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: Force of gravity on a spaceship - can't get right answer.

  1. Oct 2, 2005 #1
    This seems like it should be very easy but I keep getting the wrong answer. Here is the problem.

    "Calculate the force of gravity on a spacecraft 32000 km (5 earth radii) above the Earth's surface if its mass is 1300 kg."

    In my book there is problem very similar so I tried doing the same thing, but it did not work. What I did was:

    Force of Gravity decreases with a square of the radius so 1/5^2 = 1/25 as strong. Then just F = 1/25mg, 1/25 X (1300)(9.8) = 509.6 N. My online homework submission Webassign says that is wrong. The bookk says I can solve it doing the long way also..

    So I trie doing it the long way. F = G (mass 1 X mass 2) / (radius ^2)...soooo... G is a constant, 6.67 X 10^-11 Nm^2. mass 1 is the spaceship, so 1300 kg. mass 2 is earth, the constant being 5.98 X 10^24 kg. Change radius from km to m, so 32000 km = 32000000 m. or 3.2 X 10^7

    F = 6.67 X 10^-11 X 1300 X (5.98 X 10^24) / ((3.2 X 10^7)^2) = 506 N which is wrong as well!!!

    Please help, thank you so much!
  2. jcsd
  3. Oct 2, 2005 #2
    I think I see where you went wrong. Look at the part I made bold in your quotation below:

  4. Oct 2, 2005 #3
    Oh. So would the radius be 6 and the force of gravity would be 1/36th??

    EDIT: Yup, that was it. Thank you for the help Grogs. That one little thing that I missed screwed me up. I need to read more closely, lol.
    Last edited: Oct 2, 2005
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook