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Pulley is a device that can increase the magnitude of the effort force.
With no pulley - the effort force is similar to the load - in opposite direction.
S = F (1)
where
S = effort force (N)
F = load (N)
With a single fixed pulley the effort force is similar (or more due to efficiency) to the load.
S = F (2)
The advantage with the single fixed pulley is that the direction of force is changed - it is possible to pull down instead of lifting up.
With a single moveable pulley the effort force is half (or more due to efficiency) of the load.
S = 1/2 F (3)
With a combined moveable pulley as above - the effort force is half (or more due to efficiency) of the load.
S = 1/2 F (4)
With two pulleys and the ropes as above - the effort force is 1/3 (or more due to efficiency) of the load.
S = 1/3 F (5)