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Does this paragraph about a falling cup make sense?

  1. Dec 6, 2014 #1
    A cup with a mass/weight at the bottom was filled with cotton and an egg was place inside for a physics project. As the cup was falling, it would always re position itself to fall with the bottom/heavier half first (the bottom contains the mass). Would it make sense to explain this case this way:

    After a test drop with a plastic egg, the plastic cup had tipped over and landed on its side. This issue was resolved by placing a mass at the center of the bottom of a second cup, and placing the first cup on top. Because the bottom half of the device was now heavier, it would be less affected by air drag than the top half, and the egg-helmet would always position itself in mid-air with the heavier half at the bottom. This repositioning was a result of the collision with the cup and all the stationary air molecules in the way. The half with more mass will have more momentum to spare among the air molecules that it encounters, thus maintaining more speed than the lighter half.

    Thanks!
     
    Last edited: Dec 6, 2014
  2. jcsd
  3. Dec 6, 2014 #2

    haruspex

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    It's sort of right, but it would be better to explain it in terms of torque. Imagine the cup tilted somewhat while falling. Draw where you think the forces of gravity and drag will act.
     
  4. Dec 6, 2014 #3
    How would you explain it using torque? I'm not familiar with what it is.
     
  5. Dec 6, 2014 #4

    haruspex

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    Torque means a twisting force. If you have equal and opposite forces, but in different 'lines of action', the result will be a tendency to twist the object they're acting on.
    In the case of this cup, as you say, the mass is concentrated towards the bottom, but the drag is not. Just draw a picture of a tilted rectangle with the drag acting upwards in the middle, but gravity acting downwards near the bottom end. Do you see the twisting effect?
     
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