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Homework Help: Gravitational Potential Energy question

  1. Jun 22, 2013 #1
    Hello! I'm having some problems with a question that my professor has given me. Below is the question. I have already figured out the planet's orbital and angular-orbital velocities, but included that part due to the fact that some information was included.
    PICA-9 and PIC-10 are planets orbiting its star, PIC-2A. Find each of the planet's orbital and
    angular-orbital velocity if PIC-2A has the same mass as our Sun, PICA-9 is 1.0 A.U. away from
    the star and PICA-10 is 5.5 times further than PICA-9. What are the gravitational potential
    energies of each planet with respect to the star?

    2. Relevant equations
    Since everything must be in m/kg form, I have figured out that:
    Distance from PICA-9 and PICA-2A is 1.50X1011m
    Distance from PICA-10 and PICA-2A is 8.22X1011m
    PICA-2A mass is 1.989X1030kg

    3. The attempt at a solution
    Since the formula for GPE given to me is -GMm/r (G=gravity in space, M="the anchor" mass, m=orbiting object's mass, and r=the distance between the planets), I wasn't sure if PE=mgh would be a helpful formula in this case. Would finding the kinetic energy be helpful as well? How would I find the mass of the planets?
    Thanks for reading!
  2. jcsd
  3. Jun 22, 2013 #2


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    Hello stefiex and welcome to PF!

    Stick with PE = -GMm/r. PE = mgh is only an approximation that is valid when m is near the surface of a planet or star.

    Unfortunately, you will not be able to calculate PE without knowing the masses of the planets and there is no way to deduce those masses from the information given.
  4. Jun 22, 2013 #3
    That's what I thought. My teacher is notorious for forgetting information needed to complete the problem. Thanks for taking the time to look at it!
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