A body floats in a liquid contained in a beaker. The entire system falls freely under gravity. What is the buoyant (upthrust) force on the body due to the liquid during free fall?

Zero
Equal to the weight of the liquid displaced
Equal to the weight of the body in air
Equal to the weight of the immersed portion of the body

Correct answer: Zero

Solution

During free fall, both the liquid and the body fall with the same acceleration g. In the reference frame of the falling system (or simply noting that the net gravitational effect is zero), the effective gravity inside is zero. Buoyant force = rho * g_eff * V = rho * 0 * V = 0. There is no pressure gradient in the liquid to exert an upthrust.

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