Calculate the efficiency of the electric motor.

[1irishman]

Homework Statement

An 8.5 X 10^3 kg elevator is pulled up at a constant velocity of 1m/s by a 10kW electric motor.
Calculate the efficiency of the electric motor.

Homework Equations

P=w/t
%efficiency=work out/work in X 100%
W=mgh
d=vit+1/2at^2
v=d/t

The Attempt at a Solution

The answer in the book is 83%, but I don't understand how they got that answer. Please help.

In the meantime, this is how far I got (not too far):

vi=1m/s
m=8.5X10^3kg
vf=0

 

[xcvxcvvc]

The work done can be expressed in gravitational terms: mass * g * height. The work in can be calculated by power * time. Height can be found with velocity * time.

mass * g * velocity * time / (power * time) * 100 = mass * g * velocity / power * 100
8.5X10^3kg * 9.81 * 1 m/s / ( 10000 W) * 100 = 833%. It's off by a multiple of ten, so you gave the wrong initial values, the book published a faulty question, or (i sure hope this isn't it) I'm partly wrong. I hope this helps.

 

[1irishman]

Is the time 1 second? Is the height 1 metre? Yes, I got the mass wrong, it was suppossed to be to the power of 2 not to the power of 3.
How do I know whether to use power out/power in vs. work out/work in? Thanks!