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Question
A boy takes 3 minutes to lift a 20 litre water bucket from a 20 m deep well, while his father does it in 2 minutes.
How much work each does? Take density of water = 103 kg m-3 and g = 9.8 N kg-1.
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Solution
Work done = mgh
Given,
density of water = 103kg m -3
To convert, density of water to mass per litre
We know, 1 litre = 1000 cm3 and 1 m3 = 106 cm3
So, `10^3 kgm^-3 = (1000 " kg")/(1 "m"^3)`
`= (1000 " kg")/(10^6 cm^3)`
`= (1 kg)/(1000 cm^3)`
`= (1 kg)/(1 litre)`
Hence, 20 L of water = 20 Kg by mass
height = 20 m
Now, substituting the values in the formula for work done we get
W = 20 × 9.8 × 20
W = 3.92 kJ
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