1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Find gravitational potential energy if at point

  1. Oct 9, 2011 #1
    1. The problem statement, all variables and given/known data
    An object is moved from A to B in the gravitational field of earth. The distance between the center of earth and B is twice that of A. If the gravitational potential energy of the object at A is -2MJ, what is the gravitational potential energy of the object at B?


    2. Relevant equations
    Eg = -G*M*m / r


    3. The attempt at a solution
    Im stumped for where I should start. I need to find the mass or radius of orbit of this object but I cant figure out how to do that with just the gravitational potential energy given. Please help with a hint on where to start. I dont want the question answered. Thanks
     
    Last edited: Oct 9, 2011
  2. jcsd
  3. Oct 9, 2011 #2
    If rA is the distance from the center of the earth to A, and rb the same for B, then you know

    [itex]\frac{GMm}{r_A} = -2MJ[/itex]

    [itex]\frac{GMm}{r_B} = ?[/itex]

    Use the relation between the two lengths and the first formula to get the potential energy at B.
     
  4. Oct 9, 2011 #3
    I know that rb is 2ra

    so if I just substitute the 2ra in the first equation, do I get GMm/2ra = -2MJ/2

    Im not sure if thats algebraically correct
     
  5. Oct 10, 2011 #4
    That's correct, you basically substituted the value for the potential energy at A into the expression for it at B (with the additional half factor)
     
  6. Oct 10, 2011 #5
    lol alright. I wasnt sure about the algebra there.
    thanks alot man!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook