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It has happened many times and i have ignored it everytime. Yesterday it happened again .
I was travelling in a train and saw a fly (insect) flying near my seat.

Train was running at a speed of around of 100 km/hr. So according to the physics rules , my speed will also be 100 km/hr as i am sitting inside the train. But as far as the fly is consider, it is flying inside the train , the speed of the fly is very less as compared to the train. So why does not the fly stuck on the one side of the train ?

As the fly is not in physical contact with the train, will its speed be 120 km/hr?

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Have a look at http://en.wikipedia.org/wiki/Galilean_invariance. This is not too mathematical and explains what's going on.

The basic idea is that there is no such thing as absolute motion. For example, because the earth is rotating as I sit here typing I'm moving at about 800 miles per hour. Why am I not splattered against my computer screen? It's because everything around me is moving at the same speed, so relative to where I'm sitting I'm not moving.

In the specific case of the fly, the fly moves by beating it's wings against the air. But the air is stationary with respect to you, otherwise you'd be sitting in a 100km/hr wind. That's why you see the fly moving at whatever speed flies normally move at.

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  • $\begingroup$ Thanks. I have one more question. If we create a vaccum in the train, will it happen as i said? $\endgroup$
    – vikiiii
    Commented Mar 12, 2012 at 7:19
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    $\begingroup$ @vikiii Depends. If you created a vacuum, and placed the fly inside, it would not stick to the walls. It wouldn't be able to fly either. On the other hand, if you had a way of maintaining an open vacuum, if the fly flew in from the ouside, it would whack into the wall. $\endgroup$
    – Manishearth
    Commented Mar 12, 2012 at 7:23
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    $\begingroup$ @vikiii: the international space station moves at about 17,500 miles per hour (relative to the Earth's surface), but when an astronaut leaves the airlock for a space walk he doesn't zoom off at 17,500 miles per hour. That's because he and the ISS are moving at the same speed when he's in the airlock and they're still moving at about the same speed when he leaves the airlock. Similarly, if you fitted the fly with a space suit and pumped out all the air in the train, the fly would still be moving at about the same speed as the train. $\endgroup$
    – John Rennie
    Commented Mar 12, 2012 at 8:00
  • $\begingroup$ @John i have one more question. Suppose the window is open and the train is moving at a speed of 100 km/hr. So as soon as fly will go out of the train from the window.Will it feel the same as a sudden brake is applied on 100 km/hr train? $\endgroup$
    – vikiiii
    Commented Mar 13, 2012 at 5:09
  • $\begingroup$ Suppose first that the train is on the moon where there's no atmosphere. The fly (with a jetpack!) flies out the window and carries on at the same speed as the train, just like the atsronaut leaving the ISS. Sat in the train watching, you see the fly hovering outside the window. Now put the train on Earth. The fly goes out the window and finds itself in a 100km/h wind, and it's the wind that slows the fly. If you stick your head out of the train window you will also feel the 100km/h wind, but your head is heavier than the fly and attached to your body (!) so it doesn't slow down! $\endgroup$
    – John Rennie
    Commented Mar 13, 2012 at 6:46
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The fly is flying in the air inside the closed train. As this air moves with the train (you don't feel the wind blow), the fly can just fly in the air in your frame of reference. If it is flying with 20 km/h in the train, it is 120 km/hr relative to the earth.

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