force/acceleration.

A 10 kilogram trolley has 60 newtons right and 20 newtons left, producing 40 newtons and 4.0 metres per second squared right.
Combine forces with direction first, then use F = ma with the resultant force.Open the full-size exam diagram
Newton’s second lawF = maN, kg, m/s²

Misconception clinic

Replace the tempting answer

Read each belief, say what is wrong with it, then compare your correction.

Tempting idea: It must already move left.

Use instead: Force determines acceleration, not existing velocity. Acceleration opposite velocity decreases speed.

Tempting idea: Book weight and table support, both on book.

Use instead: Those balance on the same object. The same gravitational interaction acts on two objects.

Retrieval practice

Quick checks

1. Which is a third-law pair?

Read the feedback for every option
  1. Those balance on the same object.
  2. The same gravitational interaction acts on two objects.
  3. Those are distinct interactions on one body.
  4. Third-law forces act on different objects.

Explore the relationship

Newton law model

Change the resultant force or mass, then calculate acceleration using F = ma.

Model scope: 8463 and 8464 · Foundation and Higher

What to notice: use the readout to describe how one variable changes when the other is controlled.

Worked examples

See the method being built

Apply first and second laws in the worked reasoning, then try the changed-context questions below.

Example 1 · Find acceleration

Scope: 8463 and 8464 · Foundation and Higher

A 3.0 kg trolley has resultant force 12 N right. Calculate acceleration.

  1. Use F = ma.
  2. Rearrange a = F/m.
  3. State direction.
Show the answer

a = 12 ÷ 3.0 = 4.0 m/s² to the right.

Example 2 · Find resultant

Scope: 8463 and 8464 · Foundation and Higher

A car has 800 N driving force and 300 N resistance. Find resultant force.

  1. Forces are opposite.
  2. Subtract 300 from 800.
  3. State direction of larger force.
Show the answer

Resultant = 500 N forwards.

Example 3 · Newton’s third law

Scope: 8463 and 8464 · Foundation and Higher

A swimmer pushes water backwards. State the paired force.

  1. Name the second object.
  2. Reverse direction.
  3. Keep equal magnitude.
Show the answer

The water pushes the swimmer forwards with an equal and opposite force.

Exam precision

Use the physics precisely

Do this: Choose the object and draw its forces. Distinguish balanced forces on that object from the equal opposite forces acting on another body.

Independent practice

Exam-style questions

Write an answer before opening the marking guidance. Then edit the exact phrase or step that would gain the next mark.

1. A 1000 kg car has 1800 N driving force and 600 N resistance. Calculate its acceleration.

[3 marks]

Scope: 8463 and 8464 · Foundation and Higher

Show marking guidance and model answer

Original PhysicsUK mark scheme: Point 1 (1 mark): Resultant = 1800 − 600 = 1200 N forwards. Point 2 (1 mark): a = Fresultant/m = 1200/1000. Point 3 (1 mark): a = 1.2 m/s² forwards. Award each distinct point once. Accept equivalent scientifically correct wording. No half marks.

  1. Resultant = 1800 − 600 = 1200 N forwards. (1 mark)
  2. a = Fresultant/m = 1200/1000. (1 mark)
  3. a = 1.2 m/s² forwards. (1 mark)

Model answer: The resultant is 1200 N forwards, giving a = 1.2 m/s² forwards.

Marking notes: Carry an arithmetic error in the resultant into correct division by mass for the later method point; direction must follow the stated resultant.

2. Explain how a falling object reaches terminal velocity.

[3 marks]

Scope: 8463 and 8464 · Foundation and Higher

Show marking guidance and model answer

Original PhysicsUK mark scheme: Point 1 (1 mark): As speed rises, drag rises. Point 2 (1 mark): Resultant weight minus drag and acceleration decrease. Point 3 (1 mark): When drag equals weight, resultant/acceleration is zero and velocity stays constant. Award each distinct point once. Accept equivalent scientifically correct wording. No half marks.

  1. As speed rises, drag rises. (1 mark)
  2. Resultant weight minus drag and acceleration decrease. (1 mark)
  3. When drag equals weight, resultant/acceleration is zero and velocity stays constant. (1 mark)

Model answer: Rising speed increases drag. The resultant and acceleration fall until drag equals weight, when velocity becomes constant.

Questions pupils ask

Newton’s Laws, Resultant Force and Acceleration FAQs

Why do a third-law pair and balanced forces need different descriptions?

Third-law partners act on different bodies. Balanced forces act on the same chosen body and have zero resultant.

Useful next steps

Connect this guide to your next task

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