Physics satellite question?

  • Thread starter Thread starter Jason V
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Jason V

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If...

E(total) = -GMm/2R

Change in E = GMm/2 x (1/Ri - 1/Rf)


Can anyone explain why its 1/Ri - 1/Rf?
Ri is the initial radius and Rf is the final. And im trying to find how much energy needs to be added to the satellite to move the satellite from a 100km orbit to 200km one
 
Sure, if you explain what you mean by Ri and Rf.

Are these some sort of initial and final distances?

What's the context; what are you trying to work out?

Edit:
OK, got ya!

It's just formulaic manipulation. You're trying to work out the difference in energy between the two radii. So, you just calculate the energy at each radius and take the difference. Final level - initial level.

Hence it would be (-GMm/2Rf) - (-GMm/2Ri)

adjust for sign: = GMm/2Ri - GMm/2Rf

GMm/2 is a common factor, so we can rewrite that as:

GMm/2 * (1/Ri - 1/Rf)

Voila!
 
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