Page 133 - Understanding NCERT Science 09
P. 133
Activity ___________ 10.15 A stronger person may do certain work in
relatively less time. A more powerful vehicle
• An object of mass 20 kg is dropped would complete a journey in a shorter time
from a height of 4 m. Fill in the blanks than a less powerful one. We talk of the power
in the following table by computing of machines like motorbikes and motorcars.
the potential energy and kinetic The speed with which these vehicles change
energy in each case.
energy or do work is a basis for their
Height at Potential Kinetic E + E classification. Power measures the speed of
p k
which object energy energy work done, that is, how fast or slow work is
2
is located (E = mgh) (E = mv /2) done. Power is defined as the rate of doing
p k
work or the rate of transfer of energy. If an
m J J J
agent does a work W in time t, then power is
4
given by:
3 Power = work/time
2
W
1 or P = (10.8)
t
Just above
The unit of power is watt [in honour of
the ground
James Watt (1736 – 1819)] having the symbol
• For simplifying the calculations, take W. 1 watt is the power of an agent, which
–2
the value of g as 10 m s . does work at the rate of 1 joule per second.
We can also say that power is 1 W when the
–1
Think it over ! rate of consumption of energy is 1 J s .
–1
What would have happened if nature had 1 watt = 1 joule/second or 1 W = 1 J s .
not allowed the transformation of energy? We express larger rates of energy transfer in
There is a view that life could not have kilowatts (kW).
been possible without transformation of 1 kilowatt = 1000 watts
energy. Do you agree with this? 1 kW = 1000 W
1 kW = 1000 J s .
–1
The power of an agent may vary with time.
10.3 Rate of Doing Work This means that the agent may be doing work
Do all of us work at the same rate? Do at different rates at different intervals of time.
machines consume or transfer energy at the Therefore the concept of average power is
same rate? Agents that transfer energy do useful. We obtain average power by dividing
work at different rates. Let us understand this the total energy consumed by the total time
from the following activity: taken.
Activity ___________ 10.16
Example 10.7 Two girls, each of weight 400
• Consider two children, say A and B. N climb up a rope through a height of 8
Let us say they weigh the same. Both m. We name one of the girls A and the
start climbing up a rope separately. other B. Girl A takes 20 s while B takes
Both reach a height of 8 m. Let us say
A takes 15 s while B takes 20 s to 50 s to accomplish this task. What is the
accomplish the task. power expended by each girl?
• What is the work done by each? Solution:
• The work done is the same. However,
A has taken less time than B to do (i) Power expended by girl A:
the work. Weight of the girl, mg = 400 N
• Who has done more work in a given
time, say in 1 s? Displacement (height), h = 8 m
122 SCIENCE
Rationalised 2023-24
