# What happen when a helium balloon is floating inside a moving train?

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?

• Any more convincing answer? Commented Jul 2, 2017 at 16:32
• 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). Commented Jul 2, 2017 at 17:50