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) =PEKE=
=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