(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
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