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If we bring a floating balloon in a accelerated train, the balloon will move forward in the same direction as the acceleration as shown in the following figure.

enter image description here

However if a pendulum is hung on the train ceiling, the ball moves backward. We also get moved backward in the accelerated train.

Could you explain why the floating balloon behaves differently?

kiss my armpit
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    see http://physics.stackexchange.com/questions/86774/why-does-a-helium-filled-ballon-move-forward-in-a-car-when-the-car-is-accelerati – Christoph Jan 29 '14 at 07:18
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    Note that this will only happen if the balloon is filled with Helium or some other gas lighter than air. Now think about this: if you release a balloon and a metal ball inside a container of water, won't the balloon move upwards and the ball go downwards? Why do you think is that!? – mikhailcazi Jan 29 '14 at 07:22
  • look at this video http://www.youtube.com/watch?v=EYRuHSfR4R8 here acceleration is radial also http://www.youtube.com/watch?v=XXpURFYgR2E . The link of Christoff explains it. A matter of buoyancy. rule of thumb what floats up in air follows the balloon track, what sinks follows the pendulum. – anna v Jan 29 '14 at 09:01
  • This has been treated extensively in Christoph's link. It would be nice, though, if you could incorporate your image into that question, though ;). – Emilio Pisanty Jan 29 '14 at 13:31

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