I was wondering, within a static train, for a fly which is currently floating in the air and not moving at all, what will happen if:

  1. The train starts to accelerate and move forward. Will the floating fly experience inertia and being pushed backward?
  2. If the train is completely evacuated (without air), will the floating fly experience inertia too?
  • $\begingroup$ We had pretty much exactly the same question a while ago: physics.stackexchange.com/questions/3863 . The setup there was an astronaut in a spaceship rather than a fly on a train, but the physics is the same. $\endgroup$ – Ted Bunn Feb 9 '11 at 14:31
  • $\begingroup$ @Ted Thanks for noticing; unfortunately this is too late to close and merge will make people think when astronaut changed into a fly. $\endgroup$ – user68 Feb 10 '11 at 10:08
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    $\begingroup$ What if the windows were open on the train/bus? Would it be the same? Because the air wouldn't be moving with the train/bus it would be flowing through the windows. $\endgroup$ – user17134 Dec 24 '12 at 1:43

The fly will experience only whatever forces are acting on it, which is none in a vacuum. The same way a person on ice skates would not experience any forces on him as the train moved forward and he remained stationary. That is because there is no horizontal component of the contact force.

BTW: A fly cannot fly in a vaccum.

  • $\begingroup$ So, if the fly in non-vacuum train, it will experience inertia due to air molecular? $\endgroup$ – Cheok Yan Cheng Feb 9 '11 at 1:19
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    $\begingroup$ @Yan - if the air is moving with the train, the fly will get dragged with it due to air resistance. $\endgroup$ – ja72 Feb 9 '11 at 2:22

We needn't think about a train to develop a mental picture of the physics involved - actually, that may over-complicate it.

Have you ever caught a fly in a bottle? If so, you'll notice that the fly's motion is relatively independent of the bottle's motion - until the fly hits a wall of the bottle, of course. Moving the bottle does (practically) nothing to the fly's motion because the bottle is exerting very (very!!) little gravitational pull on the floating fly.

So the fly's motion will remain (practically) unaffected by moving the bottle, and a large train is in effect just a much larger container than the bottle, with more room for the fly to move until it hits the wall of a train.

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    $\begingroup$ You are forgetting about air -- in fact it is relatively harder to catch fly with bottle because of the air currents it generates. The lurid example are mosquitoes in the rain -- they can fly dry because flow around droplet is repelling them from its path. $\endgroup$ – user68 Feb 9 '11 at 20:22
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    $\begingroup$ But have you ever notice when you are driving your car at 120km/h, there is a fly flying slowly say (1km/h) inside your car? From outside observer, it seems that the fly is flying at speed 120+1 km/h. $\endgroup$ – Cheok Yan Cheng Feb 10 '11 at 3:13
  • $\begingroup$ @mbq - depends on the size of the bottle's neck. Perhaps if I said a large jar you'd be less concerned? (It's fairly simple to sneak up to a fly at rest and place a jar over it.. whence forth picking up the jar, you can observe that moving the jar around doesn't noticeably affect the fly's movement). $\endgroup$ – user1838 Feb 10 '11 at 14:56

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