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!

Homework Help: Weight planet problem

  1. Nov 17, 2012 #1
    1. The problem statement, all variables and given/known data

    A satellite of mass M = 683 kg is in circular orbit at height ho = 6.15x106 m above the surface of the Earth. Find:

    w, the weight of an astronaut of mass m = 75.5 kg inside the satellite.

    2. Relevant equations

    I believe it's something like:

    F = Gm_1*m_2/r²

    3. The attempt at a solution

    For the first part of the problem, I've found that the acceleration due to gravity is around 2.55 m/s².

    I used the form, but it's not right.

    F = 6.67 * 10^(-11) * 5.98 * 10^(24) * 75.5 / (6.15 * 10^(6) + 6.37 * 10^6)² ≈ 192 N
  2. jcsd
  3. Nov 17, 2012 #2
    If the satellite is in free-fall won't the astronaut weigh nothing?
  4. Nov 17, 2012 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    You found the acceleration due to gravity. What is mass × acceleration due to gravity ?
  5. Nov 17, 2012 #4
    Then, it's just F = ma, and it becomes...

    F = 75.5 kg * 2.55 m/s²
    = 192.525

    However, the answer does not seem to be right. I don't get why this happens.
    Last edited: Nov 17, 2012
  6. Nov 17, 2012 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Do they mean apparent weight?
  7. Nov 17, 2012 #6
    Yes, I believe that is what the question is trying to ask! My instructor is not good in wording the question, so I got that part wrong.
  8. Nov 18, 2012 #7
    Actually, I need to find the weight of the astronaut IN the orbitting satellite!
  9. Nov 18, 2012 #8
    Wouldn't the astronaut weigh 0 N because it's in free fall as someone just said?
  10. Nov 18, 2012 #9


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    It depends what you mean by "weigh". If you stood the astronaut on a set of scales then yes, it would be 0. But if you mean the force exerted by gravity then your original answer is correct. In my view, the 0 answer comes about by using a non-inertial reference frame, which makes it as misleading as discussing centrifugal force.
  11. Nov 18, 2012 #10
    My original answer is correct if the question doesn't ask for apparent weight.

    BUT I am not sure if the answer is 0 or not. The astronaut is in the orbiting satellite.
  12. Nov 18, 2012 #11
    Haha. Very funny. Then, the answer is 0.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook