Free GCSE Physics lesson: Equations and Units
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Free Lessons -> GCSE / Key Stage 4 -> Physics -> Equations and Units

Lesson 26 · GCSE / Key Stage 4 · Physics

GCSE Physics equations and unit conversions

Practise choosing equations, rearranging them, converting units and checking physics answers sensibly.

Qualification: GCSE Subject: Physics Equations

Calculation skills

Lesson overview

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

FocusEquation selection, rearranging and unit conversion
Time45-60 minutes
EquipmentCalculator, ruler and equation sheet if your course uses one.
Practical linkNo separate practical method focus
Maths tagsM1 substitution with units, M2 rearranging/equations, M3 unit conversions

What you will learn

  • Choose an equation from the quantities in a question.
  • Rearrange simple GCSE Physics equations accurately.
  • Convert common units such as kJ to J, cm to m and minutes to seconds.
  • Check final answers with units and sensible size.

Equation practice set supplied on this page

Use the mixed equation prompts to practise identifying quantities first, then substituting values with units.

Physics equations and unit conversions infographic

Infographic explaining GCSE Physics equations and unit conversions, including choosing equations, rearranging before substituting values, SI unit conversions, a V equals I R worked example and a graph-gradient check.
Use this visual to choose equations, rearrange safely, convert units and check answers with units and graph evidence.Download visual

Clear explanation

The safest calculation routine is: identify the quantity asked for, write down the known values, choose the equation, rearrange if needed, substitute with units, calculate and check the answer.

Unit conversion matters because GCSE Physics equations usually expect standard units. For example, 2.5 kJ is 2500 J, 30 cm is 0.30 m and 4 minutes is 240 s.

If an answer looks impossible, check powers of ten, whether you used seconds rather than minutes, and whether the equation was rearranged the right way round.

Worked examples

Convert before calculating

A device transfers 3.6 kJ in 2 minutes.

Convert 3.6 kJ to 3600 J and 2 minutes to 120 s.

power = energy transferred ÷ time = 3600 ÷ 120

Answer: The power is 30 W.

Quick checks

Choose an answer, then check your thinking.

1. What should you do before substituting values into an equation?

2. What is 0.45 kJ in joules?

Practice questions

Question 1

Convert 75 cm into metres.

Reveal answer and marking guidance

Answer: 0.75 m.

Marking: Credit dividing by 100.

Question 2

A motor transfers 4800 J in 60 s. Calculate power.

Reveal answer and marking guidance

Answer: 80 W.

Marking: Credit power = energy ÷ time and 4800 ÷ 60 = 80 W.

Question 3

Rearrange speed = distance ÷ time to make distance the subject.

Reveal answer and marking guidance

Answer: distance = speed x time.

Marking: Credit multiplying both sides by time.

Question 4

A student gets a car speed of 4000 m/s in a school problem. What should they do?

Reveal answer and marking guidance

Answer: Check units, substitution and equation choice because the answer is not sensible for a normal car.

Marking: Credit reasonableness check and a specific likely error such as minutes not seconds.

Practice ladder

FluencyRecall the key definition, unit, equation or model before using the lesson questions.
ApplicationApply equation selection, rearranging and unit conversion 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

  • Using minutes when the equation needs seconds.
  • Forgetting to convert centimetres to metres.
  • Rearranging by guessing rather than doing the same operation to both sides.
  • Writing a number without the final unit.

Extension challenge

Create a three-column table for five equations: quantity asked for, values given, and final unit.

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 equation selection, rearranging and unit conversion.

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 Circuit Calculations.

Previous lesson: Exam RoutineBack to GCSE Physics lessonsNext lesson: Circuit Calculations