Pulleys & Mechanical Advantage
Read pulley diagrams correctly, count the rope segments that actually support the moving load, and connect mechanical advantage to force and distance.
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Follow the rope, not just the pulleys
Its axle stays fixed. Ideally it mainly changes the direction of the pull; IMA = 1.
The pulley travels with the load. More than one rope segment can share the load.
In a massless rope over frictionless pulleys, each segment of that continuous rope has the same tension.
Count tension-carrying rope parts that pull upward on the moving block—not every visible rope segment.
IMA equals the number of supporting tension segments in these pulley systems.
If IMA = 4, the ideal effort is one-fourth the load, but the free end moves four times the load distance.
MOVES → SUPPORTS → DIVIDE
MOVES: identify the pulley/block that rises with the load. SUPPORTS: count the rope tensions pulling upward on that moving block. DIVIDE: for an ideal system, divide the load by that count to get the required pull.
Fast check: A fixed pulley used only to redirect the free end does not magically add mechanical advantage. Ask whether it adds another upward tension force on the moving block.
Read the rigging before calculating
The load is supported by one rope tension. For a 100-N load, the ideal pull is 100 N. The useful change is direction.
Two rope segments support the moving pulley. For a 200-N load, 2T = 200, so the pull is 100 N.
Four equal tensions support the moving block. A 400-N load therefore needs an ideal pull of 100 N.
The upper fixed pulley makes the free end convenient to pull downward, but the moving block is still supported by two rope segments. IMA remains 2.
With four supporting segments, raising the load 0.50 m requires pulling 2.0 m of rope in the ideal case.
Common traps
Counting pulleys instead of supporting segments: mechanical advantage comes from supporting tensions. Counting the free end automatically: count it only if it actually pulls upward on the moving block. Giving a fixed pulley IMA 2: a single fixed pulley has IMA 1. Forgetting the distance tradeoff: less force means more rope must move. Using ideal results for real hardware: friction and other losses increase the required effort.
Can you read five pulley forms?
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Quick clarifications
Does every pulley reduce the force?
No. A fixed pulley can simply redirect the force.
Why are the tensions equal?
That is the ideal-rope model used here: a massless rope and frictionless pulleys.
What exactly should I count?
Count the tension forces that directly support the moving block/load. The diagram matters.
What changes in a real pulley?
Friction, pulley mass, rope stiffness, and other losses mean the actual effort is usually greater than the ideal value.
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