k-x.
A modified type of this phenomenon stops as quickly as possible when its characteristic equation only involves one double root, a "critical" situation. In another situation, this phenomenon can be easily modelled by setting "I alpha" equal to "negative m-g-l-theta" using the small-angle approximation. Modelling it involves solving a second-order differential equation with no first-derivative termlike (*) "m d-squared-x d-t-squared equals negative k-x." It occurs when the displacement is proportional to the restoring force, as in Hooke's law. For 10 points, name this oscillation demonstrated by springs and pendula.
A modified type of this phenomenon stops as quickly as possible when its characteristic equation only involves one double root, a "critical" situation.
simple harmonic motion [accept simple harmonic oscillation; accept harmonic oscillator; prompt on "oscillation"]