A bread gives a boy of mass 40 kg an energy of 21 kJ. If the efficiency is 28%, then the height that can be climbed by him using this energy, is :

Options

A. 22.5 m
B. 15 m
C. 10 m
D. 5 m

Answer

B

Explanation

To climb a height h, the boy utilizes potential energy = mgh
In order to climb, he will use the efficient energy
Energy of one bread = 21 KJ = 21 `xx 10^2` J
Efficiency of boy = 28%
Hence, energy consumed by boy
`=(28)/(100)xx21000 = 5880" "J` ...(i)
From law of conservation of energy. this energy is utilized in giving potential energy mgh, where is acceleration due to gravity. `therefore mgh = 40 xx 9.8 xx h...(ii)`
From equation Egs. (i) and (ii) we have
`implies 40xx9.8xxh=5880`
`implies h=(5880)/(40xx9.8)=15` m