Advertisements
Advertisements
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.
Advertisements
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.
RELATED QUESTIONS
When does a force perform work?
A weight lifter lifted a load of 200 kgf to a height of 2.5 m in 5 s. Calculate:
- the work done, and
- the power developed by him. Take g = 10 N kg-1
A max X goes to the top of a building by a vertical spiral staircase. Another man Y of the same mass goes to the top of the same building by a slanting ladder. Which of the two does more work against gravity and why?
What is the SI unit of work?
State the energy changes which take place when A toy car with a wound spring moves on ground.
A force is applied on a body of mass 20 kg moving with a velocity of 40 ms−1. The body attains a velocity of 50 ms−1 in 2 seconds. Calculate the work done by the body.
How long should an electric motor, of power 2 H.P. operate, so as to pump 5 m3 of water from a depth of 15m. [Take g = 10 N kg−1]
