shrooms
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!
Confused2
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.
shrooms
Confused2, yes, units for power is watts, typo error and you are absolutely right!! thanks so much for your help!! merry christmas!!
PhysOrg scientific forums are totally dedicated to science, physics, and technology. Besides topical forums such as nanotechnology, quantum physics, silicon and III-V technology, applied physics, materials, space and others, you can also join our news and publications discussions. We also provide an off-topic forum category. If you need specific help on a scientific problem or have a question related to physics or technology, visit the PhysOrg Forums. Here you’ll find experts from various fields online every day.