Calculating Gravitational Potential Energy: A to B

[natural]

Forces & Fields

(a) calculate the gravatational potential pf a point A on the surface of the Earth
(b) what is the gravatational potential of a point B, 1000km above A?
(c) How much energy is expended if an object of mass 10kg were to just reach point B from A?

I know the eq. for gravatational potential energy is U=-GMm/r ...but what do i use as my 2nd mass ? please help I am really lost

 

[Astronuc]

There is a difference between gravitational potential and gravitational potential energy.

Gravitational potential is the potential energy per unit mass of an object due to its position in a gravitational field. The gravitational potential due to a point mass:

E_g = -GM/r, where M is the mass of the object responsible for the gravitational field, whereas

gravitational potential energy is given by

mE_g = -GMm/r

Please refer to this page - http://hyperphysics.phy-astr.gsu.edu/hbase/gpot.html

and the following plates including - http://hyperphysics.phy-astr.gsu.edu/hbase/gpot.html#gpi .

 

[natural]

Are my calculations correct? just want to make sure>
a) -6.67*10^11 * 5.98*10^24/6.37*10^6 = -6.26*10^29

b)-6.67*10^11 * 5.98*10^24/ 6.37*10^6+1000*10^3 = -5.41*10^29

C)-6.67*10^11*5.98*10^24*10/6.37*10^6+1000*10^3 = -5.41*10^30