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Work and Power

When a force is applied to an object which results in a change in motion or physical state of the object, work is done. If you detect the motion or physical state of the object changing you know work must be done.

Work causes a change in energy.

Work is the product of a force and the displacement of the object due to that force. The catch is that the force and the displacement must be along the same line. If the force is perpendicular to the motion, no work is done. If the force is at an angle to the motion then only the component of the force along the motion is used.

EX. Calculate the work done in each of the following situations:

You can see that the unit for work is the combination of force and displacement (Nm). This has been redefined as the Joule (J) in honor of Henri Joule.

Power is a measure of the rate at which the energy changes or the work is done. When something is more powerful either it can do the same amount of work in less time, or it can do more work in the same time.

Power is calculated by dividing the energy change by the time, or the work done by the time.

P = DE / T P = W / T

The unit used for power would be a J/s, but this has been redefined as the watt.

1 watt = 1 J / s

A kilowatt (Kw) is often used for large amounts of power. 1 Kw = 1000 w

EX. If you lift a 1.5 Kg book from the table to a shelf 0.5 m high in 0.75 s,
a) what is the amount of work done, and
b) what power is required?

          a) W = Fd       You have to exert a force on the book equal to its weight to lift it to the shelf.
                                                         F = mg = 1.5 Kg(9.8 m/s²) = 14.7 N
               W = 14.7 N (0.5 m)
                   = 7.35 Ns = 7.35 J

b) P = W / T = 7.35 J / 0.75 s = 9.8 J/s = 9.8 w

The power of a device is important. Engines and motors are rated by their power so that they can be properly matched to a specific task. If the device does not have the capability of providing the required power either it won?t be able to do the work, or if it can it won?t be fast enough.

For your information, the term ?horsepower? is the power unit in the english system. It actually was defined by the inventor James Watt as the amount of work a standard draft horse could do in a second of time. One horsepower is the same power as 746 watts.

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