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
          |========|=========|
          |========|=========|
          |========|=========|
          |________|_________|

With regard to a given framework, an acceleration of that framework is equivalent to an gravitational field in the opposite direction, and vice versa.  So an acceleration to the right is equivalent to a gravitational attraction to the left.  The lead would "sink" to the left and the cork would "float" to the right, in the frame of reference of the tank.

Relative to the earth, I'm sure everything would be accelerated to the right. Just the lead would not go as fast and the cork would go faster, than the tank as a whole.

Posted by Charlie on 2006-01-05 13:25:03