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Homework Help: Problem question on power calculation

  1. Mar 26, 2007 #1
    Here's the question and my answer doesn't match what the book gives as you can see in the below workings out

    1)A 300 KG weight is lifted with a block and tackle to a height of 4m in 1 minute and 20 seconds A) Neglecting friction, calculate the power used? B)What’s the efficiency if the actual power used is 24KW?

    My working a) Power=work/time To find work firstly force must be found
    Force=300 KG X 9.81m/s= 2,943 N
    Work=Force X Distance= 2,943 N X 4M=11,772 Joules
    Power=11,772/60 seconds=196.2 Watts

    B)Efficiency=Power out/Power in X 100
    196.2/24000 X 100

    Book answers= A)=15W B)=62 percent
  2. jcsd
  3. Mar 26, 2007 #2
    are you sure your data is correct? your working is correct except for the part where you should have put 80 instead of 60 seconds. plus the part about 15 W and 24 KW doesnt match...
  4. Mar 26, 2007 #3
    yep those are the answers the book gives. Why would I divide by 80 seconds and not 60?
  5. Mar 26, 2007 #4
    Because the work is done "in 1 minute and 20 seconds" = 60sec+20sec = 80sec.
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