# Revision and exam help

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## Doing calculations

• Some are simple one-step calculations and are worth perhaps a couple of marks. Give the formula you are using, show the calculation and give the answer clearly. Ensure your workings are all clear.
• Some calculations are longer and require an intermediate step. Ensure that you clearly give intermediate results. For example calculating a cross-sectional area is often a necessary stage before finding out stress (the force per unit area.) Even if you make a mistake at the end, you could still gain marks if you have done the cross-sectional area correctly. Obviously these marks cannot be given to you if you didn’t show that stage clearly!
• If you make a mistake cross it simply and redo it, but never cross out work unless you replace it by something else in case you lose possible marks contained in the calculations.
• If you are giving a numerical answer, remember to give the appropriate S.I. unit.
• Write the answer to a reasonable number of significant figures rather than everything from your calculator display. A reasonable number if significant figures is the same as the number of significant figures in the information provided. Two points of caution – do calculations to at least one extra significant figure in order to avoid rounding errors, and just because an answer seems to come out to a nice number don’t forget the significant figures. For instance, if you work out velocity from information given to 3 significant figures and the answer is 6 on your calculator, don’t write down 6 ms-1. The answer should be 6.00 ms-1.
• Don’t give up a whole question just because you get stuck early on. If you can’t do an early part, write down a reasonable guess at an answer and write it down for that first part. Then clearly use that answer in later calculations. The examiner can still award you full marks for “error carried forward” (e.c.f. on some mark schemes) where you use your answer correctly. It is really important to write the guessed answer down where it should have first been given, not just later when you need to use it. Also you must show your calculations exactly as advised so all the method is clear.