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.

Answer 1

(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.