ELECTRICAL POWER
 

You might decide to spare yourself the tiring labor of cranking a winch to lift heavy objects over and over just to perform experiments to demonstrate the nature of power. Anyhow, it’s hard to measure mechanical power directly, although it can be calculated theoretically. You might connect an electric motor to the winch, as shown in Fig. 1. If you then connect a wattmeter between the power source and the motor, you can measure the power directly. Of course, this assumes that the motor is 100 percent efficient, along with the other assumptions that the rope does not stretch and the pulley has no friction. All these assumptions are, of course, not representative of the real world. A real pulley does have friction, a real rope will stretch, and a real motor is less than perfectly efficient. As a result, the reading on a wattmeter connected as shown in Fig. 1 would be greater than the figure we would get if we used the scheme in Problem 8-8 to calculate the power.

Fig. 1. Electrical power can be measured directly when a motor is used to drive a winch to lift a heavy object.