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Helicopter in a box: Third law

  1. Feb 9, 2009 #1
    A thought-experiment. (This is not homework!) Supposing someone puts a radio-controlled helicopter in a CLOSED clear-plastic box. They then put the box plus chopper on a sensitive scale and recorded the weight. Say 50N for the sake of argument. Now suppose they started up the chopper and instructed it to hover in the center of the box. How would the scale reading change?

    1) If I think in terms of internal forces, there should be no changes --- all internal forces balance. It should remain at 50N.

    2) But if I think in terms of impulses I would expect the scale reading to change at lift off and then return to its original reading of 50N.

    What am I missing? I wonder if anyone has actually done this.
  2. jcsd
  3. Feb 9, 2009 #2


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  4. Feb 9, 2009 #3


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    I believe you are right that it would change at liftoff - the helicopter is accelerating upwards, causing an upwards acceleration in the overall center of mass of the system, increasing the force against the scale at liftoff. However, once the helicopter reaches an equilibrium (hover), the force against the scale will go back to precisely what it had been prior to the liftoff.
  5. Feb 10, 2009 #4


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    It's been done:


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