Question

A boy takes 3 mins to lift a 20 litre water bucket from a 20 m deep well,while his father takes 2 mins . compare their work and power developed by them

Answer 1

Given,
Time taken by boy to lift bucket = 3 mins = 180 seconds
Time taken by father to life bucket = 2 mins = 120 seconds
Depth of well = 20 m
Density of water = 1kg /litre
1 litre = 1kg
20litres = 1*20= 20kg

Mass of lifted water = 20kg
Taking g = 10m/s^2

Force of water = mg
                        = 20 * 10 = 200 N

Power developed by boy = Work/time
                                        = Force * displacement / time (W = FS)
                                        = (200 * 20/180)watt
Power developed by father = Work/time
                                            =  Force * displacement / time (W = FS)
                                            = (200 * 20/120)watt

Ratio of power developed by boy to that of father 
=(200 * 20/180)watt/(200 * 20/120)watt = 120/180 = 2:3 (Ans )

Answer 2

Answer:

Explanation:Volume of water = 20 l = 20/1000= 0.02 m^3

Mass of water = Density × Volume = 10^3×0.02 = 20 kg

(i) Work done by boy = mgh = 20×9.8×20 = 3920J

Work done by father = mgh = 20×9.8×20=3920J

Compare work done =>

3920 : 3920

= 1:1

(ii) Time taken by boy = 3 min = 180 s

Time taken by father = 2 min = 120 s

Power developed by the boy = Work done /Time = 3920/180

Power developed by the father = Work done /Time = 3920/120

Comparing power devolped =>

3920/180 : 3920/120

=2:3

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