07_InstSolManual_PDF_Part15 - Work and Energy 7-15 7.68 Set...

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7.68. Set Up: The watt represents power as joules per second; so convert the unit of year to seconds for part (a). For part (b), the rate of consumption is simply the power divided by the estimated number of consumers. Solve: (a) (b) 7.69. Set Up: From Problem 7.68, the U.S. consumes of power per year. The area required in part (a) may then be calculated as However, for an array efficiency of 25%, the radiative power rate must be multiplied by 0.25 to account for power losses. For part (b), since the array is square, and the length of a side is simply Solve: (a) (b) Reflect: An array of this size is approximately equivalent to the state of Rhode Island and thus feasible. 7.70. Set Up: The thermal energy is produced as a result of the force of friction, The average thermal power is thus the average rate of work done by friction or where is based on the initial and final velocities. Solve: Reflect: The power could also be determined as the rate of change of kinetic energy, where the time is calcu- lated from and
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