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: Gravitational force

  1. Aug 25, 2008 #1
    1. The problem statement, all variables and given/known data
    COnsider two identical 1kg blobs of water on opposite sides of the Earth, one on the side facing the moon and the other on the side farthest away from the moon.

    a)calculate the gravitational force of the moon on the blob on the side of the earth closest to the moon. (considering that the distance i have for earth to moon is centre to centre.)

    b) calculate the force of the moon on the blob on the farthest side away from the moon.

    Distance from Earth to moon = 3.84E8
    Radius of Earth = 6.37E6
    G= 6.67259E-11
    Mass of Earth = 5.98E24
    mass of Moon = 7.36E22

    2. Relevant equations


    3. The attempt at a solution

    seeing how the blobs of water on the surface i assumed i would have to subtract the radius of the earth from the distance to give me a more accurate distance from blob to moon.
    3.84E8-6.37E6 = 3.78E8.

    now factored that into equation

    F=6.6725(5.98E24x 7.36E22)/3.78E8^2

    thats a huge number does that seem correct..

    for b) all i changed was the distance which i added the radius of the earth instead which gave 3.9E8

    i hope this is right....
  2. jcsd
  3. Aug 25, 2008 #2


    User Avatar
    Homework Helper

    You are likely off by the factor of the mass of the Earth.

    They only asked about attraction to the moon. The other mass that you are considering are the 1 kg blobs.

    As to your distances You would do well to take the Earth moon distance and add a radius of earth to one and subtract for the other.

    Good luck.
  4. Aug 25, 2008 #3
    Oh ok i see now i was actually calculating the attraction of the earth to the moon instead of the blob to the moon
    cheers thanks for that.
  5. Aug 25, 2008 #4
    ok now i get Fnear= 3.4e-5
    and Ffar= 3.2e-5

    that sounds much better i think.
  6. Aug 26, 2008 #5
    Quick question are the units for the gravitational force N ??? seeing how in the equation there are 2 kg / m^2
  7. Aug 26, 2008 #6


    User Avatar
    Homework Helper

    Check out the units of the gravitational constant.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook