A mass on the end of a spring can oscillate vertically, horizontally or both at the same time.
Experimental observations are presented and simple explanations are given for the observed motion of
the mass, including the well known case where the frequency of oscillation in the vertical direction
is twice that for pendulum motion. In the latter case, measurements show that the horizontal
restoring force on the mass is a nonlinear function of the horizontal displacement, causing the
horizontal amplitude to grow with time. However, the vertical oscillation frequency then decreases
due to a decrease in the vertical restoring force, causing the horizontal amplitude to decrease with
time. The net result is a periodic transfer of energy between vertical and horizontal motion.