IGCSE Physics / Practical and maths skills
Using equations: substitute, rearrange, evaluate revision notes
Edexcel International GCSE Physics (4PH1), chapter 29: Practical and maths skills, specification point M3B, M3C, M3D. Free lesson.
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Physics calculations follow five steps: choose the equation, substitute values, rearrange, evaluate and give the unit. Setting out each step clearly helps you earn marks, even if a later step is wrong.
How to set out a calculation
The equation list gives you the equations, so marks come from using the right one and showing your method. Substitute means put the values in place of the letters. Convert units first if needed, then substitute before rearranging. This can earn the substitution mark even if your rearrangement is incorrect.
How to rearrange when the wanted letter is on the bottom
Pressure is force divided by area. If the wanted letter is in the denominator, rearranging makes it divide the other values. Check whether your answer is a sensible size as well as checking the algebra.
How to make v the subject when it is squared
Kinetic energy is KE = 1/2 × m × v². To make v the subject, first clear the half, then divide by m. This leaves v², so you must take the square root of the whole expression to find v.
How to answer a show that question
Use the given measurements and the relevant equation to calculate the value. Do not substitute the value you are being asked to show. Give more figures in your answer than the rounded value in the question, so the calculation can be checked. For a speed asked for as about 6 m/s, give a value such as 6.1 m/s or 6.12 m/s, not just 6 m/s.
How to calculate mass from kinetic energy
When the wanted value is m, substitute the known speed and kinetic energy into the equation first. Simplify the numbers, then solve for m.