- rigid bar pivoting on a fulcrum, producing mechanical advantage
- one of six simple machines, alongside wheel, wedge, screw, pulley, inclined plane
- three classes defined by relative positions of effort, load, and fulcrum
- class 1: fulcrum between effort and load (seesaw, crowbar)
- class 2: load between fulcrum and effort (wheelbarrow, nutcracker)
- class 3: effort between fulcrum and load (tweezers, human forearm)
- Archimedes: “give me a place to stand and I will move the Earth”
- principle extends to hydraulic systems, pump mechanisms, and digital logic gates
- energy input equals energy output minus friction losses