Posted by **jess** on Friday, October 8, 2010 at 7:06am.

A 715 kg object is lifted from the surface of the earth to a height of 880 kg above the surface of the earth.

Calculate the increase in gravitational potential energy using the exact expression and compare it with the approximate expression using g=9.81. Give the magnitude of the difference between the two expressions divided by the exact result (that is the relative error) as your answer. Use 6.0x10^24 kg, 6.4x10^6 m and 6.67x10^-11 for the earth mass, earth radius and the Gravitational Constant G, respectively.

- physics -
**bobpursley**, Friday, October 8, 2010 at 8:48am
The exact PE is:

PE= GMe*m/r where r is the distance from the center of the Earth.

So compute it for both r, then subtract.

What bothers me about this question, is that you are asking for a comparision to 9.81, a three digit expression, but then using "exact" constants to two significant (three in the case of G) digits. That of course, is sheer nonsense.

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