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Homework Help: Falling bodies experiment

  1. Jan 7, 2005 #1
    I used atwoods machine to test the affect of mass on the speed of falling bodies. My research said objects fall at the same rate no matter their weight. When I tested this on Atwoods machine heavier masses fell faster. Can someone tell me why?
  2. jcsd
  3. Jan 7, 2005 #2


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    Is this what you're referring to? Atwood's machine

    Did you neglect pully mass?
  4. Jan 7, 2005 #3
    From the pictures I looked at, on some of the larger machines air resistance would play a minor (but still calculatable). This and the mass of the pully and the tension/strechting of the string would all contribute greatly to a source of error.
    Last edited: Jan 7, 2005
  5. Jan 7, 2005 #4

    I used a similar machine. Mine had two pulleys. I would not think pulley mass would have an effect because I used the same pulley and set up for each mass I dropped. I first balanced two masses. then I added 5 grams of weight to one side and measured the time it took to fall 1 meter. I then added 5 more grams and measured the time it took to fall 1 meter. the heavier the mass the less time it took to fall.
  6. Jan 7, 2005 #5
    air resistance not be a source of error because I used lumps of clay with the same surface area.
  7. Jan 7, 2005 #6

    Doc Al

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    The greater the mass difference, the greater the net force acting on the two masses. As the mass difference approaches infinity, the acceleration approaches g. What else would you expect? It is not an error! These masses are constrained (they are attached to each other via a pulley); they are not freely falling. Check out the link that Q_Goest provided.
  8. Jan 7, 2005 #7


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    Adding to Doc Al's comment, I must say:
    1) Your argument for why you can neglect the pulley mass is completely wrong.
    You can do so only if
    a) Frictional forces are negligible ("reasonable")
    EDIT: That is, about the axis.
    b) the effective mass of the pulley (i.e, its moment of inertia divided by its squared radius) is much smaller than the sum of the two masses.

    This is quite a different statement than yours.

    2) Secondly, Q-Guest's link shows that (given that you may neglect the pulley mass) if the mass RATIO is constant, then the acceleration remains constant as well.
    That is something you ought to verify (I doubt you'll be able to disprove it)
    Last edited: Jan 7, 2005
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