A rectangular wooden block of density 800 kg/m³ and mass 2 kg is pushed fully under water and then let go. Ignoring viscous drag, what is the block's initial acceleration? (g = 10 m/s²)

1.25 m/s² downward
2.5 m/s² upward
1.25 m/s² upward
2.5 m/s² downward

Correct answer: 2.5 m/s² upward

Solution

When fully submerged, the upthrust equals the weight of displaced water, which exceeds the block's weight since the block is less dense. The net upward force divided by the mass gives an upward acceleration a = g*(rho_w - rho_b)/rho_b.

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