Average power as rate of gravitational potential energy gain

Steps:

• Total mass = bar (20 kg) + two weights (80 kg each) = 180 kg
• Gravitational potential energy gained, ΔU = mgh = 180 kg × 10 N/kg × 2.5 m = 4500 J
• Time taken, Δt = 0.5 s
• Average power, P = ΔU / Δt = 4500 J / 0.5 s = 9000 W = 9 kW

Why D is correct:

• Power equals work done against gravity divided by time, where work is mgh for vertical lift.

Why the others are wrong:

• A: Overestimates by factor of 10, likely from dividing by 0.05 s instead of 0.5 s.
• B: Underestimates, possibly using only bar mass (20 kg) for mgh/t.
• C: Close but wrong mass (e.g., 800 kg total) leading to 80 kW.

Final answer: D