Question

A windmill is used to raise water from a well. The depth of the well is 5.0 m. The windmill raises 204 kg of water every day. What is the average useful power extracted from the wind? A. 12 mW B. 120 mW C. 690 mW D. 1700 mW

Answer 1

The work done in raising 204 kg through a distance of 5 m is

W = (mass) x (gravity) x (height) =

(204 kg) x (9.8 m/s²) x (5 m) =

9,996 kg-m²/s² = 9,996 joules

Power = (work) / (time) . That's 9,996 joules per day. But we always like to
describe power in (joules per second), and there's even a special unit for that . . .
1 joule per second is called "1 watt".

(9,996 joules/day) x (1 day / 86,400 seconds) =

(9,996 / 86,400) joules/second = 0.1158 watt (rounded)

or 115.8 milliwatts

or 120 milliwatts (very rounded)