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Unformatted text preview: homework 12 FIERRO, JEFFREY Due: Mar 1 2008, 11:00 pm 1 Question 1, chap 8, sect 5. part 1 of 1 10 points An outboard motor propels a boat through the water at 13 mi / h. The water resists the forward motion of the boat with a force of 23 lb. How much power is produced by the motor? Correct answer: 0 . 797334 hp (tolerance 1 %). Explanation: First, let us change the velocity to ft/s. = (13 mi / h) parenleftbigg 5280 ft 1 s parenrightbigg parenleftbigg 1 hr 3600 s parenrightbigg = 19 . 0667 ft / s . Since the boat moves at a constant velocity, the power output of its motor is P = F v u = (23 lb) (19 . 0667 ft / s) = (438 . 534 ft lb / s) parenleftbigg 1 hp 550 ft lb / s parenrightbigg = 0 . 797334 hp . Question 2, chap 8, sect 5. part 1 of 2 10 points A 1990 kg car starts from rest and acceler ates uniformly to 18 m / s in 16 . 2 s . Assume that air resistance remains con stant at 307 N during this time. Find the average power developed by the engine. Correct answer: 30 . 3794 hp (tolerance 1 %). Explanation: m = 1990 kg , v i = 0 m / s , v f = 18 m / s , and t = 16 . 2 s . The acceleration of the car is a = v f v i t = v f t since v i = 0, so a = 18 m / s 16 . 2 s = 1 . 11111 m / s 2 . Thus the constant forward force due to the engine is found from summationdisplay F = F engine F air = ma F engine = F air + ma = 307 N + (1990 kg) ( 1 . 11111 m / s 2 ) = 2518 . 11 N . The average velocity of the car during this interval is v av = v f + v i 2 , so the average power output is P = F engine v av = F engine parenleftBig v f 2 parenrightBig = (2518 . 11 N) parenleftbigg 18 m / s 2 parenrightbiggparenleftbigg 1 hp 764 W parenrightbigg = 30 . 3794 hp . Question 3, chap 8, sect 5. part 2 of 2 10 points Find the instantaneous power output of the engine at t = 16 . 2 s just before the car stops accelerating. Correct answer: 60 . 7587 hp (tolerance 1 %). Explanation: The instantaneous velocity is 18 m / s and the instantaneous power output of the engine is P = F engine v f = (307 N)(18 m / s) parenleftbigg 1 hp 764 W parenrightbigg = 60 . 7587 hp . homework 12 FIERRO, JEFFREY Due: Mar 1 2008, 11:00 pm 2 Question 4, chap 8, sect 2....
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This homework help was uploaded on 04/03/2008 for the course PHY 303K taught by Professor Turner during the Spring '08 term at University of Texas at Austin.
 Spring '08
 Turner
 Force, Work

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