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Homework Help: Gravitational Force

  1. May 31, 2008 #1
    Hi all, I'm new here. So anyways, I have been studying for exams by going through my old tests, and I found a question from one of them that I cant solve...I hope someone can help me.

    1. A person stands on a set of bathroom scales which have been calibrates in newtons. The scales read 500N (assume 3 sig figs)
    A)What would the reading be if the same person stood on the scales on a planet where the gravitational field strength, g is 14 N/kg?

    B) If this planet had a mass of 7.0 x 10^24 kg, what would its radius be?

    C) What mass would this person weigh at an altitude of 2.8 x 10^6 m above the planet's surface?

    2. Relevant equations

    Well I used the equations for

    F = G (mass planet) (mass object) / d^2

    C) I don't know what equation to use...

    So what I did:

    for A) I used Fg=mg

    Fg = mg
    500 = m (9.8)
    m = 51

    then I substituted for the planet.

    Fg = m(14)
    Fg = (51)(14)
    Fg = 714

    For B) I used F = G (mass planet) (mass object) / d^2

    Fg = 6.67 x 10^-9 *7.0x10^24 / d^2
    d^2 = 4.669
    d = 2160789.4 km

    For C) given height, I dont know what equation to use.

    So really, I don't know whether I used my equations correctly, so I would appreciate someone to point out my errors.
  2. jcsd
  3. May 31, 2008 #2
    Hi. The mass of a person should always be constant when he/she moves away from the surface of the planet. I believe the verb "weigh" here does not imply the weight of the person.
  4. May 31, 2008 #3

    Doc Al

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    Staff: Mentor

    Looks good.

    Did you leave out the mass of the person?

    Use the same equation that you used for B. What would be the person's distance to the center of the earth?
    Last edited: May 31, 2008
  5. May 31, 2008 #4

    Doc Al

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    Staff: Mentor

    I assume that the word mass was an error and that the question should read: What would this person weigh...
  6. May 31, 2008 #5
    So, would the mass of the person be unchanged, at 51 kg? I am not sure on what you meant when you said use the same equation as "C".

    Do you mean using Fg = mg? If so, how do I factor the given altitude into the equation?
  7. May 31, 2008 #6

    Doc Al

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    Staff: Mentor

    Oops... I meant: Use the same equation as used in "B".
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