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Let's assume a helium balloon is suspended into the floor of a moving train. We already know how it will react during acceleration, deceleration, and constant speed as illustrated in figure no 1, 2, and 3. However, what will happen to the balloon if we detach the balloon from train floor and attach something that is exactly the same weight as it's floating force thus making it "floating" in the train cabin. Assuming that we do that after train has stopped accelerating (moving at constant speed) as illustrated in figure no.4. Will it stay where it was detached inside the cabin or will it hit the back of the cabin as the train is moving forward at a constant speed?

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  • $\begingroup$ Any more convincing answer? $\endgroup$
    – Albert Tobing
    Commented Jul 2, 2017 at 16:32
  • $\begingroup$ I think Eti is correct: Assuming that you are at constant speed (and that the air is not moving [no wind], in thermal equilibrium, the train is not taking a curve, ...) you don't get any movement of the ballon -- it stays where it was, with respect to the train. However, once the train accelerates, the logic is analog to to (1) and (2). $\endgroup$
    – NotMe
    Commented Jul 2, 2017 at 17:50

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First of all, weight which attached shouldn't be equal to floating force since rope and balloon have mass too. But let's say balloon is floating for us. Assuming that you have travelled with train at least once. You know that air molecules are not just back side of the train but everywhere. You can apply the same logic for floating balloon too.

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Ideally, there will be no external force acting on the balloon in the horizontal direction so it will move along with the train in accordance with Newton's First Law.

However in reality, there will be collisions with gas particles present in the coach which will give it some random velocities.

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  • $\begingroup$ Why do think the ballon is cutting through air? What air? $\endgroup$
    – nasu
    Commented Jul 2, 2017 at 14:21
  • $\begingroup$ I thought the gas particles would cause some retardation but then realised that they are randomly moving inside the train. $\endgroup$
    – User
    Commented Jul 2, 2017 at 14:40

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