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Worksheet #37 – Heat Engines and Refrigerators Supplemental Instruction Iowa State University Leader: Course: Instructor: Date: Wesley PHYS 221 Whisnant 04/29/2014 Review Questions 1. (51.2%) A motor has a power output of 45 kW when it rotates at an angular speed of 80 rpm. A drum of radius 1.2 m and negligible mass is attached to the motor shaft. A rope is wrapped around the drum and attached to a hanging weight. What is the weight, in N, if it is being lifted at a constant speed? 2. A rod with mass 1.00 kg and length 1.00 m is allowed to rotate about an axis located at its center of mass (50.0 cm mark) on a horizontal, frictionless table (see sketch). A small point mass(m = 200 g), moving with an initial velocity of 2.00 m/s in the direction indicated in the figure, hits one end of the rod and sticks to it. What is the angular velocity of the rod/mass system immediately after the collision? 1060 Hixson-Lied Student Success Center 515-294-6624 sistaff@iastate.edu http://www.si.iastate.edu Do NOT plug in numbers until you have solved for the desired variable! Try to solve each problem algebraically first. Today’s Topics 3. (Heat Engine) A gasoline engine takes in 1.61*104 J of heat and delivers 3700 J of work per cycle. The heat is obtained by burning gasoline with a heat combustion of 4.60*104 J/g. a. What is the thermal efficiency of this cycle? b. How much heat is discarded in each cycle? c. What mass of fuel is burned in each cycle? d. If the engine goes through 60.0 cycles per second, what is the power output in kilowatts? 4. (Refrigeration) A refrigerator has a coefficient of performance of 2.25, runs on an input of 95 W of electrical power, and keeps its inside compartment at 5 ºC. If you put a dozen 1.0-L plastic bottles of water at 31 ºC into this refrigerator, how long will it take for them to be cooled down to 5 ºC? You may ignore any effects of the plastic from the bottles.