Power

Mathematically

 
P = W/ T

P

Power [W]

W

Work Done[Nm]

T

Time Taken [S^-1]

Units

 

Power is measured in the SI system in Joules per Second [Js^-1] or Watts.
In the imperial system power is measured in pounds foot per second [lb ft s^-1].

Horsepower

We often discuss engine power in terms of Horsepower, but what does it mean ?
A 19th century engineer called James Watt carried out an experiment to calculate how powerful a horse is. He attached a 220lb weight to a system of pulleys and ropes down a shaft and measured how far the horse could raise the weight in one minute.
The weight moved 150ft.
So in 1 minute a horse does 150 * 220 = 33000 lb ft of work, this is equivalent to 550lbs one foot in one second, which dimensionally is lb ft s^-1 (and fits the units of power described above).

Moving a 550lb mass 60 feet in one minute or a 33000lb mass 1 foot in one minute would both take 1hp.

If the horse Watt used had been down to the gym and was fitter, we would have a different definition of Horsepower today !