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Consider two boys A and B carrying 10 bricks each to the top of a house. Let us imagine, A does it in 10 minutes, B does it in 15 minutes. As each of the boys has carried the same number of bricks through the same height, they have done the same amount of work. However, if it is asked who between A and B is more powerful, certainly you will say that A is more powerful than B. This is so because A does the same work as B does. So, the rate at which the boy A finished the work is greater than that of B. Thus, power relates work and time.

Power is the rate of doing work. Thus,
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For a particle acted upon by a constant force Description: 63159.png, causing displacement Description: 63155.png, the work done given by Description: 66160.png.
Hence, power can be written as
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where Description: 63193.png is the constant velocity acquired by a particle due to Description: 63159.png. Thus,
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The particle moves with a constant velocity when the applied force just overcomes the frictional force. The rate at which energy is produced or utilized is called power.

Units of Power
The SI unit of power is watt (W). This is named after James Watt, the British inventor of steam engine.
If one joule of work is done in one second, then the power is said to be one watt.
The watt is frequently used to measure electric power. However, it can be used to measure any kind of power.

In many branches of engineering, the power output of machines is expressed in terms of horse power which is defined as 550 foot pound per second (a horse pulling a weight of 550 lb through 1 ft in 1 second).
  • 1 watt = 1 J s−1
  • 1 kilo watt = 103 W
  • 1 mega watt (MW) = 106 W
  • 1 horse power (HP) = 745.7 W (say 746 W)

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