# physics grav fields

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#1
Im stuck on 17 (b) (ii) (3)
and 17 (c) (ii) of this paper

https://www.ocr.org.uk/Images/528085...ng-physics.pdf

mark scheme:

https://www.ocr.org.uk/Images/529274...ng-physics.pdf

For 17 (b) (ii) (3) , How did they get the y intercept as lg(GM) ?

and for 17 (c) (ii) , I got this equation: M(Earth) / M (Mars) = Radius of earth ^2 / Radius of mars ^2

but i dont know if this is correct

if anyone can help me out id be very grateful. Thanks
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2 months ago
#2
We know g = GM/(r^2)
Try taking logs of both sides, you should come out with the y = mx + c form, with y being log(g) and x being log(r).

And looking at their answer for the other part, GM(Earth)/(re)^2 = GM(Mars)/(rm)^2, G cancels on both sides, and by multiplying by re ^2 and dividing by M(Mars) we get M(Earth) / M(Mars) = (re)^2/(rm)^2 which is what you got, so it is correct well done.
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