7.1 Work
7.2 Energy
7.3 Forms of Energy
7.4 Power
7.5 Key Formulas
7.6 Practice Questions
7.7 Explanations
Power is an important physical quantity that frequently, though not always, appears on SAT II Physics. Mechanical systems, such as engines, are not limited by the amount of work they can do, but rather by the rate at which they can perform the work. Power, P, is defined as the rate at which work is done, or the rate at which energy is transformed. The formula for average power is:
Power is measured in units of watts (W), where 1 W = 1 J/s.
A piano mover pushes on a piano with a force of 100 N, moving it 9 m in 12 s. With how much power does the piano mover push?
Power is a measure of the amount of work done in a given time period. First we need to calculate how much work the piano mover does, and then we divide that quantity by the amount of time the work takes.
Be careful not to confuse the symbol for watts, W, with the symbol for work, W.
Instantaneous Power
Sometimes we may want to know the instantaneous power of an engine or person, the amount of power output by that person at any given instant. In such cases, there is no value for to draw upon. However, when a steady force is applied to an object, the change in the amount of work done on the object is the product of the force and the change in that object’s displacement. Bearing this in mind, we can express power in terms of force and velocity:
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