The adjacent Figure shows a fixed pulley used by a boy to lift a load of 400 N through a vertical height of 5 m in 10 s. the effort applied by the boy on the other end of the rope is 480 N.

(a) What is the velocity ratio of the pulley ?
(b) What is the mechanical advantage ?
(c) Calculate the efficiency of the pully.
(d) Why is the efficiency of the pulley not 100% ?
(e) What is the energy gained by the load in 10 s?
(f) How much power was developed by the boy in raising the load ?
(g) The boy has to apply an effort which is greater than the load he is lifting. what is the justification for using the pulley ?

Explanation

(a) When the effort moves a distance d downwards, the load moves a distance d upwards.
Velocity ratio=`("displacement of effort")/("displacement of load")=(d)/(d)=1`
(b) Mechanical advantage=`("load")/("effort")`
or `M.A.=(400)/(480)=(5)/(6)=0*833`
(c) Efficiency `eta=(M.A.)/(V.R.)=(0*833)/(1)=0*833` or 83.3%
(d) The efficiency is less than 100% because some energy is wasted in overcoming the friction of the pulley bearings.
(e) Energy gained by the load
=load`xx`displacement of load
`=400xx5=2000J`
(f) Power developed by the boy
`=("effort"xx"displacement")/("time")`
`=(480xx5)/(10)=240W`
(g) Use of pulley helps in changing the direction of the applied force to a convenient direction (instead of upward direction to downward direction). so one may also use his/her own weight as effort.