Free GCSE Physics lesson: Newton's Laws
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Free Lessons -> GCSE / Key Stage 4 -> Physics -> Newton's Laws

Lesson 15 · GCSE / Key Stage 4 · Physics

Newton's laws and resultant forces

Use Newton's laws to explain motion and calculate force, mass and acceleration.

Qualification: GCSE Subject: Physics Forces

Forces

Lesson overview

This lesson introduces the core physics idea, the useful equipment and the calculation or data skills used on this page.

FocusNewton's laws and F = m a
Time45-60 minutes
EquipmentCalculator and force diagram practice.
Practical linkNo separate practical method focus
Maths tagsM1 substitution with units, M6 ratio and percentage

What you will learn

  • Explain balanced forces and steady motion.
  • Use force = mass x acceleration.
  • Link resultant force to acceleration.
  • Describe action-reaction force pairs carefully.

Force scenarios supplied on this page

Use the trolley, car and skater examples to practise resultant force, acceleration and force-pair language.

Newton's laws and resultant forces infographic

Infographic explaining GCSE Physics Newton's laws and resultant forces, including balanced forces, force equals mass times acceleration, resultant force calculations, free-body diagrams and action-reaction force pairs.
Use this visual to connect balanced forces, resultant force, acceleration and Newton's third-law force pairs.Download visual

Clear explanation

Newton's first law says an object remains at rest or moves at steady velocity unless a resultant force acts.

Newton's second law links resultant force, mass and acceleration: force = mass x acceleration.

Newton's third law describes force pairs: when one object exerts a force on another, the second object exerts an equal and opposite force on the first object.

Key diagram

Free-body diagram showing a forward resultant force A box has a larger driving force arrow to the right and a smaller friction arrow to the left, so the resultant force is to the right. drive 80 N friction 30 N normal contact weight
Diagram: unequal horizontal arrows show why the resultant force is forwards while vertical forces are balanced.

Worked examples

Calculating resultant force

A 1200 kg car accelerates at 2 m/s2.

force = mass x acceleration

force = 1200 x 2 = 2400

Answer: The resultant force is 2400 N.

Quick checks

Choose an answer, then check your thinking.

1. A box moves at steady velocity. What is the resultant force?

2. A 5 kg object accelerates at 3 m/s2. What is the resultant force?

Practice questions

Question 1

Calculate the force needed to accelerate 8 kg at 2.5 m/s2.

Reveal answer and marking guidance

Answer: 20 N.

Marking: Credit F = m a and 8 x 2.5 = 20 N.

Question 2

A 60 N resultant force acts on a 12 kg object. Calculate acceleration.

Reveal answer and marking guidance

Answer: 5 m/s2.

Marking: Credit a = F ÷ m and 60 ÷ 12 = 5 m/s2.

Question 3

Explain why a cyclist can move at steady speed even while pedalling.

Reveal answer and marking guidance

Answer: The forward driving force balances resistive forces, so resultant force is zero.

Marking: Credit balanced forces and steady velocity.

Question 4

State the force pair when a foot pushes backwards on the ground.

Reveal answer and marking guidance

Answer: The ground pushes forwards on the foot with an equal and opposite force.

Marking: Credit forces on different objects.

Practice ladder

FluencyRecall the key definition, unit, equation or model before using the lesson questions.
ApplicationApply newton's laws and f = m a to an unfamiliar device, practical setup or data description.
Practical interpretationUse evidence, graph features, uncertainty, method quality or conclusion wording where the question asks you to evaluate.
Maths skillM1 substitution with units

Answers and marking guidance

The exact practice answers are hidden under each question so you can try first. For this lesson, marks come from using the correct physics model, choosing the right equation where needed, keeping units with values, and explaining changes with precise words such as transfer, resultant force, acceleration, evidence and uncertainty.

Common mistakes

  • Saying steady motion needs a forward resultant force.
  • Using weight instead of mass in F = m a without care.
  • Putting action-reaction forces on the same object.
  • Forgetting acceleration units.

Extension challenge

Draw a free-body diagram for a car accelerating, cruising and braking, then label the resultant force each time.

Reveal answer

Example answer: A strong extension response names the physics model, uses accurate units and explains why the evidence supports the conclusion.

Exam-board guidance

Short board notes only. Learn the core physics above first.

AQA GCSE Physics

AQA GCSE Physics: often rewards clear physics explanations, correct equations, units and practical evidence for newton's laws and f = m a.

OCR GCSE Physics

OCR GCSE Physics: often values precise definitions, clear working, graph interpretation and links between models and evidence.

Pearson Edexcel GCSE Physics

Pearson Edexcel GCSE Physics: often combines the concept with equation use, data handling and practical interpretation.

Eduqas GCSE Physics

Eduqas GCSE Physics: learn the core explanation and practise applying it to unfamiliar contexts, data and practical questions.

WJEC Wales

WJEC Wales: often expects accurate terms, units and evidence-based explanations using the shared physics idea.

CCEA GCSE Physics

CCEA GCSE Physics: connect the idea to your current unit and use the same practical method language your class uses.

Next lesson

Next, continue with Stopping Distances and Momentum.

Previous lesson: Motion GraphsBack to GCSE Physics lessonsNext lesson: Stopping Distances and Momentum