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A Fly in a Moving Car

  1. Jul 2, 2009 #1
    In a car moving forward at 60 mph a fly is buzzing freely. If the car suddenly comes to a complete stop, does the fly crash into the windshield?
     
  2. jcsd
  3. Jul 2, 2009 #2
    Why don't you test it out?
     
  4. Jul 2, 2009 #3
    Of course.
     
  5. Jul 2, 2009 #4

    Ivan Seeking

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    Smoke, however, will move towards the back of the car. Why?
     
  6. Jul 2, 2009 #5
    Same reason it floats upwards.
     
  7. Jul 2, 2009 #6
    Same direction as an inflatable balloon I would guess : because it's afraid of the windshield of course.
     
  8. Jul 2, 2009 #7
    What a deep statement... indeed gravity is also an acceleration, and we're all afraid of falling.
     
  9. Jul 2, 2009 #8

    Ivan Seeking

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    Okay, how about the classic: If a bird is sitting inside and on the bottom of a sealed box, and the bird begins to fly - hovers in place inside of the box - does the box lose weight?
     
  10. Jul 2, 2009 #9
    The flying bird will release heat, increasing the temperature of the box which will radiate energy away : the box does loose weight until the bird dies of hunger.
     
  11. Jul 2, 2009 #10
    I'm thinking that the wing beats create a downward air pressure that increases the weight of the box.
    edit -- hmm...that's probably wrong since there is no source of new air...
     
  12. Jul 2, 2009 #11
    The mass "m" of the box itself doesn't change, and the height certainly doesn't change significantly enough to change the value of little "g" (even if it's bouncing around), so the weight of the box doesn't change. Now if the box was originally sitting on a scale, and you wanted the net force of the box/bird system on the scale, that's a different question.
     
  13. Jul 2, 2009 #12
    That would only be true if smoke is heavier than air. But is it?

    Test it! have somebody who smokes (not me) blow a bubble filled with smoke and compare the speed at which it sinks with that of a normal bubble.

    Then why does smoke rise? Not because it is lighter than air.
     
    Last edited: Jul 2, 2009
  14. Jul 2, 2009 #13
    Yes the box on the scale would see a decrease ~ the bird is no longer part of the boxes structure.
     
  15. Jul 2, 2009 #14
    The net downward force of the air on the bottom of the box has to equal the net upward force of the air on the bird. There'll be slight fluctuations, as the bird flaps and falls, but on average no weight will be lost.
     
  16. Jul 2, 2009 #15

    tiny-tim

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    I think it's because the smoke is hot, and heats the air, and the smoke is then carried on the updraft of hot air.

    That's why smoke will be attracted to a chimney even if the chimney is not directly above … it follow the air current. :smile:
    I dunno :confused: … won't the fly reach terminal velocity very quickly … alternatively, won't the air suddenly become very "viscous"?
     
  17. Jul 2, 2009 #16

    Pythagorean

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    actually, it is part of the boxes structure through the air in the box. If the bird lifts up off the box, he will have to be exerting force downwards (on the air) some of which will push down on the box, registering on the scale.

    Of course, the full weight of the bird probably won't show up on the scale as some of the air molecules push against the sides of the box, which doesn't register on the scale. I don't know how they would relate mathematically, but I'm guessing you'd have to use navier-stokes or some what crap.
     
  18. Jul 2, 2009 #17

    Ivan Seeking

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    Really?

    Btw, I have tried it.
     
    Last edited: Jul 2, 2009
  19. Jul 2, 2009 #18

    Ivan Seeking

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    :rolleyes: Alright, the box is sitting on a scale and the bird begins to fly inside of the box... :tongue:

    It was intended as a tricky question but not a trick question. :biggrin:
     
    Last edited: Jul 2, 2009
  20. Jul 2, 2009 #19
    Can you provide the accuracy of the scale, an order of magnitude for the proper mass of the box (not counting air inside), the inner volume of the box, the mass of the bird, and also the temperature and pressure if they are not standard. Then we could make an actual statement : this is not measurable...
     
  21. Jul 2, 2009 #20

    Pythagorean

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    just keep it theoretical

    V = volume
    m = mass of bird
    M = mass of box

    etc...
     
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