Year 12s: what will be the first way you research your future options? >>

Unit 4 multiple choice question.

Brainfart. How do I do this type of question?

On inspection, B seems like the obvious answer, as g is proportional to M, so the star of mass 4M will have a stronger gravitational field strength per unit distance that the star of mass M, the point where they have the same value of g will be closer to the star M than the star 4M.

Mathematically, you know they have the same gravitational field strength at this point, and g= GM/r^2 , so you can set up an equation, where the distance of the star 4M from the point y is d-y. Rearranging the equation to find y/d gives a value of 1/3, so B is the answer.

Sorry if that makes very little sense, but I guess the main thing to realise is that B is the only logical answer, and if you can see that, you don't need to bother with the rearranging etc. which takes a fair bit of time.