what is power defined as in physics

What is power?

 In physics power is defined as rate with which work is done. Power can be calculate in watts or in horse power if you are dealing with electrical power. power have difference equation use in calculation of it , depend on the parameters available just like equations of motion  and what you are calculating its power.  There are difference types of power, like electrical power, generator power, motor engine power and so on.

 Power =  \frac {work done or energy spent}{time taken}

That is  \frac { w }{ t } =f\times \frac { s }{ t } =f\times v

Therefore power =f\times v where v = velocity, and f = force.

The SI unit of power is called the watt (w) which is the rate of transfer of energy of one Joule per second. That is      1W = 1JS-1 , the large units are the Kilowatt (kw) and megawatt (mw)

1kw = 103w, 1mw = 106w

Sometimes, we use horse power (h.p)  unit, to measure power.

Note that   1h.p  =  746watt = 0.75kw

The units of energy that is sometime used for electrical energy is called the kilowatt-hour (kwh). This is the energy used by an appliance with a power of one kilowatt in one hour.

1kwh  =   1000w  x 3600S = 3.6 x 106J = 3-6MJ


  • Calculate the power of pump which lifts 500kg of water through a vertical height of 4 meters in 5 seconds.  (assuming g =10MS-2)
  • A car travelling at a constant speed of 20ms-1 overcomes a resistance of 300n. What is the horse power of the engine? (take  p =3/4kw )

Answers to questions above

  • , solution to number one question,

Recall that power = p=\frac { work\quad done }{ time\quad taken }

that is force x distance/time

p=f\times \frac { s }{ t }

but F=500N, S = 4, and time = 5s

So work done = 500 x 4 = 2000J, therefore

p=\frac { 2000 }{ 5 } =400w

  • To answer number 2 question, let use

Engine power =

\frac { work\quad done }{ time\quad taken } =f\times v

Where f = force and v = velocity, therefore

Engine power = 300 x 20 = 6000watts = 6kw


= 1h.p

6kw    =  8h.p.





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