Problem question on power calculation

[wewelsburg]

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

 

[xiankai]

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 doesn't match...

 

[wewelsburg]

yep those are the answers the book gives. Why would I divide by 80 seconds and not 60?

 

[mbrmbrg]

yep those are the answers the book gives. Why would I divide by 80 seconds and not 60?

Because the work is done "in 1 minute and 20 seconds" = 60sec+20sec = 80sec.