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Pulleys
Pulley basics, blocks and tackles
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Pulley is a device that can increase the magnitude of the effort force.
Without Pulley
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)
Single Pulley
Fixed Pulley
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.
Movable Pulley
With a single moveable pulley the effort force is half (or more due to efficiency) of the load.
S = 1/2 F (3)
Combined Pulleys
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)
General Equation for Blocks and Tackles
The general effort force equation for block and tackle to raise or pull a load can be expressed as
S = F / μ n
= m g / μ n (6)
where
S = effort force (N)
F = load, often weight (N)
m = mass (kg) (when lifting a mass)
g = constant of gravitation (9.81 m/s2) (when lifting a mass)
μ = mechanical efficiency of the system (equal to one for an ideal frictionless system, a fraction less than one for real-world systems with energy losses due to friction)
n = number of ropes between the sets of pulleys
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