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Beaker ball balance problem

This video introduces an interesting problem of two balls placed in beakers of water.  A ping pong ball is fixed to the bottom of one beaker of water and an acrylic ball suspended into another beaker of water.  Both beakers contain the same amount of water and both balls displace equal amounts of water (they have the same volumes).  The beakers are placed on a balance.  What motion is seen?

The beaker containing the acrylic ball moves downwards, the other upwards.  This is explained in terms of the additional force on the string holding the ping pong ball in place. The resultant downwards force is reduced.


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