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Homework Help: Gravitational Attract

  1. Oct 28, 2013 #1
    1. The problem statement, all variables and given/known data
    The force of gravity at Earth's surface on an astronaut is 634N. What is the force of gravity on the same person at each of the following distances, in multiples of Earth's radius, from the centre of Earth?
    a) 2 b)5 c)10 d)17.2

    I have no idea how to solve this, can someone please point me in the right direction of how to solve this question.
  2. jcsd
  3. Oct 28, 2013 #2
    Seems to me that this is a problem involving Newton's law of universal gravitation:

    F=(g x mass 1 x mass 2) / (r^2) with g being the gravitational constant, 6.67 x 10^-11.
    You can solve for the astronaut's mass, and use this equation (assuming the Earth's mass and radius as given, just look this up) to solve for the force of gravity on the astronaut.

    Correct me if I'm wrong, I'm only another AP Physics student.
    Last edited: Oct 28, 2013
  4. Oct 28, 2013 #3
    You don't need earth's mass and radius and also G.
    We have F=GM1M2/R2

    As according to your problem M1 and M2 are not going to change

    Use F is inversely proportional to R2

  5. Oct 28, 2013 #4
    I don't understand how you came up with that equation.
  6. Oct 28, 2013 #5

    Now according to the problem it is asked to find the force on the man when its distance from the center of earth is 2R,4R......etc

    So in the formula GMearthMman/R2 we are going to change the distance R.Right?

    As everything else in the formula is going to remain constant except R
    We can say that F is dirctly proportional to 1/R1

    so let F1 be the force of gravity on the man at a distance R from the centre of earth.
    Let F2 be the force of gravity on the man at a distance R2 from the centre of earth.

    So F1=GMearthMman/R2.................1

    divide equation 1 by 2 you will get my formula.

    For your firsr problem R1=2R and F1=634

    put in the above equation you will get F2
    Last edited: Oct 28, 2013
  7. Oct 29, 2013 #6


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    Please don't post complete solutions. Just give the questioner a nudge in the right direction.
  8. Oct 29, 2013 #7

    Oh,sorry for that.I will take care next time.
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