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In mechanics, how do you now if you need to give answer to 2sf or 3sf?

I suppose common sense is to put it to 2sf if g = 9.8 is used in the calculation, but the mark schemes take that very lightly. Sometimes it says to get the accuracy mark it needs to be to 2sf and sometimes to 3sf. Here's an example from JAN 11.
In part a) it's this:
P = 12× (5 + 75g sin 5°) = 828 7.
P = 830 W
And it's saying it has to be 830 (2sf) and nothing else to get the A1 mark. And that makes sense, but then part b) says to give it to 3sf and nothing else to get the mark even though g = 9.8 was used again ( I couldn't copy part b, bear with me). I only noticed this now and it's scaring me. I don't want to lose marks on sig figs so any help is appreciated.
If you think looking at the actual mark scheme could help here it is (question 3):
https://ca99c64778b62ba7e7b339967029e090c5733c3a.googledrive.com/host/0B1ZiqBksUHNYOXdJSDFGUUZPRUE/January%202012%20MS%20-%20M2%20Edexcel.pdf
i dont know about edexcel specifically but the very front page of the exams always say answers to so many sig figs if not specified in the question.
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mik1a
10
If not specified anywhere, good practice is to have your answers inherit the precision from the numerical values in the question.

For example, if the question tells you that x = 4.523, then you assume 4s.f.

If you use too few, then you are introducing a rounding error and discarding good data. If you use too many, then you give a false indication that your answer is more precise than it really is.

Think of every number as a rounded number. So 2 is interpreted as "somewhere between 1.5 and 2.5". The number 2.4 is interpreted as "somewhere in between 2.35 and 2.45". This is how the number of significant figures can implicitly indicate the precision of the measurement.
(edited 8 years ago)

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