**Power and its units**

Machines may be classified by the speed with which they transfer energy or do work thps-there are motor-car engines of small “power”, as they are rated, or large power (Power is defined as the rate of transfer of energy, energy transferred

or average power = time taken

The SI unit of power is called the watt (W) and is a rate of transfer of energy of joule per second Thus, 1 W = I Jjs

Larger units used are the kilowatt (kW) and the megawatt (MW) 1 kW = 1000 W (or 103 W) \ J 1 MW = 1 000 000 W (or 106 W

**Example.** Calculate the power of a pump which can lift 200 kg of water through a vertical height of 6 m in 10 s. (Assume g = 10 mjs2.)

Force overcome Distance work done (energy transferred)

Time taken

energy transferred

Power = time taken

= 200 x 10

=6m

= 200 x 10 x 6 J

= 10 s

200 x 10 x 6

10

1200W

1.20 kW

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