Homework Help: Physics

Posted by lenny on Sunday, June 17, 2012 at 3:34am.

1. A 2.5kg rock is dropped from a 350m cliff. It hits the ground with a speed of 60 m/s. What percent of its mechanical energy was lost to air resistance?
-the answer is 48%

2. A motor uses 5500J to lift a 35kg object a vertical distance of 12m. What is the efficiency of the motor?
-the answer 0.75

I need help, Thanks!

Physics - Elena, Sunday, June 17, 2012 at 4:25am
1. PE = W(fr)+KE
W(fr) =PE-KE=
=mgh - mv²/2 =
=2.5(9.8•350 - 60²•/2)=
= 4075 J.
PE =mgh = 2.5•9.8•350=8575 J.
W(fr)/PE =4075/8575 =0.48.

2. PE =mgh = 35•9.8•12 =4116 J.
k = PE/W =4116=5500 =0.75

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