Power

Power is also part of the energy subject and is just as important.  To a petrol head it is always the first thing asked about a vehicle and horse power is a brilliant pub discussion.  

A measure of the rate at which work is done or the rate of transfer of energy, therefore defined as:

Power = Work Done

              Time Taken

We already know that work done is Force x Distance moved, and from the previous unit that velocity is Distance moved therefore by substituting this into the power 'Time Taken' Equation mechanical power becomes:

Power = Force x Velocity

 P = Fv

Denoted by W (Watt)

1W   =   1J/s   =   1Nm/s

Like the joule, the Watt can also refer to other forms of energy.   Because the joule is the unit not only of work but of energy in general, an alternative definition of the watt is a rate of transfer of energy of 1 joule per second.  Thus the watt is also the unit of electrical power and of rate of heat transfer.

Watt is horsepower?

James Watt was a brilliant Scottish inventor and mechanical engineer renown for his development of steam power.  He coined the term HORSEPOWER after determining the rate at which a horse could pull coal.  It was a term he used to sell engines to manufacturers as an alternative to horses which were then used for any heavy moving.  Horses were expensive and took time from the workers who had to maintain their living and feeding regimes.  Horses were used to grind mill stones, pull coal trucks and plow fields.  It indicates the rate at which an engine can deliver work.


If an engine can push 33,000 Lb of something one foot in one minute, we say that is a one-horsepower engine.

Example

If an engine drives a car against a total resistance of 1.2KN over a distance of 250m in 30s, what power is being developed at the crankshaft?

If the vehicle is to overcome a resistance of 1.2KN  the engine must provide a force of 1.2KN at the crankshaft.


Power developed at the crankshaft:

 Power  = Force x distance moved

             Time taken

                Power  = 1200 x 250                Nm

                         30                      s 

 =  10,000W or 10KW

Now that you have read and absorbed the information on work, energy, power and potential and kinetic energy pages please have a go at this quiz.  It is beneficial to you to do this without looking up the information each time, and if you would like an email with feedback and your results please enter a correct email. 

Energy Work & Power Quiz

Thankyou