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 ?

The adjacent Figure shows a fixed pulley used by a boy to lift a load

1.	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 ?
Answer» Solution :(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)=0833` (c) Efficiency `eta=(M.A.)/(V.R.)=(0833)/(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.