A PSSC roller skate cart is mounted so that it can roll freely on a flat glass plate. It is held in position by two long horizontal coil springs. One end of one of the coil springs is attached to an eccentric crank on a one-twelfth horsepower Boston Gear Co. motor that is controlled by a Ratio-troll. The original control dial of this Ratio-troll is replaced by a vernier dial. The free period of oscillation as well as the damping constant of the oscillation can be measured before the motor is turned on. The damping can be increased above its small minimum value by placing a permanent magnet above some horizontal aluminum plates that are supported on the cart. The mass of the cart can be changed by placing weights on it. This setup enables the student to obtain, very reproducibly, the amplitude of the oscillation of the cart vs. frequency of the driving oscillation. It is possible to make qualitative observations on the relative phase of the cart and the driving crank.