Question

A 60N motor winds a cable around a drum. (I) If the cable lifts a 3-ton load of bricks to a height of 12cm in 5 sec, calculate the efficiency of the motor. (II) At what rate is work done against friction?

Answer 1

Given:

$F=60\:\rm kN$

$m=3000\:\rm kg$

$h=0.12\:\rm m$

$t=5\:\rm s$

(a)

$$\eta=\frac{mgh}{Fh}=\frac{mg}{F}=\frac{3000*9.8}{60000}=0.49=49\%$$

(b)

$$P=\frac{mgh}{t}=\frac{3000*9.8*0.12}{5}=706\rm\: W$$