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Homework Help: Gravity question - help please?

  1. Jan 9, 2015 #1
    Can anyone help me with this question?

    1. The problem statement, all variables and given/known data

    "Describe how the weight of an object changes as it moves from the surface of the Earth to the surface of the moon. At what point between them is the resultant gravitational force on an object zero? Where is its acceleration zero?"

    2. Relevant equations

    (distance between Earth and moon = 3.8e8 m
    Mass of earth = 81 x mass of moon
    Mass of earth = 6e24 kg
    Radius of earth = 6.4e6 m
    Gravity on earth's surface = 9.8 m/s
    Universal gravitational constant = 6.7e-11 N m^2 kg^-2)

    3. The attempt at a solution

    Thanks in advance!!
    Last edited by a moderator: Jan 9, 2015
  2. jcsd
  3. Jan 9, 2015 #2


    Staff: Mentor

    Welcome to PF!

    You need to show your work before we can help.

    Also I moved your post to the proper forum and edited it adding the homework template.
  4. Jan 9, 2015 #3
    Thank you, but I don't understand - what do you mean by "you need to show your work"? Apologies!
    Last edited: Jan 9, 2015
  5. Jan 9, 2015 #4


    User Avatar

    Staff: Mentor

    It means that the 3rd section of the HH Template is for you to start working on the problem. Use the relevant equations and write out the equation for the pull of the two gravity forces on the object as a function of distance along the path from the Earth to the Moon.
  6. Jan 9, 2015 #5
    Ah, sorry. I just read the Terms of Use.

    Well, the only thing I can seem to be able to do is to use the formula:

    GM1m / x^2 = GM2m / y^2
    Where M1 is the mass of the earth, M2 is the mass of the moon.

    The only way I got that was from manipulating other formulae, and by looking at the law of gravitation.

    I named the unknown point (the point at which the resultant gravitational force is zero) point P, and the distances between P and the earth/P and the moon distances x and y, respectively. I tried to use algebra in solving it but I'm at a loss. I'm not very experienced on the topic, sorry to seem stupid! It's just bugging me so much that I don't know where to go.
  7. Jan 9, 2015 #6


    User Avatar

    Staff: Mentor

    It looks like you've set it up right, with x+y = total distance between Earth and Moon, right? Can you maybe solve for the ratio of x/y? Knowing that ratio and the total distance, you could solve for the point P...
  8. Jan 10, 2015 #7
    Ah - that would help immensely. I'll try that now. Thank you!
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