  Friction is a complex phenomenon that is of paramount importance in everyday
life. We present an easy-to-build and inexpensive experiment illustrating
Coulomb's law of kinetic friction. The so-called friction, or Timoshenko,
oscillator consists of a plate set into periodic motion through the competition
between gravity and friction on its rotating supports. The period of such an
oscillator gives a measurement of the coefficient of kinetic friction \mu_k
between the plate and the supports. Our prototype is mainly composed of a
motor, LEGO blocks, and a low-cost microcontroller, but despite its simplicity
the results obtained are in good agreement with values of \mu_k found in the
literature.
