See how a block and tackle system can increase your mechanical advantage. You will need two broomsticks, a rope, and two helpers. A. Give each helper a broom stick and have them stand facing each other holding the broomstick firmly in both hands in a horizontal position. The broomsticks should be at the same level, about two feet apart.

B. Tie one end of the rope to one of the broomsticks. (You may need to have one of your helpers hold the knot firmly in one hand as he or she holds the broomstick to help anchor it.)

C. Wrap the rope around the other broomstick, as in a simple pulley system, and pull on the free end of the rope. Your helpers should resist your pulling. What happens? Can you pull the broomsticks together?

D. Now weave the rope back over the first broomstick, so you have a double pulley. Pull on the free end. What happens now? Can you pull the broomsticks together? What is your mechanical advantage?

E. Try weaving the rope over the broomsticks one or two more times. Describe what happens, and state the mechanical advantage of each situation.

F. Write a summary of what happened and an explanation using the terms:

An internally reversible refrigerator has a modified coefficient of performance accounting for realistic heat transfer processes of where qin is the refrigerator cooling rate, qout is the heat rejection rate, and is the power input. show that copm can be expressed in terms of the reservoir temperatures tc and th, the cold and hot thermal resistances rt,c and rt,h, and qin, as where rtot rt,c rt,h. also, show that the power input may be expressed as 1.39 a household refrigerator operates with cold- and hot-temperature reservoirs of tc 5 c and th 25 c, respectively. when new, the cold and hot side resistances are rc,n 0.05 k/w and rh,n 0.04 k/w, respectively. over time, dust accumulates on the refrigerator’s condenser coil, which is located behind the refrigerator, increasing the hot side resistance to rh,d 0.1 k/w. it is desired to have a refrigerator cooling rate of qin 750 w. using the results of problem 1.38, determine the modified coefficient of performance and the required power input w under (a) clean and (b) dusty coil conditions. internally reversible refrigerator qout qin w high-temperature reservoir low-temperature reservoir th th,i tc,i tc high-temperature side resistance low-temperature side resistance w qin th tc qinrtot tc qinrtot copm tc qinrtot th tc