Worksheet 37 - Heat Engines and Refrigerators

Worksheet #37 – Heat
Engines and Refrigerators
Supplemental Instruction
Iowa State University
PHYS 221
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?
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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
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
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.