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Gravitational potential question, nasty!

  1. Nov 14, 2009 #1
    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, [tex]\phi[/tex]=Work done/mass
    =GMeMm/r


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

    Ep= m([tex]\Delta[/tex][tex]\phi[/tex])
    =(-67.72--59.03)(700)
    =-6083J

    2) G=gMM/r2

    RE: [tex]\phi[/tex]= GM/r

    therefore, g=[tex]\phi[/tex]/r

    But this r=Re + height?

    And this is where I'm stumped.
     
  2. jcsd
  3. Nov 14, 2009 #2
    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.
     
  4. Nov 15, 2009 #3
    At a ht of 400 000m. But they did not give me the r of the earth. Do I need to find it?
     
  5. Nov 15, 2009 #4
    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.
     
  6. Nov 15, 2009 #5
    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..
     
  7. Nov 18, 2009 #6
    So..anyone?
     
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