What named machine uses two masses over a pulley to study Newtonian acceleration?

The Atwood machine uses two masses connected by a string over a pulley to study Newtonian acceleration. It provides a controlled laboratory model for relating weight differences, tension, mass, and acceleration.

English mathematician George Atwood invented the apparatus in 1784. In the idealized version, the pulley and string are massless, the string is inextensible, and friction is ignored. If the masses are unequal, the heavier mass moves downward while the lighter mass rises with the same magnitude of acceleration.

For masses m₁ and m₂, the ideal acceleration is a = g(m₁ − m₂)/(m₁ + m₂), assuming m₁ is the heavier mass. This makes the acceleration smaller than free fall when the masses are close in value, giving students more time to measure the motion. Equal masses produce equilibrium in the ideal model.

Real Atwood machines include pulley inertia, axle friction, air resistance, and a string with some mass. Those effects make the two tensions unequal and slightly alter the ideal formula, but the apparatus remains a classic demonstration of Newton’s second law and constant-acceleration motion.