Summary
A simple machine changes the size or direction of a force: the lever, the inclined plane (and its variants, the wedge and the screw), the wheel and axle and the pulley. None of them saves work: what is gained in force is paid for in distance. A lever obeys F · dF = R · dR, and depending on what sits in the middle it is first class (the fulcrum, like a seesaw), second class (the load, like a wheelbarrow) or third class (the effort, like tongs).
Mechanisms transmit or transform motion. Gears and belt-driven pulleys pass it between shafts and change the speed: a small wheel driving a big one is a reduction system, slower and stronger. The crank and connecting rod, the rack and pinion and the cam turn rotation into straight-line or back-and-forth motion.
Key points
- The simple machines are the lever, inclined plane, wedge, screw, wheel and pulley.
- A machine does not save work: less force, more distance.
- Law of the lever: F · dF = R · dR.
- The class of lever depends on what sits in the middle.
- A fixed pulley changes direction; a movable one halves the force.
- A small wheel driving a big one: reduction (slower, stronger).
- Gears: n1 · z1 = n2 · z2; belt pulleys: n1 · d1 = n2 · d2.
- Crank and rod, rack and pinion and cams turn rotation into straight-line motion.