Hi everyone, i'm new here and hope someone can enlighten me as to how to solve the following problem.

An electric crane uses 8.0A at 150V to raise a concrete block of 350kg at 12m/min. Determine the efficiency of the system.

power = current x voltage
so P = 1200J
this should be the input right?

as for the output, i can only work out the kinetic energy and not able to find the power output.

so wondering how do i go about solving this question? thanks in advance!

Hi shrooms,

Power comes in Watts .. that's Joules/second. NOT Joules ..

On the electrical side

Power = 8 x 150 = 1200 Watts.

Power is rate of doing work

Work is force times distance moved.

The downward force on 350Kg is 350 x 'g' Newtons where 'g' is the acceleration due to gravity at the Earth's surface. (I assume you are an Earth resident). Local Earth 'g' is about 9.8 metre/second/second

That 12m/min needs to be turned into distance per second

speed is 12/60 m/sec

So rate of doing work (work done per second) = 350 x 9.8 x 12 / 60 Watts

= 686 Watts.

Efficiency = Pout/Pin = 686/1200 = 0.57

or 57% if you like the look of that better.

Could be completely wrong of course .. hopefully some clues in there though. Check!

Best wishes,

-C2.

Confused2, yes, units for power is watts, typo error and you are absolutely right!! thanks so much for your help!! merry christmas!!