What happen to a Fly floating in the air when the train start moving

  • Thread starter yccheok
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  • #1
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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 start to accelerate and moving forward. Will the floating fly experience inertia and being pushed backward? Does the inertia caused by air molecular?

2) If the train is complete vacuum, will the floating fly experience inertia too?
 

Answers and Replies

  • #2
901
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It would move forward because the air from the back of the plane will be forced slightly to the front due to acceleration of the train
 
  • #3
Drakkith
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Everything in the train would experience something like you feel when you accelerate in a car. The fly, being very light, would be greatly affected by the air in the train. So it would feel the same thing, but depending on the acceleration of the train it shouldn't have a problem flying around still. The air would be "pushing" on the fly and causing it to accelerate with the train.

What do you mean by the train being in vacuum? Is there air inside the train? If not, the fly would experience no force until the back of the train collided with it and caused it to accelerate.
 
  • #4
Drakkith
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It would move forward because the air from the back of the plane will be forced slightly to the front due to acceleration of the train

I don't think the fly would be pushed forward as a fly is denser than the surrounding air.
 
  • #5
901
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I know it happens for a balloon.
 
  • #6
1. The fly pushes the air with its wings to remain aloft. When the train moves, it brings along the air, so along goes the fly.

2. If there is no air in the train, the fly will be crawling.
 
  • #7
Drakkith
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I know it happens for a balloon.

That is because the baloon is less dense than the surrounding air. The reason it moves forward during acceleration is the same reason that it floats to begin with. Acceleration = gravity. Or something like that lol.
 
  • #8
901
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1. The fly pushes the air with its wings to remain aloft. When the train moves, it brings along the air, so along goes the fly.

2. If there is no air in the train, the fly will be crawling.

In reference to 2, touché.
 
  • #9
901
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Less dense? It doesn't float (that is a balloon filled with CO2).
 
  • #10
Drakkith
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Less dense? It doesn't float (that is a balloon filled with CO2).

I've never had a baloon filled with CO2, only helium, so I couldn't say anything on that. I don't know the density of CO2 compared with air.
 
  • #11
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If you have a glass of water sitting in the stationary train, then the water level in the glass will be parallel with the floor of the train. When the train starts to accelerate the water level in the glass will tilt. The side closest to the front of the train will tilt down, and the side closest to the rear will tilt up. The fly, which is flying in mid air, will tilt in this same way.
 
  • #12
Drakkith
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If you have a glass of water sitting in the stationary train, then the water level in the glass will be parallel with the floor of the train. When the train starts to accelerate the water level in the glass will tilt. The side closest to the front of the train will tilt down, and the side closest to the rear will tilt up. The fly, which is flying in mid air, will tilt in this same way.

I don't believe it would. The water tilts because the water wants to stay put when the train, and then the glass, starts moving. Since the glass doesn't let the water stay still the water "tilts" as it is pressed to the back of the glass. A similar effect can be seen by simply holding a glass of water and turning at an angle to the ground. Gravity pulls the water straight down, but the glass is tilted in relation to the force.
 
  • #13
901
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You know what, my mistake, I believe it is an He balloon.
 
  • #14
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I would expect the fly to remain fairly stationary as the train accelerates.

The force the air exerts on the fly as it is pushed forward by the train accelerating will be tiny.

It wont be until the pressure behind the fly builds to the point that it can push the fly forward.

So overall, I'd say the fly would appear to remain still and eventually hit the back of the train.
 
Last edited:
  • #15
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Drakkith said:
I don't believe it would. The water tilts because the water wants to stay put when the train, and then the glass, starts moving. Since the glass doesn't let the water stay still the water "tilts" as it is pressed to the back of the glass. A similar effect can be seen by simply holding a glass of water and turning at an angle to the ground. Gravity pulls the water straight down, but the glass is tilted in relation to the force.
The water level is always perpendicular to the direction of acceleration. It's just a basic accelerometer. When the train is stationary, the direction is straight up. That direction changes when the train accelerates. The greater the acceleration of the train, the greater the tilt angle. It doesn't even have to be a glass of water. If you stand up in the train when it accelerates, you will lean forward with the same angle as the water level. Everything on the train is affected the same way, including the air and the fly - and helium filled balloons.
 
  • #16
rcgldr
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Ignoring the slight amount of movement of air due to the accleration, the total force on the fly will increase when the train accelerates, so the fly will have to compensate for the increase in force, and based on it's reaction time, it will move backwards for a bit until it compensates.

For example, if the train accelerates at 1 g, then the fly's apparent weight increases by a factor of 1.4142 (sqrt (2)), and the direction of the apparent weight is 45 degrees downwards and backwards.
 
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