Question

(a) Give reason for the following: 

(i) The kinetic energy of a freely falling object increases, yet it holds law of conservation of energy. 

(ii) A girl fills up 10 pages of a notebook in order to practice sums, yet she has not done ‘work’ in terms of Science / Scientific concept. 

(iii) Work done by gravitational force on an object moved along a horizontal path, is zero. 

(b) Find the energy in kWh consumed in 24 hours by two electric devices, one of 100 W and other of 500 W. 

Answer 1

(a)(i) According to the law of conservation of energy, energy can neither be created nor be destroyed but can be transformed from one form to another. Here, the increase in kinetic energy is due to transformation of potential energy into kinetic energy i.e. 

Loss in P.E. = gain in K.E. So, the conservation of energy holds good. 

(ii) There has been no displacement of the pages of the book so work done is zero. This is because W = F × s and when s = 0 then W = 0. 

(iii) As the angle between the gravitational force and the displacement is 900 . Hence, W = 0. 

(b) Energy consumed= Power × time 

Given,

Power of first device, P₁= 100W and Power of second device, P₂= 500W 

Time taken for both the devices, t = 24 h 

Total energy consumed= P₁t + P₂t = [100 × 24 + 500 × 24] 

= [100 + 500] × 24 = 600 × 24 = 14400 Wh = 14.4 kWh