In a closed glass tank which is filled with water to the brim, there are two spheres, one of cork and one of lead. The cork sphere is fixed to the bottom by a thread to prevent it from rising further; the lead sphere hangs from a thread fixed to the top of the tank. The lengths of the threads are such that the lead sphere is hanging just above the cork sphere. Explain what happens from the point of view relative to the tank and from the point of view relative to the Earth, if the tank is now suddenly moved to the right.
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|========O=========| <----- lead sphere
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|========O=========| <----- cork sphere
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|________|_________|
I know (from recent experience) that driving with helium balloons in a car is quite an experience. Everytime I took a turn, the balloons actually flew to the inside of the turn (displaced by the heavier air which flew to the outside). And braking hard would send the balloons to the back of the car.
So from the frame of reference of the tank, I would say that the cork would move in the direction of sudden movement, and the lead would move in the opposite direction.