PULLEY

 

Pulley systems make it easier to lift a load, because each part of the system supports part of the load’s weight. Examine these three pulley systems. In which system do you use the least force to lift the load? In which system do you move the load over the greatest distance?

One wheel

A one wheel pulley allows the direction of force to be changed. The force required – and work done – remains the same.

If M1 > M2

M1 moves down, causing M2 to rise. This causes downwards acceleration of load M1 and upwards acceleration of load M2

 

 

and

 

 

 

 

 

 

If M2 > M1

 

 

 

 

 

Two wheels

Effort=1/2 the load!

If a pulley system is perfectly efficient the mechanical advantage and the velocity ratio are both equal to the number of pulleys.

 

 

 

Three wheels

Notes: The theoretical mechanical advantage of a system is the ratio  of the force that performs the useful work to the force applied, assuming there is no friction in the system.

Velocity ratio is defined as the ratio of the distance moved by effort and load, in the same time interval.