Question:

When the pendulum bob is lowered into the water, does the container topple? Why?

Answer:

Is the right side heavier because the bob is on the right side? Or is the right side lighter because there is less water on that side?

Archimedes Approach

Guess what, the “downthrust” due to the bob is exactly equal to the missing water’s weight. This is because upthrust is always equal to the weight of fluid displaced. So both sides are heavier by the same amount.

Fluid Pressure Approach

Notice that the water level is always horizontal? When the bob displaces the water, the water always redistributes itself such that the water level is remains constant. This means that the pressure forces exerted by the water on the bottom of the container remains uniform.

Water has this amazing ability to spread its weight uniformly!

Upon Landing

The balance is broken when the bob touches the bottom of the container. There is no mechanism to automatically counter the additional downward normal contact force exerted by the bob on the container.

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