201_Physics ProblemsTechnical Physics

201_Physics ProblemsTechnical Physics - Chapter 7 P7.41 203...

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Chapter 7 203 P7.41 energy power time For the 28.0 W bulb: Energy used = 28 0 1 00 10 280 4 .. W h kilowatt hrs af ej total cost =+ = $17. $0. $39. 00 280 080 40 kWh kWh bg For the 100 W bulb: Energy used = ×⋅ 100 1 00 10 1 00 10 43 W h kilowatt hrs # bulb used = × = 100 10 13 3 4 . . h 750 h bulb total cost × = 13 3 420 1 00 10 080 60 3 . $0. . $0. $85. kWh kWh Savings with energy-efficient bulb =−= $85. $39. $46. 60 40 20 *P7.42 (a) Burning 1 lb of fat releases energy 1 9 4186 171 10 7 lb 454 g 1 lb kcal 1 g J 1 kcal J F H G I K J F H G I K J F H G I K J The mechanical energy output is 1 71 10 0 20 7 c o s ×= J a f nF r θ . Then 3 42 10 0 6 .c o s ° J nmg y 342 10 50 98 80 0150 3 42 10 5 88 10 6 63 . × J k g m s s t e p s m J 2 n n where the number of times she must climb the steps is n = × × = 588 10 582 6 3 . . J J . This method is impractical compared to limiting food intake.
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