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

• yccheok
In summary, the floating fly in a static train will experience inertia and be pushed backward when the train starts to accelerate and move forward. This is caused by the air molecules in the train being pushed along with the train and exerting a force on the fly. In a complete vacuum, the fly would not experience inertia until it is pushed by the back of the train colliding with it, causing it to accelerate forward. However, the fly would not be affected much by the acceleration due to its light weight and the lack of air resistance. The water level in a glass or a person standing in the train would also tilt in the direction of acceleration, demonstrating the same effect as the fly.

#### yccheok

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?

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

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.

Kevin_Axion said:
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.

I know it happens for a balloon.

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.

Kevin_Axion said:
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.

Dr Lots-o'watts said:
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.

Less dense? It doesn't float (that is a balloon filled with CO2).

Kevin_Axion said:
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.

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.

TurtleMeister said:
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.

You know what, my mistake, I believe it is an He balloon.

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 won't 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:
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.

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.

Last edited:

## 1. What happens to the fly when the train starts moving?

When the train starts moving, the fly will experience a force pushing it in the direction opposite to the train's motion. This is due to the air particles inside the train moving with the train, creating an area of high pressure at the front and low pressure at the back. As the fly is lighter than air, it will be pushed towards the back of the train.

## 2. Does the fly remain floating in the air?

Yes, the fly will remain floating in the air, but its position will change due to the force exerted on it by the moving train.

## 3. Will the fly get sucked into the engine of the train?

No, the fly will not get sucked into the engine of the train. The force exerted on it by the moving train is not strong enough to overcome the fly's ability to resist movement.

## 4. What happens to the air particles around the fly?

The air particles around the fly will also experience a force from the moving train, causing them to move in the same direction as the train. This creates an area of high pressure in front of the fly and low pressure behind it, causing it to move towards the back of the train.

## 5. Will the fly experience any change in velocity?

Yes, the fly will experience a change in velocity as the train starts moving. This is due to the force exerted on it by the moving train, causing it to accelerate in the direction of the train's motion.