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Homework Help: Energy conservation + planets

  1. Mar 16, 2008 #1
    1. The problem statement, all variables and given/known data
    Two particles of masses M1 = 81000 kg and M2 = 9400 kg are initially at rest an infinite distance apart. As a result of the gravitational force the two masses move towards each other. Calculate the speed of mass M1 and mass M2 when their separation distance is 26.5 km.

    2. Relevant equations

    3. The attempt at a solution
    I think you have to use energy-conservation equations here for each mass

    1/2mv1^2 - (GM1M2)/R = 1/2mv2^2 - (GM1M2)/R

    so, for mass M1, v1=0 and (GM1M2)/R on the left = 0 because R = infinite. then you get

    1/2m1v2^2 = (GM1M2)/R --and solve for v2, but this doesn't seem to be correct..
  2. jcsd
  3. Mar 16, 2008 #2
    Lets say that the 2 masses were infinitely separated. The potential energy of the system would be 0J. However, an attractive force acts on them and they are brought closer together, in which the separation between them 26.5 km. Because it is an attractive force, the potential energy of the system is negative. What you do is find the change in potential energy. The initial value is 0J and the final value would be when they are the given distance apart. Using -dU = dK (d being delta, U being gravitational potential energy, and K being kinetic energy), you would solve for the kinetic energy of each mass. From that, you solve for velocity.
  4. Mar 16, 2008 #3
    isn't that basically what i did? calculated the changes to solve for velocity?
  5. Mar 17, 2008 #4

  6. Mar 17, 2008 #5
    M1 / M2 <10 so I don't think you can say that.

    -GM1M2/r is the potential energy of BOTH the masses.

    At the start, potential and kinetic energy is 0. at a distance r the total energy is:

    [tex](1/2)m_1v_1^2 + (1/2)m_2v_2^2 - Gm_1m_2/r[/tex]

    This won't allow you to calculate [tex]v_1[/tex] and [tex]v_2[/tex] by itself. You also need conservation of momentum.
  7. Mar 17, 2008 #6
    I never told him to ignore the masses.

    DID I?
  8. Mar 17, 2008 #7
    Well I didn't think you said that. I just don't think that M1>>M2 is either true or helpful.
  9. Mar 17, 2008 #8
    It is helpful for sure, especially when energy consideration comes into play.
  10. Mar 17, 2008 #9

    D H

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    Staff Emeritus
    Science Advisor

    It is not helpful for this problem. First, the approximation m1>>m2 is incorrect here, as m2 > m1/10. Second, the approximation implicitly makes the velocity of the larger mass zero, and the problem asks for both v1 and v2.

    This, on the other hand, is very good advice.
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