A block and tackle system of pulleys has velocity ratio 4.
What is the value of the mechanical advantage of the given pulley system if it is an ideal pulley system?
Topic: Ideal machine
Answer
Given: block and tackle system with velocity ratio VR = 4.
For an IDEAL machine there is no friction and the moving parts are weightless, so the efficiency is 100 %: η = (MA ÷ VR) × 100 = 100 % MA ÷ VR = 1 MA = VR
MA = 4
Mechanical advantage of the ideal pulley system = 4
"Ideal" is the whole question. It means no energy is wasted, so the efficiency is 100 % and the mechanical advantage equals the velocity ratio exactly.
In a REAL block and tackle the MA is always less than the VR, because some effort goes into overcoming friction at the axles and lifting the weight of the lower pulley block itself. That is why real efficiencies are 80–90 %, not 100 %.
For a block and tackle the velocity ratio equals the number of strands supporting the load — so VR = 4 means four strands.