Gravitational potential question, nasty

[Boogeyman]

Homework Statement

Values for the grav potential due to the Earth are given below:

dist from Earth's surface/m : grav potential/MJ kg^-1
0 : -62.72
390 : -59.12
400 000 : -59.03
410 000 : -58.94
Infinity : 0

1. If a satellite of mass 700kg falls from a ht of 400 000m to the Earth's surface, how much potential energy does it lose?

2. Deduce a value for the Earth's grav field at a ht of 400 000m.

Homework Equations

g=F/m

grav potential, \phi=Work done/mass
=GM_eM_m/r

The Attempt at a Solution

1)I think I got out this part.

E_p= m(\Delta\phi)
=(-67.72--59.03)(700)
=-6083J

2) G=gMM/r²

RE: \phi= GM/r

therefore, g=\phi/r

But this r=R_e + height?

And this is where I'm stumped.

 

[anubis01]

well at what height above the surface of the Earth are you trying to find g at? The total radius will be the radius of the earth+the height at which you are trying to find the g value for.

 

[Boogeyman]

well at what height above the surface of the Earth are you trying to find g at? The total radius will be the radius of the earth+the height at which you are trying to find the g value for.

At a ht of 400 000m. But they did not give me the r of the earth. Do I need to find it?

 

[anubis01]

the radius of the Earth is a physical constant, you can just look up the value (6378.1km) and use that to solve your equation.

 

[Boogeyman]

the radius of the Earth is a physical constant, you can just look up the value (6378.1km) and use that to solve your equation.

I don't think you could do that. These questions usually give you everything you need to solve them..besides you wouldn't be able to do that in an exam..

 

[Boogeyman]

So..anyone?