Displacing a tensioned string creates an unbalanced restoring force that drives oscillation.

Evidence

If the bead is displaced vertically, part of the force from each side of the filament points down, as seen in figure 8.2. For small displacements, the magnitudes of the forces from each filament to either side of the bead are the same as before, but now the forces are no longer exactly opposite. … The vertical part of the force does not find a canceling force; this is the restoring force. When the bead is released, the bead accelerates downward according to Newton’s law, frestore = Ma, where frestore is the net force on the bead, M is the mass, and a is the acceleration, in the direction of the net force. — Eric J. Heller, Why You Hear What You Hear- An…, 770

Categorization

• XREF: The restoring-force mechanism mirrors Hooke’s law for springs and shades into the larger theme of how geometry, not increased tension, produces the vibratory force.