Getting every mark in a calculation revision notes

Edexcel International GCSE Physics (4PH1), chapter 30: Exam technique, specification point M3C. Free lesson.

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Show each stage of a physics calculation so you can earn method marks even if your final answer is wrong. Convert units, use earlier answers carefully, and show how you read values from graphs.

How each line earns marks

A calculation can earn marks for choosing the right equation, substituting values, rearranging, evaluating and giving the correct unit. Show these stages clearly, because a correct final answer without working may not earn every mark. For specific heat capacity, use ΔQ = m × c × ΔT. Substituting the values earns a mark, even if a later step is wrong. Rearranging to make c the subject can earn a separate mark. The unit may have its own mark, so include it.

Convert units before substituting

Power is energy transferred per second, so the time in a power calculation must be in seconds. Convert minutes before using the equation, or the calculated energy will be wrong. Five minutes is 300 s, not 5 s. Use W = P × t, where W is energy transferred in joules (J), P is power in watts (W) and t is time in seconds (s).

Keep marks with error carried forward

Error carried forward means you may still earn marks in a later part if you use an earlier wrong answer correctly. This only works when your working is shown, so complete every part even if you think you made a mistake. Use the answer from the earlier part in the later calculation. Show the substitution and the rest of your method.

Read values from a graph

Some calculations begin with a value taken from a graph. On a velocity-time graph, read the velocity by going up from the time value to the graph line, then across to the velocity axis. Draw read-off lines to show where your value came from. At 4 s, the trolley's velocity is 3.0 m/s. Use the graph value in the kinetic energy equation, with the speed squared.

Use the answer from the first step

In a two-step calculation, the result from the first step can be used in the second. For a falling object with no air resistance, the lost gravitational potential energy becomes kinetic energy. First calculate the gravitational potential energy lost. Set this equal to the kinetic energy, then solve for speed.