3. You have to choose between a 225W fridge that costs 1150 dollars and a 425W m

Question

3. You have to choose between a 225W fridge that costs 1150 dollars and a 425W model that costs 850 dollars. The first runs 11% of the time and the second runs 20% of the time. If electricity costs 9.5 cents/kWh, after how many days you break even? 4. Consider an inclined plane as in Fig.4 with two blocks of masses m -2kg (on the plane) and m2 6kg (hanging) connected by string and pulley. The string is long 12m and the angle of inclination is = 30°. The block on the plane is at a height 4m above the ground. The second block is hanged at an initial height of h 3m from the ground. One can use this device to throw the first block up Figure 4 in the air. What is the maximum height reached by the block m1?

Explanation / Answer

Fridge 1 :

power rating 225 W , runs 11% of the time

energy consumed per 1 hr = 225*0.11 = 24.75 Wh

Fridege 2:

power rating 425 W , runs 20 % of the time

energy consumed per 1 hr = 425*0.2 = 85 Wh

difference in energy consumtion = 85 - 24.75 = 60.25 Wh

cost of energy 9.5 c per kWh

$ savings     = 60.25 *9.5 /1000 = 0.5724 $ per hr of running

diff. in initial cost   = 1150 - 850 = 300 $

break even period = 300/0.5724 = 524.13 hrs

The intial additional cost on Fridge 1 is recovered over a period of 524.13 hrs of usage through power savings and there after the power saving are more.

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