Define power. How will you differentiate between kW and kW h? If the velocity of a ball is tripled, what will be the ratio of
(a) its initial kinetic energy and final kinetic energy
(b) initial momentum and final momentum.
OR
Four men lift a 250 kg box to a height of 1 m and hold it without raising or lowering it.
(a) How much work the men do in lifting the box?
(b) How much work do they do in just holding it?
(c) Why do they get tired while holding it? (g = 10 m s–2)
Power is defined as the rate of doing work. Power measures the rate of work done i.e, how fast or how slow the work is done.
Power = work/ time
kW defines how much energy a device uses or generates in a given amount of time, kWh defines how much energy that devices actually used or generates.
kW is a measure of power while kWh is a measure of energy.
Kilowatt hour = kilowatt × hour
or kWh = KW × hr
kW = kWh/h
(a) Let Initial mass of ball be m, initial velocity v
Initial kinetic energy = 1/2 mv2
Final velocity of ball = 3v
Final kinetic energy = 1/2 m (3v)2
= 9 (1/2 mv2)
= 9 initial K.E
= 
= 1: 9
(b) Momentum p = mv
i.e. momentum is directly proportional to velocity
initial momentum = mv
final momentum = m (3v)
= 3(mv)
= 3 × initial momentum
Therefore, the ratio of initial and final momentum is given as 1:3
OR
Given the mass of box, m = 250kg
Gravity, g = 10m/s2
height, h = 1m
Work done = mgh
= 250 × 1× 10
= 2500J
(a) Zero work is done in holding the box because it does not move while holding it.
(b) To hold the box the men are applying force against gravity. This requires muscular efforts. Faster blood circulation is required hence their body gets tired.