Solution of Work and Energy (Page No - 164 & 165) - Physics by Lakhmir Singh, Class 9 | Extra Documents & Tests for Class 9 PDF Download

Page 164
Solution 46
Mass of body = 1 kg
g = 10 m/s² 

I. At h= 5m; v= 0 m/s

PE = 10 x 5 = 50 J

KE = 0 J

at total Energy = PE+KE = 50 J

ii. at h = 3.2 m; v =10 m/s

PE = 0 J

total Energy = PE+KE = 50 J

The total energy in all three cases is constant . this prove the law of conservation of energy

Solution 47
Since from point A total energy = PE + KE = 80 J
And according to the law of conservation of energy Total energy remains constant
a) PE = 0
Total energy = PE + KE = 80 J
KE = 80 J PE = 80 J
b) At point B KE = 48 J
Total energy = PE + KE = 80 J
PE = 80 J KE = 80 – 48 = 32 J
c) Law of conservation of energy 

Page 165
Solution 48
No. of steps = 28
Height of each step = 28 cm
Total height = 20 x 28 = 560 cm = 5.6 m
Mass of student = 55 kg
g = 9.8 m/s²
time = 5.4 s
Work done = m x g x h = 55 x 9.8 x 5.6 = 3018.4 J 

Solution 49
Weight of box = 100 N
Height = 1.5 m
work done = m x g x h = 100 x 1.5 = 150 J
potential energy = m x g x h= 100 x 1.5 = 150 J
iii. weight of 4 boxes = 400 N
time = 1 minute = 60 s
work done = 400 x 1.5 = 600 J 

Solution 50
a) Electrical energy to sound energy
b) Sound energy to electrical energy
c) Electrical energy to light (and heat) energy
d) Chemical energy to electrical energy to light
energy(and heat energy)