Mechanical systems, an engine for example, are not limited by the amount of work they can do, but rather by the rate at which they can perform the work. This quantity, the rate at which work is done, is defined as power.
From this very simple definition, we can come up with a simple equation for the average power of a system. If the system does an amount of work, W, over a period of time, T, then the average power is simply given by:
From this equation, we can derive another equation for instantaneous power that does not rely on calculus. Given a force that acts at an angle θ to the displacement of the particle,
Since = v ,
|P = Fv cosθ|
The unit of power is the joule per second, which is more commonly called a watt. Another unit commonly used to measure power, especially in everyday situations, is the horsepower, which is equivalent to about 746 Watts. The rate at which our automobiles do work is measured in horsepower.
Power, unlike work or energy, is not really a "building block" for further studies in physics. We do not derive other concepts from our understanding of power. It is far more applicable for practical use with machinery that delivers force. That said, power remains an important and useful concept in classical mechanics, and often comes up in physics courses.
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