Question

Give reasons for the following :

(a) In a single fixed pulley, the velocity ratio is always more than the mechanical advantage.

(b) The efficiency of a movable pulley is always less than $100\%$.

(c) In case of a block and tackle system, the mechanical advantage increases with the increase in the number of pulleys.

(d) The lower block of a block and tackle pulley system must be of negligible weight.

Answer 1

a)
In a single fixed Pulley, the velocity ratio is always more than a mechanical advantage because of single fixed Pulley cannot be considered as a hundred per cent efficient machine so some amount of input energy may be lost as heat and in overcoming the friction
b)
The single movable is not 100% because:-

Some amount energy is lost in moving parts.

Some amount of energy is lost as a result of friction between the moving parts
c)

Mechanical advantage in block and tackle system increases with increase in pulleys as the number of string increases and the amount of tension applied to them in decreases

d)

The lower block of this pulley system must have a negligible weight.

Actual Mechanical advantage (M. A.) is given by

M. A. = n –

Where W' = weight of the lower block

Smaller is the value of W', more will be the mechanical advantage.