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Free fall with heavy box

  1. Oct 7, 2009 #1
    hello,

    consider a man falling from a roof with a heavy box in his hands. what should he do with the box?
     
  2. jcsd
  3. Oct 7, 2009 #2

    A.T.

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    Hold it over his head!

    Nahh... just kidding. I would push it downwards and thus accelerate myself upwards shortly before impact.
     
  4. Oct 7, 2009 #3
    but why shortly before impact?
     
  5. Oct 7, 2009 #4

    Dale

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    Because (assuming that the box has the same drag coefficient as the person) that will maximize the energy lost to drag. Of course, it will have to be a very tall building for drag to matter much.
     
  6. Oct 7, 2009 #5
    consider that case without drag
     
  7. Oct 7, 2009 #6

    Dale

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    Then the timing does not matter.
     
  8. Oct 7, 2009 #7

    A.T.

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    I think timing does matter without considering drag.

    Think of the extreme case: You are strong enough to give the box so much momentum downwards, that you stop falling just before impact. Applying the same momentum to the box at the begin of the fall will just make you fly higher and fall even deeper.
     
  9. Oct 7, 2009 #8

    Dale

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    Good point. I think you are right, but I would have to work it out to be sure.
     
  10. Oct 7, 2009 #9

    A.T.

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    Here a simulator (the box is just like rocket fuel):
    http://lander.dunnbypaul.net/
     
  11. Oct 7, 2009 #10
    to minimize the "imapact velocity" one should through the box downwards as late as possible. this can be easy shown with a velocity vs time diagram and considering, that v*t is constant.
     
  12. Oct 8, 2009 #11
    ya..i think you should drop it as late as possible assuming you have enough strength in the end. I take it that you have nothing but the box, so once you come to stop momentarily after throwing, you should really make sure you don't fall long enough
     
  13. Oct 8, 2009 #12
    You can exert a far larger force using your leg muscles than using your arm muscles. While in free fall, you should put your feet on the box and jump at the last moment.
     
  14. Oct 8, 2009 #13
    It would be interesting to calculate the force that is needed to slow down the velocity to zero.
     
  15. Oct 9, 2009 #14
  16. Oct 9, 2009 #15

    A.T.

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    Sure, humans are to weak. It is hard enough to jump off the ground with the massive earth as support. Pushing down a box or even an elevator cabin might give you enough force to cancel out the gravitational acceleration for a moment, but you would need much more to achieve an upwards acceleration that would slow you down significantly.
     
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