The Atwood machine (or Atwood's machine) was invented in 1784 by Rev. George Atwood as a laboratory experiment to verify the mechanical laws of uniformly accelerated motion.

It consists of two masses, m₁ and m₂, connected by an inelastic massless string over an ideal massless pulley. When m₁ = m₂, the machine is in stable equilibrium regardless of the position of the weights. Unequal masses will accelerate with . Inversely, gravitational acceleration g can be derived from timing the movement of the weights. Atwood machines are a common classroom demonstration used to illustrate principles of physics.

Atwood's original illustrations show the main pulley's axle resting on the rims of another four wheels, to minimize friction forces from the bearings. Many historical implementations of the machine follow this design.

An elevator with a counterbalance approximates an ideal Atwood machine and thereby relieves the driving motor from the load of holding the elevator car — it has to overcome only weight difference and inertia of the two masses. The same principle is used for funicular railways with two connected railway cars on inclined tracks.