Answer in Physics for Hassan #85821
If the restoring force is proportional to vertical displacements from the equilibrium position, then the motion would be purely damped harmonic motion. The shape is most important in order to do that. If the leaf were shaped like a simple cylinder with its axis vertical, then the above statement is true for leaf and then performs a damped harmonic motion.
Since the leaf is going up and down and generating a wave having equal wavelength in each interval passing by, so we can say leaf is executing simple harmonic vertical motion