Atwood Machine Calculator

About this calculator

An Atwood machine is the classic physics setup of two masses, m1 and m2, connected by an inextensible string over a frictionless, massless pulley. Applying Newton's second law to both masses gives a constant acceleration of a = (m1 − m2)g / (m1 + m2), directed so the heavier mass falls.

The string tension follows from the same equations of motion: T = 2·m1·m2·g / (m1 + m2), a value less than the weight of either mass individually, which is exactly why the system accelerates instead of staying in static equilibrium.

Since Newton's time, the Atwood machine has been a standard teaching tool for demonstrating Newton's second law with two coupled bodies, because the low acceleration (compared to free fall) makes it easy to measure directly in an introductory physics lab.