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Two persons lift 4 boxes each through a height of 5 m. This means that they do the same amount of work. However, one person is stronger and powerful and he lifts all of his box in one go. Other person is rather less strong and less powerful, lifts her box two at a time. We use the word powerful to describe someone who can do the work quickly. Power is the rate of doing work or converting energy. 


Power = Work done or energy converted/Time taken

From the equation above you can see that power is measured in 1 J/s. But we give power its own unit called the watt. 1 watt is equal to the arte of working of 1 J/s. Power changes can be very large; Then we use the units of K W (Kilowatt or 1000 W) and MW (MEGAWATT OR 106 w).

Unit of power : 

The power was earlier measured in a unit called HORSE POWER.

            One horse power = 746 watt

One kilowatt of power supplied for one hour amounts to one Kilowatt-hour of energy. The Kilowatt-hour is used as a commercial unit energy.

            One unit = One Kilo-watt hour

                        = 3600000 Joules.

Body Power : 

A girl measured its personnel power output by running up a flight of steps. She takes 8.4 s to run up a flight of steps. Her mass is 14 kg. What power does she develop. She has done some work to lift her weight of 140 N through a height of 6m.

        Work done = force * distance

                       = 140N * 6m = 840J

              Power =  work done/time

                       =  840/8.4s = 100 W.  

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