A block of wood is pushed far below the surface in a tall column of water and then let go. As it rises, it moves

with a constant upward acceleration
with an upward acceleration that decreases
with a deceleration (slowing down)
at a constant (uniform) velocity

Correct answer: with an upward acceleration that decreases

Solution

Just after release the only forces are buoyancy (constant) and weight (constant), giving an initial upward acceleration. As the block speeds up, the viscous/drag force grows and opposes motion, so the net upward force and hence the acceleration decreases (until terminal velocity).

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