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Q & A: Fly Inertia

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Most recent answer: 10/22/2007
If a fly were hovering in a moving car just behind the windscreen (Windshield in the US), and the driver of the car were to brake sharply, would the fly’s inertia cause it to hit the windscreen, or would it remain in it’s original position? and why? My old chemisty teacher asked me this question years ago and it’s bothered me since Many thanks
- Doug (age 24)
The fly might well hit the windshield (windscreen UK) due to its inertia. The only reason it might not is that flies have a lot of friction with the air. Since the air will not be rushing through the windshield, that friction will impede the fly's motion toward the glass. However, if it started close to the glass, it will indeed hit.

Mike W.

Flies are heavier than air and so this argument is exactly right. If you held a helium balloon in a similar situation, the heavier air will by its inertia be pressed against the windscreen, and a bouyant force will push the helium balloon backwards in the car.


(published on 10/22/2007)

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