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Can't figure out this question for the life of me.

Two planets X and Y are in concentric circular orbits about a star S. The radius of the
orbit of X is R and the radius of orbit of Y is 2R.
The gravitational force between X and Y is F when angle SXY is 90º, as shown in the
diagram.
What is the gravitational force between X and Y when they are nearest to each other?
http://pmt.physicsandmathstutor.com/download/Physics/A-level/Past-Papers/AQA-Old/Unit-4/June%202016%20QP%20-%20Unit%204A%20AQA%20Physics%20A-level.pdf

The question is no9 on the paper, I've tried working out the distance between Xand Y then using and tried other stuff but am getting nowhere, any hep would be much appreciated
Original post by Radioactivedecay
Two planets X and Y are in concentric circular orbits about a star S. The radius of the
orbit of X is R and the radius of orbit of Y is 2R.
The gravitational force between X and Y is F when angle SXY is 90º, as shown in the
diagram.
What is the gravitational force between X and Y when they are nearest to each other?
http://pmt.physicsandmathstutor.com/download/Physics/A-level/Past-Papers/AQA-Old/Unit-4/June%202016%20QP%20-%20Unit%204A%20AQA%20Physics%20A-level.pdf

The question is no9 on the paper, I've tried working out the distance between Xand Y then using and tried other stuff but am getting nowhere, any hep would be much appreciated


so you need to find the ratio of the distances from X to Y at the two positions mentioned.
the ratio of the forces will be connected to the ratio of the distances in a well know way.
(edited 6 years ago)
Original post by the bear
so you need to find the ratio of the distances from X to Y at the two positions mentioned.
the ratio of the forces will be connected to the ration of the distances in a well know way.


But how, I tried it and got nowhere.
Original post by Radioactivedecay
But how, I tried it and got nowhere.


use Pythagoras to find the distance XY in the diagram.

when the planets are aligned you can easily find their new separation.

divide the two distances to get a ratio .... then raise this to a certain power.
Original post by the bear
use Pythagoras to find the distance XY in the diagram.

when the planets are aligned you can easily find their new separation.

divide the two distances to get a ratio .... then raise this to a certain power.


Oh, I see, so the distance XY is root3 R and when I use the inverse square law I get 3F. thank you very much
Original post by Radioactivedecay
Oh, I see, so the distance XY is root3 R and when I use the inverse square law I get 3F. thank you very much


:hat2:

your welcome

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