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Just a simple question but trickier than I expected.

Let us suppose that there is a fly hovering in the moving train. The train moves at a speed much higher than the hovering of the fly. However, the fly moves with the train and does not hit its walls.

Could anyone explain the exact reason for this? Is it because of the air inside the train and the fly is very light? Then if the inside the train is a vacuum, devoid of air, would the fly hit the wall?

Qmechanic
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Keith
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  • But... if the window were open, the rushing air would splatter it on the wall, even on a perfectly windless day, no? – Cosmas Zachos Mar 22 '20 at 18:56
  • Well for one, without air, the fly couldn't fly in a vacuum since its wings use air resistance to counteract gravity. – Andrew Mar 22 '20 at 18:57
  • Ok, for the vacuum case, instead of fly, think of anything that can stay above the ground. – Keith Mar 22 '20 at 19:00
  • I assume that there is no window. – Keith Mar 22 '20 at 19:01
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    When the fly takes off from a surface on the train, it has and retains the speed of the train and its movements are + and - changes from that speed (as if the fly took off from a non-moving train). Same would apply if the inside were a vacuum (if we imagine it could fly in a vacuum).

    If the train were to suddenly speed up, then a backward displacement of the fly might be noticeable.

     – Not_Einstein Mar 22 '20 at 19:06

1 Answers1

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Let us suppose that there is a fly hovering in the moving train. The train moves at a speed much higher than the hovering of the fly. However, the fly moves with the train and does not hit its walls.

The air inside the train moves with the same speed as the train. Therefore the hovering fly has no relative speed with respect to the air. For the fly this is the same situation as if the train (and the air inside) stands still.

Thomas Fritsch
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