In physics, power is the rate at which work is done. Mathematically, it is the work done on an object divided by the time being considered. While work only considers the force that acts to cause a displacement, power considers the time it takes for the force to cause the displacement. Power can also be described as force multiplied by velocity, since velocity is displacement divided by time.
For the same amount of work, power and time are inversely proportional: with more power the same amount of work can be done in less time. Machines are given power ratings for this reason, as although machines are designed and built to do work, the time frame in which they do the work is also important.
For example, a car's horsepower rating is a measurement of power. If a car's 40-horsepower engine accelerates it from 0 to 60 mph in 16 seconds, then the same car with four times the power - a 160-horsepower engine - can accelerate it to 60 mph in 4 seconds, which is one-fourth the time.
In the metric system, power is expressed in Watts and work is expressed in Joules, with one Watt equating to one Joule-per-second. A single unit of horsepower is equivalent to 750 Watts.