What’s the Atwood machine?

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The Atwood machine was built to demonstrate and verify certain laws of motion under laboratory conditions. The machine is named after the man who invented and built it, the Reverend George Atwood. Atwood’s machine essentially consists of two masses suspended by a string on a pulley and is used to show the state of constant acceleration experienced by both masses when the masses are not equal to each other. Today, any machine similarly constructed for this purpose is still called an Atwood Machine, and machines of this type are still commonly used in teaching to demonstrate certain laws of physics.

By applying Newton’s second law of motion, it can be calculated that when two masses suspended by a string on a pulley are unequal, the acceleration will result when the greater mass falls and the lesser mass is lifted, and that this acceleration will be constant for both the masses. This is what would happen under ideal conditions and it doesn’t take into account friction or the tendency of any string or thread to stretch. Both of these factors can be calculated and factored into any measured observation of any demonstration of this law using an Atwood machine.

The basic construction of an Atwood machine is simple. A vertical stand with a pulley mounted on one arm allows the two masses to be suspended from the pulley by a single rope. The pulley can be mounted in any way, as long as the strings hang vertically, but in the original version and 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 each mass travels during the demonstration or experiment.

The machine provides students and scientists with a way to confirm by demonstration and to better understand Newton’s second law of motion and other principles of physics and mechanics. The two masses to be hung from the ends of the string, for example, will not move by themselves as long as they are equal. Changing one of the masses so that they are not equal will result in an acceleration up and the other down, both with an equal and constant velocity that does not change regardless of the size of either mass. A very large mass and a very small mass will both accelerate at the same constant rate as two masses that differ only slightly, and this machine allows you to demonstrate this fact under laboratory conditions.

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