TBR Physics Chapter 3 Work, Energy Question on PE!

[DeMoNdOgDFM]

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This question is on Question #49 for the end of Chapter 3 Question Set. The question talks about two different satellites at different distances from the Earth and is asking for the ratio of U (gravitational PE).

I understand that U is prop. to 1/r (via the equation). However, why isn't the answer 2 to 1? Don't you have to add the R for the Earth to each of the individual satellite distances to get the overall distance from the earth's center? That would make it 4R:2R or 2:1.

Someone help! Thanks.

 

[jamilabella20]

Hi
the QUESTIONS is 3R so the answer is
Ua/Ub= Rb/Ra= 3:1

 

[DeMoNdOgDFM]

I'm sorry that doesn't assist me in answering my specific question.

 

[DeMoNdOgDFM]

Anyone?

 

[milski]

R and 3R are the orbital radii for both satellites, not the distances from the Earth's surface. They are the radii themselves, no need to involve the radius of anything else.