The Atwood machine revisited using smartphones

The Atwood machine is a simple device used for centuries to demonstrate the Newton's second law. It consists of two supports containing different masses joined by a string. Here, we propose an experiment in which a smartphone is fixed to one support. With the aid of the built-in accelerometer of the smartphone the vertical acceleration is registered. By redistributing the masses of the supports, a linear relationship between the mass difference and the vertical acceleration is obtained. In this experiment, the use of a smartphone contributes to enhance a classical demonstration.