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    I've been going through some past papers and I've been losing marks for not using the correct number of significant figures in my answers. In maths, it was generally accepted that 3 sig figs were the most appropriate when necessary. However, in this physics course, it seems that they only want 2 sig figs most of the time.

    Anybody know why this is? Should I definitely use 2 sig figs in the exam?

    This question comes up quite a lot. Some boards don't seem to have a consistent policy for physics.
    The usual answer is that the result should have an "appropriate" number of sig figs.
    "Appropriate" is determined by the number of sig figs in the data in the question. Strictly speaking, the data in the question should all be given to the same number of sig figs (3 usually) and you give your answer to the same. However, if the data is given to a mixture of 2 and 3 sig figs, you cannot meaningfully give your answer to a precision greater that the least number of sig figs in the data.
    Part of the confusion is that people confuse "number of decimal places" with "number of significant figures". It is not the same thing.
    Most of the boards I've looked at point out that certain calculations, such as those using atomic mass values to calculate mass deficit and thus energy in nuclear reactions, are given to 5 sig figs. The answer should be expressed accordingly.
    Keep all intermediate values that are used later in the calculation to their maximum precision.
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Updated: January 28, 2010

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