This applet shows several quantities of interest for a horizontal spring-mass system, often studied in physics. The motion is frictionless, but an artificial velocity-dependent damping is provided. Available are plots of the displacemnt, velocity, acceleration, potential energy, and kinetic energy, all as functions of time. These plots can be enabled with checkboxes, to permit study of their various relationships. Variables controlled by sliders include the mass (m), linear spring constant (k), damping parameter (alph), and initial displacement (L). The period of motion and damping ratio are shown. The time basis is in seconds. The energy graphs are arbitrarily scaled by 0.10 to place them better on the display. Use the L parameter to adjust the graphs to a convenient size. The mathematical basis for the applet is developed in three attached PDF documents; the study of that material is highly recommended, although the math level may be somewhat advanced for some students.