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Significant figures

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(edited 8 years ago)
Original post by PotterPhysics
Hi there, I can solve physics problems alright but I always have this problem with sig figs. I know that you have to state your answer to the same number of sig figs as the piece of data with the lowest sig figs (this is correct right?), but where I have trouble is problems like this:

What is the force between two bodies of mass 10^5 tons at a distance of 100m apart?

How am I meant to know the number of sig figs in the data? The data given is 100m and 10^5 tons, but these don't specify how much significant figures they are. If it was say 1.0*10^5 tons I would know it was to two sig figs because of the 1.0 there.

Please help!



Welcome to TSR
Exam boards will generally accept an answer to 3 sig figs for all questions except for one question on the paper where they are specifically testing you on this topic. Then the question will state that you should use the correct number of sig figs in your answer and the question itself will give you data with a specific number of sig figs to enable you to do it. There is only one such question testing this on any one paper and you get one mark for doing it correctly. You can only lose a maximum of one mark on a paper for getting this wrong.

In your example I would assume 100 is 3 sig figs (it could be) and 105 tonnes is 1.00 x 105 then give your answer to 3 sig figs unless the question specifically asks you otherwise.
Original post by PotterPhysics
Thanks! I was taught completely differently from this. Or maybe I confused another lesson where we were doing practicals, I can't remember now. But anyway that has simplified my problem a lot!


Yes my answer is referring to questions in theory papers, not practicals.
In practicals you generally deal with absolute and %age uncertainties, which is a somewhat different matter.
Original post by PotterPhysics
Yeah thanks, makes sense now.

I have another question. In > 1 step questions where you have to use the answer from a previous part, how can i store the full "un-truncated" answer to use in the enxt part. Or is it fine to use the "truncated to 3 sig figs" version for the next part?

Example: If part (a) had the answer 6.857867565...*10^24, and part (b) required to use this number, would you plug in 6.86*10^24 into the calculator or the full result? If it's the full result how can I store it most easily for the next parts?

Thanks!


Express the 1st answer to 3 sig figs (or whatever is required) but use the full, most accurate value if you need it for a further calculation to avoid rounding errors. This is true almost anywhere in science.
Again, exam boards usually make allowance in the mark scheme for an answer that has been obtained from the intermediate result with rounding error.

How to store it?
-write it down?
-in the calculator memory?

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