Oscillating Ruler
When displaced from its vertical equilibrium position, this plastic ruler oscillates back and forth because of the restoring force opposing displacement. When the ruler is on the left, there is a force to the right, and vice versa.
Source
Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:
"OpenStax College, College Physics. October 6, 2012."
http://cnx.org/content/m42240/latest/?collection=col11406/1.7
OpenStax CNX
CC BY 3.0.