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M1 edexcel marking question

Can I leave my answer in the exact form if it doesn't specify the number of sig figs in the question ? I'm used to doing that , I find it easier and more accurate but I once saw a comment in the mark scheme that says sth like "Penalise accuracy too great once only" so I'm confused

Reply 1

old_engineer

As far as I am aware, genuinely exact answers should be fine. But you need to watch out for answers that look exact but in fact aren't. As a simple example, say one of your answers is sqrt(g/49). If you use g = 9.8ms^-2, your calculator will tell you that the answer is sqrt(5)/5. But if you give the answer as sqrt(5)/5 you will lose marks because g = 9.8 is an approximation, meaning that no calculation that uses g = 9.8 can possibly yield an exact result. If you have used g = 9.8 or any other approximation in your working, you must give the answer in non-exact form, to two or three sig figs.

Reply 2

Moe_00

I've lost marks in mechanics before for leaving it in exact form. Apparently real world results can't be that accurate so you need to round to 2/3 sig fig. My teacher says to be safe write the answer in exact form then write next to it the rounded value.