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Relative Motion
Name: Robert S.
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 2000-2001
Question:
If i am in a parked car with the windows rolled up, and
there is a fly hovering in the air next to me, if the car accelerates
rapidly from rest, is it possible that the fly will hit the rear window?
or will it continue to hover next to me as the car accelerates?
Replies:
It would depend if your windows are rolled up or not, If the windows are
rolled up then the fly would continue to hover next to you, because when you
accelerate all the air in the car with you would also accelerate, this would
be a single reference frame. If the windows were down or you would have a
convertible with the top down and a rear windscreen and accelerated it would
be possible to catch the fly in the rear windscreen because the air next to
your head would remain in place as the car moved, although the fly would
probably start flying to avoid the upcoming object.
Michael Baldwin
If the inside of the car contains a vacuum the fly will hit the back window.
Otherwise this motion will be affected by the fluid friction of the air. The
lighter the fly, the more this effect. Try it by throwing a ball in the air
when accelerating.
Larry Krengel
Since the air moves with the car the fly will also, though its inertia may
cause it to move somewhat relative to you. You could try this experiment with
a helium filled balloon in the car. Weight it down to the point where it is
just bouyant and drive around while a passenger observes it.
Greg Bradburn
In most cases, the fly will hit the rear window. For the fly to move with
the car, something must push the fly. The fly must experience just as much
acceleration as the car. The air being pushed forward by the rear window
will provide a noticeable force on the fly, but not enough to cause a rapid
acceleration. The fly would have to flap its wings so as to accelerate
forward. While hovering, the fly is flapping its wings so as to have no
acceleration. This same motion would be enough to hover in a fast-moving
vehicle, so long as the fly does not have to accelerate or overcome a wind.
In this situation, the fly has to accelerate rapidly.
Kenneth Mellendorf
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Update: June 2012
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