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Homework Help: Free Fall Question!

  1. Sep 22, 2008 #1
    The moon's gravity is about one sixth of the Earth's. (Basically, one sixth of 9.80m/s^2). If two objects were dropped at the same height on the Earth and on the Moon, the time it would take for the object to reach the surface of the moon compared to the Earth is...
    a) 6
    b) 36
    c) Square root of 6

    I can't figure it out. I tried using the formula:
    v^2 = -2g(Yo-Y)
    but i got some really weird answers. Any help please? :S
  2. jcsd
  3. Sep 22, 2008 #2


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    Well, the question is asking you about the time it takes to fall. So you should start by using an equation that contains time.
  4. Sep 22, 2008 #3
    oh yeah whoops, wrong formula.. i tried using y = v0*t - 0.5gt^2

    sorry my bad
  5. Sep 22, 2008 #4


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    well, what is v0 in this problem? Hint: the object is initially being held at rest at some height and then is just dropped/released.
  6. Sep 22, 2008 #5
    so the initial velocity is 0m/s..
  7. Sep 22, 2008 #6
    then we have y = 0.5gt^2
    and when i did it for both the Earth and the Moon, i got two really weird answers that aren't part of any of the choices..
  8. Sep 22, 2008 #7


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    SHOW us what you did.
  9. Sep 23, 2008 #8
    Step back a bit. There are two scenarios,

    1 on Earth) y(1) = v0(1) X t(1) + 9.8 X g X t(1)^2
    2 on Moon) y(2) = v0(2) X t(2) + (9.8/6) X g X t(2)^2

    Now, what do you know about y(1), y(2), v0(1), and v0(2)?
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