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What is the change in potential energy of the moon if it comes 4.0 cm closer to the e

  1. May 14, 2012 #1
    earth?

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

    Mass of moon - 7.3 x 10^22 kg
    Mass of earth - 6.0 x 10^24
    distance between moon and earth - 356400 km
    change in distance 4 cm
    2. Relevant equations



    3. The attempt at a solution

    I just need to check if my answer is correct, I get some thing to the power 28, using the equation GMm/r

    Is my answer reasonable?
     
  2. jcsd
  3. May 14, 2012 #2
    Re: What is the change in potential energy of the moon if it comes 4.0 cm closer to t

    You should get something to the 28 for the potential energy at 356400 km. Now move it 4cm and find a new potential. You will need to keep a lot of digits because most will subtract out when you find the difference between the two potentials.
     
  4. May 14, 2012 #3
    Re: What is the change in potential energy of the moon if it comes 4.0 cm closer to t

    I get something to the power 28 when I use the formula:
    for r in meters,
    -GMm/r - ( - GMm/(r+4x10^-2)

    so is the value I got wrong then?
     
  5. May 14, 2012 #4
    Re: What is the change in potential energy of the moon if it comes 4.0 cm closer to t

    By the way do I need to find the potential (i.e. GM/r) or the potential energy (GM m(moon) )/r ?
     
  6. May 14, 2012 #5
    Re: What is the change in potential energy of the moon if it comes 4.0 cm closer to t

    -GMm/r

    This itself should be about 10^28 if you do an order of magnitude estimate

    - GMm/(r+4x10^-2)

    This also be around 10^28 and be very close to the previous value.

    It should be so close because r changed by only about 0.00001% This means that there should be a large number of digits that subtract out between the two numbers.

    Your initial post said potential energy, so I think that's what you need. You wouldn't need the moon's mass to find potential.
     
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