The diagram shows a fixed pulley used by a boy to lift a load of 400 N through a vertical height of 5 m is 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 pulley.
(d) Why is the efficiency of the pulley not 100%?
(e) 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 load")/("displacement of effort")`
`=d/d =1`
(b) Mechanical advantage (M.A.) = `("Load")/("Effort")`
`=(400)/(480)=5/6`
= 0.833
(c) Efficiency = `("Mechanical advantage")/("Velocity ratio") =(0.833)/(1)`
= 0.833 or `0.833 xx 100%= 83.3%`.
(d) The efficiency is less than 100% because some energy is wasted in overcoming the friction of the pulley bearings.
(e) Use of pulley helps in changing the direction of the applied force to a convenient direction.