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A couple of weeks ago I was travelling in a car (120 km/h approximately) and I saw a fly flying in front of me (inside the car, near my nose, windows closed). I wonder how was that possible.

Does it mean is really flying at 120 km/h or the fly is being affected by some kind of gravity/force?

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Outside the car? How far in front of you was it? – David Z Jan 31 '12 at 23:46
Sorry, I updated my question – César Jan 31 '12 at 23:50
It flies 120 km/h if it was a convertible without windshield. Did you feel like you where in a hurricane in your car? If not then why should the fly? – Alexander Feb 1 '12 at 0:00
Because the fly can fly. I can't – César Feb 1 '12 at 0:06
up vote 7 down vote accepted

Basically from the frame of observation as your car:

The fly was inside your car, so its speed with respect to the car is zero. Its just as much inside the car as you are. Both are travelling at 120 with respect to any observer on the road. ut with respect to anyone inside car you both are just sitting inside the car.

So the speed of fly with respect to you is $v=0\,\frac{m}{s}$, with respect to some observer on the road is $120\,$km/h.

Its no more than a tissue paper you might keep near the steering wheel, in front of you.

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What if the windows were opened? – Jake Oct 2 '14 at 4:10

For the same reason that when you are in a train that moves at 120 km/h, you can walk down the aisle without running at 120 km/h yourself.

In your case, you are walking on a floor that is moving at 120 km/h alredy. In the case of a fly, it is flying in air that is moving at 120 km/h.

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The other answers provide a good explanation of the situation.

But,your observation might have been different if the car was constantly accelerating but the fly was flying at a constant speed.Then,it would have crashed with the back glass after sometime,as the car is moving forward faster than the fly is moving forward.

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