# Homework Help: Gravitational potential question, nasty!

1. Nov 14, 2009

### Boogeyman

1. The problem statement, all variables and given/known data
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.

2. Relevant equations

g=F/m

grav potential, $$\phi$$=Work done/mass
=GMeMm/r

3. The attempt at a solution
1)I think I got out this part.

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

2) G=gMM/r2

RE: $$\phi$$= GM/r

therefore, g=$$\phi$$/r

But this r=Re + height?

And this is where I'm stumped.

2. Nov 14, 2009

### 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.

3. Nov 15, 2009

### Boogeyman

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

4. Nov 15, 2009

### 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.

5. Nov 15, 2009

### Boogeyman

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..

6. Nov 18, 2009

So..anyone?