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.

           __________________
          |        |         |
          |========|=========|
          |========|=========|
          |========|=========|
          |========O=========|  <----- lead sphere
          |==================|
          |========O=========|  <----- cork sphere
          |========|=========|
          |========|=========|
          |========|=========|
          |________|_________|

(In reply to First Thoughts by Rollercoaster)

Ah, but you now have a different situation than the helium balloon in the car.  Now you are dealing with water which, unlike the air in your car, cannot be compressed.  So, I'm thinking that the only force then that would act on the two spheres is inertia which would cause them both to move to the left relative to the tank....  maybe.

Posted by Sing4TheDay on 2006-01-05 11:31:51