Gravitational Potential Energy of the earth

[kt102188]

Homework Statement

How high above the surface of the Earth must a mass be located so its gravitational potential energy is only 17.00% of its value on the surface?

Homework Equations

PEg=(-GMm)/r, PElocal= mgy where y is height the object can fall

The Attempt at a Solution

I've tried a couple of different things but all my variables cancel out.

(-GMm)/r1=0.17((-GMm)/r2

r1/0.17=r2 If I randomly set r1 to be 1 r2= 5.88235 but that doesn't make sense because if you raise an object by 5m it would have a greater PEg

Any suggestions on where to go?
Thanks!

 

[tiny-tim]

Welcome to PF!

Hi kt102188! Welcome to PF!

r1/0.17=r2 If I randomly set r1 to be 1 r2= 5.88235 but that doesn't make sense because if you raise an object by 5m it would have a greater PEg

But the radius of the Earth is a lot larger than 1m.

(and remember, the question assumes that the PE at ∞ is zero )

 

[kt102188]

Thank you! I was totally thinking of this wrong. Actually using the radius of Earth got the right answer.