Question

A boy of mass 50 kg runs up a staircase of 45 steps in 9 s. If the height of each step is 15 cm, find his power. Take g = 10 m s–2

NCERT Class IX
Sciences - Main Course Book

Chapter 11. Work and Energy

Answer 1

Given, mass of body, m = 50 kg

h = 45 x15 = 675 cm = 6.75 m

t = 9 s, g = 10 m

PE = mgh = 50 x 10 x 6.75 = 3375 J

Power of the body = PE / time = Energy / time = 3375 / 9 = 375 W

Answer 2

Given :

Mass (m) = 60 Kg

Time (T) = 9 sec

To Find :

Power

Solution :

Total displacement of the boy (d) = 45 × 15

                                                       = 675 cm

                                                       = 6.75 m

∴ Work Done by the boy = Force (F) × Displacement (d)

                                         = mg × d

                                         = 50 × 10 × 6.75

                                         = 3375 J

Power = $\frac{Work (W)}{Time (T)}$

           =  $\frac{3375}{9}$

           =  375 W

Therefore, the power is 375 W.