Discussion
Lifting \( 20 \) tonnes per hour through \( 18 \) m requires a theoretical power of
Answer: D
Correct answer: 0.98 kW
The theoretical power is the rate of doing work against gravity:
\[ P = \frac{mgh}{t} = \frac{20\,000 \times 9.81 \times 18}{3600} = \frac{3\,531\,600}{3600} = 981\ \mathrm{W} \]
so about \( 0.98 \) kW. This is the irreducible minimum, ignoring friction, bucket digging resistance and drive losses. A real installation might need three or four times as much, which is why an efficiency figure must be applied before a motor is selected.
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