Condition for balance
In On the Equilibrium of Planes, Archimedes proved that weights balance at distances inversely proportional to their magnitudes: F₁d₁ = F₂d₂. He is credited with "Give me a place to stand and I will move the earth." Mechanical advantage is d₁/d₂; what is gained in force is lost in distance, so work is unchanged.
Classes of lever
- First class: fulcrum in the middle (seesaw, scissors, claw hammer); the head nodding on the atlas.
- Second class: load in the middle (bottle opener, wheelbarrow); rising onto the toes. Always gains force.
- Third class: effort in the middle (tweezers, chopsticks); the biceps flexing the elbow. Needs more force but gains speed and range.
Biomechanics
Most of the musculoskeletal system works as third-class levers, trading force for fast, wide movement. Holding a load close to the body shortens the load arm and reduces strain on the lower back.
References
- Heath, T. L. (Ed.). (1897). The Works of Archimedes. Cambridge University Press.
- Halliday, D., Resnick, R., & Walker, J. (2018). Fundamentals of Physics (11th ed.). Wiley.
- Hall, S. J. (2019). Basic Biomechanics (8th ed.). McGraw-Hill.