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The Atwood machine consists essentially of two masses suspended from a string over a pulley and is used to show the state of constant acceleration experienced by both masses when the masses do not equal each other. 

The basic construction of an Atwood machine is simple. A vertical stand with a pulley mounted on an arm allows the two masses to be suspended from the pulley by a single string. The pulley can be mounted in any fashion, as long as the strings hang vertically, but in the original version and in most other machines of this type, the pulley is mounted so that its axle rests on and is surrounded by as many as four other wheels, in an attempt to reduce friction as much as possible. Any version of Atwood's machine will also have the means to measure the distance traveled by each mass during the demonstration or experiment.

By applying Newton's second law of motion, it can be calculated that when the two masses suspended by a string over a pulley are unequal, acceleration will result as the larger mass falls down and the smaller mass is pulled up and that this acceleration will be constant for both masses.