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Homework Help: Weight on hypothetical planet

  1. Jul 15, 2008 #1
    1. The problem statement, all variables and given/known data

    The mass of a hypothetical planet is 1/100 that of Earth and its radius is 1/4 that of Earth. If a person weighs 150 lb on earth, what would he weigh on this planet

    2. Relevant equations

    I think F = G*m1*m2 / r^2

    3. The attempt at a solution

    I believe F = 150lbs (im not sure)

    So in that problem do i do G (Me/100)(Me)/ (Re/4)^2

    no im not doign something right. I really wish my professor would have covered this.
    Last edited: Jul 15, 2008
  2. jcsd
  3. Jul 15, 2008 #2


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    You have his weight on Earth, so you'd need to find his mass by dividing by acc. due to gravity on Earth.

    Then use the info to find the accel. due to gravity on the planet,g'.

    New weight will be mg'.

    Remember how to find gravitational field strength?
  4. Jul 15, 2008 #3
    Since my proffessor is way behind, he said Gravitational fields is one thing that we will not cover on test/final. Does this mean this question is invalid?
  5. Jul 15, 2008 #4


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    ah my bad. You don't need to go through all of this.

    Just use


    which is what you were doing. But one of the masses is the planets mass while the other is the mass of the person (this is what you need to find).
  6. Jul 15, 2008 #5
    150lb = 68 kg

    F = G*5.98x10^24*68 / 1592500^2 = 107 N ?
  7. Jul 15, 2008 #6


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    One mass is the mass of the planet= 5.98E24 /100

    for r did you put (radius of the earth)/4 and then square?
  8. Jul 15, 2008 #7
  9. Jul 15, 2008 #8


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    Then it should be correct.
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