Name: Robert S.
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?
It depends in part on what the fly has in mind as the car accelerates.
A helium filled balloon is simpler since it has no mind of its own!
The balloon will go forward as the car accelerates. This is because
the "up" direction points more forward in an accelerating car. You
could try dropping a stone in an accelerating car to show that the
"down" direction points backward.
I have enjoyed watching helium filled balloons behaving the opposite
of what one expects objects to do as a car accelerates and brakes. Be
careful, though, that the balloon doesn't interefere with the driver's
safe operation of the car! One can argue that balloons behave in this
way because of their "negative" mass. Their mass is not really
negative, of course, but their density is less than the surrounding
The reason the balloon knows the "up" direction points forward in an
accelerating car is that the air in the car moves backward (slightly)
so that the air pressure and density are higher towards the back of
the car. The balloon responds to the change in density of the air.
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Update: June 2012