This month's Challenge Question: Specs & Techs from GlobalSpec:
Two identical containers are
filled with water to the same height. Suspend a steel ball from the ceiling
such that the ball will remain inside the left container. Next take a hollow
plastic ball with the same volume as the steel ball and keep it immersed inside
the right container. What will happen to the balance pans? Are they going to
remain at the same height?
And the answer is:
Because the water levels remain the same, we
might think the pressure at the bottom of each container will be same. After
all, the pressure exerted by a liquid on the bottom of the container is
proportional to the height of the liquid.
From this point of view this is
correct but we must consider also the forces, if any, exerted on the bottom of
the containers. For the left container, where the steel ball is suspended in
the liquid, the ball does not exert any force because it is not touching the
bottom. However the suspended plastic ball exerts an upward force on the bottom
of the right container, moving it upward.
Therefore the pan with the plastic rises and the left pan (with the steel ball) lowers.
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