What Is The Atwood Machine?

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The Atwood Machine is a device that demonstrates the basic principles of acceleration and dynamics. You’ll mostly see Atwood machines in Physics laboratories and classrooms. It consists of two objects with different masses that hang vertically from a frictionless pulley that has a very small, negligible mass.

This machine helps to demonstrate the mechanical laws of motion with constant acceleration and illustrates key principles of classical mechanics.

Atwood Machine Structure

The Atwood machine is a very common device in physics labs. Its structure is quite simple (because it only contains 2-3 components). This machine is typically composed of a string, a pulley and a system of masses. The most basic Atwood machine consists of two objects that are connected by a light, inextensible cord that passes over a pulley (which is frictionless). Below is a simplified diagram of a typical Atwood machine:

An ATWOOD's machine concept

The diagrammatic representation of the Atwood machine.

Atwood Machine in Labs

As mentioned earlier, an Atwood machine is a common sight in Physics labs and classrooms. The image above was a diagrammatic representation of the machine, but now, let’s take a look at the real deal:

Real atwood's machine

(Photo Credit : Iskolaellátó.hu)

The Man Behind the Atwood Machine

The Atwood machine, the pulley-wheel arrangement consisting of two different masses, is named after George Atwood, an English mathematician back in the 18th century. He was a tutor at Trinity College, Cambridge and invented the machine in 1784.

Significance of the Atwood Machine

The Atwood machine proves quite helpful in understanding and demonstrating the laws of accelerated motion and a few other related concepts. Newton’s’ laws of motion, for instance, can be easily demonstrated by the Atwood machine in a lab.

As you can imagine, when the two objects on the machine are of equal masses, then the system will be in equilibrium and no motion of any kind will take place. However, if there is a very small difference in their masses, then their acceleration will be small and, as such, can be easily measured. This is what makes the Atwood machine quite useful to determine acceleration due to gravity (g).

The acceleration of two bodies can be calculated using the following formula:

With a modified Atwood experiment, you can also demonstrate the third law of motion. Let’s imagine that you have a suspended mass attached to a string that goes over a pulley to a cart on a flat track, as pictured below.

A modified Atwood experiment.

This setup clearly demonstrates the third law of motion, i.e., for every action, there is an equal and opposite reaction. For instance, consider the weight of the cart and the suspended mass (object), which is the result of Earth’s gravitational pull on both the cart and the mass.The other factor in this system is the Earth, which is being pulled upward towards the cart and the mass.

Similarly, you can also demonstrate the first and second laws of motion using the Atwood machine.


  1. Kenyon College
  2. La Salle University
  3. Stony Brook University, New York
  4. Texas State University
  5. Boston University Physics
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About the Author:

Ashish is a Science graduate (Bachelor of Science) from Punjabi University (India). He spends a lot of time watching movies, and an awful lot more time discussing them. He likes Harry Potter and the Avengers, and obsesses over how thoroughly Science dictates every aspect of life… in this universe, at least.

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