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Question
A girl of mass 40 kg runs a height of 80 stairs, each 25 cm high with a load of 20 kg on her head in 25 sec. If g is 10 m/s2, find:
(i) Gravitational force acting on the girl.
(ii) Work done by her.
(iii) Useful work done by her.
(iv) Her power in watt.
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Solution
(i) Gravitational force acting on the girl = Her mass in kg × g
= 40 × 10 = 400 N.
(ii) Total gravitational force = (Mass of girl + load) × g
= (40 + 20) × 10 = 600 N.
and Distance or height =`(80xx25)/100` = 20 m.
∴ Work done by her = 600 N × distance
∴ Total work done = 600 N × 20 m = 12000 J.
(iii) Useful work done = Load × g × Distance covered
= 20 × 10 × 20 = 4000 J.
(iv) Power = `("Total work done")/("Time")=12000/25"W"` = 480 W.
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