Physics, visually · Work, Energy & Power · Concept 06
Levers and Pulleys
How can a child lift a heavy stone with a long stick, or one person lift a car engine with a chain hoist?
Simple machines do not give you free energy. They let you pick: a small force over a long distance, or a big force over a short one.
- Lever tab. Put the load 1 m from the pivot and the effort 3 m away. How big a push balances 60 kg? Then slide the effort closer.
- Pulley tab. Raise the number of supporting ropes from 1 to 4. The pull drops to a quarter, but how much rope must you pull?
- Ask yourself: in both tabs, is the work (force × distance) you put in smaller than the work you get out?
Force × distance stays the same. Trade one for the other.
A lever balances when load × its distance = effort × its distance (moments). A pulley with n supporting ropes divides the force by n but makes you pull n times more rope. Nothing is free.
Crane jibs and balance arms
A counterweight on the short side of a crane balances the load on the long side: the same moment rule as the lever.
Chain hoists and block-and-tackle
A hoist with four load-bearing chains lets one person lift four times what they could pull directly, slowly.
Gripper fingers and toggle clamps
Robot grippers use lever arms to multiply the cylinder force at the finger tip, or to speed up its motion.