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Fly floating in a car?

  1. Jul 21, 2011 #1
    fly floating in a car? :)


    Last day we were arguing with my colleagues what will happen with a fly flying in a car when the car suddenly stops :)

    Will it go forwards (or backwards when the car is accelerating)?
    Or will it stay stationary?

    Suppose the following:
    - the car can stop (and accelerate) *really* fast
    - the fly is floating (no horizontal movement)
    - the car is filled with air (what will happen if there is vacuum?)

    Or another similar experiment: Suppose you have glass bottle and the fly is floating in the middle of it. You throw the bottle from the 6th floor :) Will it smash on the bottom of the bottle when it reaches the gound? Bottle is not deformed!

    Please answer only if you are 100% sure what will happen and your answer is Physically valid. No guessing please :)

    P.S. Surely there are similar topics here but I couldn't find any.
  2. jcsd
  3. Jul 21, 2011 #2
    Re: fly floating in a car? :)

    what are you getting at?
    will it help to substitute the fly in the bottle with a marble?

    Clearly, if substitute the fly with a marble inside the bottle (you drop the marble first into the bottle and when it reaches the middle of the bottle, you drop the bottle too) ...when it reaches the ground, the marble is going to continue its trajectory and crush against the floor...the only reason why possibly a fly or mosquito wouldn't be subjected to the same is because they are much lighter and have a higher drag-coefficient/weight ratio and so, they would simply be nicely stopped by the air itself, like an air bed, if you will.
  4. Jul 21, 2011 #3
    Re: fly floating in a car? :)

    For the car situation if the car suddenly accelerates from stop to say... 100km/h then the fly will be *flung* to the back of the car; or to be more physically correct the back of the car will come and 'hit' the fly. Once the acceleration stops then the fly could stabilize itself within the column of air in the car and be stationary relative to the inside of the car. From the outside it would look like the fly is moving at 100km/h with the car. When it stops the same thing will happen, the fly will be 'flung' into the windshield. Or rather the fly will be continuing at 100km/h while the car is slowing down, it wouldn't be able to slow down in time before it hits the windshield.

    Same sort of concept for the fly in the bottle. When you drop the bottle the fly will be forced to the top, if the bottle is closed then it will basically be stuck at the neck/cap of the bottle until it is no longer accelerating. At this point the fly will be traveling at the same free-fall speed as the bottle and be able to maneuver as it wishes within the column of air in the bottle. Once the bottle smashes into the ground so will the fly, probably dying in the process.
  5. Jul 21, 2011 #4
    Re: fly floating in a car? :)

    OK lets continue with the car. It's easier I think. :)

    So when the car stops really quick the air in it also stops. The fly is in the air (not touching the car body). And the forces on the fly are from the *air* only.

    Part of my colleagues thought that the only way the fly can be moved significantly is the car to stop so hard and the G to be so bigger that it creates different pressure at the back and at the front part of the car coupe. Like the air to be moved to the front part of the car... and since the fly stays in the air...
    I don't know :) That's why I'm asking
  6. Jul 21, 2011 #5
    Re: fly floating in a car? :)

    Clever-Name > so yes, the fly won't be stopped by the air in front/back of it because the fly is much denser that the air :)
  7. Jul 21, 2011 #6


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    Re: fly floating in a car? :)

    Think it through. If you start with a fly that is stationary with respect to a *moving* car, then it is moving with respect to a stationary car. Therefore, when the car stops abruptly, the fly will still be moving relative to the (now stationary) car. It's all Newton's first law in this case .. if the fly is moving initially, then it will continue moving in the same direction until acted upon by an external force. It's really no different then the forces you feel from your seat back or seat belt when riding in a car that accelerates and decelerates.

    [EDIT] If you're confused because the air seems to be "stationary", there are two important things you are neglecting. 1) the inertia of a fly is FAR greater than the inertia of an air molecule, and inertia is what determines how much force you must exert on something to change its velocity. 2) Air molecules are constantly in motion, and are colliding with the fly from all sides at speeds much greater than the relative speed of the car ... therefore the forces from these collisions all average out.
  8. Jul 21, 2011 #7
    Re: fly floating in a car? :)

    Correct, the air wont have a significant effect on the fly. It might seem like the air would have some sort of effect on what's happening but think of it this way. When you accelerate (windows up) do you feel a breeze? I sure don't.
  9. Jul 21, 2011 #8


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    Re: fly floating in a car? :)

    Flies don't float, they fly. That distinction is critical here.
  10. Jul 21, 2011 #9
    Re: fly floating in a car? :)

    like I said....I think you guys are getting too theoretical with newton's laws and are forgetting about drag! Flys are not very aerodynamic at all, legged, big-eyed and hairy and are very very light....I think drag coefficient alone is going to keep the fly pretty healthy.
  11. Jul 21, 2011 #10
    Re: fly floating in a car? :)

    Well I would just like to add that a fly can't fly in a vacuum in case that was not mentioned.
  12. Jul 21, 2011 #11
    Re: fly floating in a car? :)

    By the way, what do you think is the last thing that goes through the fly's mind as it smashes against the windshield?
  13. Jul 21, 2011 #12
    Re: fly floating in a car? :)

    It's a*s? :)
  14. Jul 21, 2011 #13
    Re: fly floating in a car? :)

    So the real question is, can a volume of static air produce enough drag on a fly to slow it from 60 mph to 0 mph in less than a few feet? I say no. The bug hits the windshield because of its own inertia when you slam on the breaks.
  15. Jul 21, 2011 #14
    Re: fly floating in a car? :)

    the air may be static at the beginning, but as soon as the car comes to an immediate stop, now you have a fly that is initially supposedly moving at 60mph!

    what's the momentum of the fly? mv? how much is m?
    like I said before, I bet the fly has one ugly drag coefficient and even bet that the fly's terminal velocity is not even close to 60 mph!...
  16. Jul 22, 2011 #15


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    Re: fly floating in a car? :)

    For a fly, you cannot neglect the viscosity of the air. Now it depends on if you are talking about a fruit fly or a large fly, but the Reynolds number for a small insect is in the range 10 to 100. The length of the car cabin is probably greater than 100 times the length of the fly, so the fly should equalize with the air before hitting the wall of the car.
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