  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.
