Ncert Exercises & Solutions

A person of mass 60 kg wants to lose 5 kg by going up and down a 10 m high stairs. Assume he burns twice as much fat while going up than coming down. If 1 kg of fat is burnt on expending 7000 kcal, how many times must he go up and down to reduce his weight by 5 kg?

Hint: The total energy burnt will be equal to the rate of energy burnt multiplied by the weight loss.

Step 1: Find the total energy burnt.

Given, the height of the stairs =h=10m

The energy produced by burning 1 kg of fat = 7000 kcal

∴

The energy produced by burning 5 kg of fat =

5 \times 7000 = 35000 k c a l = 35 \times 10^{6} c a l

Step 2: Find the energy burnt in one cycle.

The energy utilised in going up and down one time

= m g h + \frac{1}{2} m g h = \frac{3}{2} m g h

= \frac{3}{2} \times 60 \times 10 \times 10

= 9000 J = \frac{9000}{4.2} = \frac{3000}{1.4} c a l

Step 3: Find the no. of cycles.

∴

Number of times, the person has to go up and down the stairs

= \frac{35 \times 10^{6}}{(3000 / 1.4)}

\frac{35 \times 1.4 \times 10^{6}}{3000}

= 16.3 \times 10^{3} t i m e s

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