Restoring force, momentum, and equilibrium
(a) The plastic ruler has been released, and the restoring force is returning the ruler to its equilibrium position. (b) The net force is zero at the equilibrium position, but the ruler has momentum and continues to move to the right. (c) The restoring force is in the opposite direction. It stops the ruler and moves it back toward equilibrium again. (d) Now the ruler has momentum to the left. (e) In the absence of damping (caused by frictional forces), the ruler reaches its original position. From there, the motion will repeat itself.
Source
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"OpenStax College, College Physics. October 6, 2012."
http://cnx.org/content/m42240/latest/?collection=col11406/1.7
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