Editing Reaction (physics) (section)

===Mass on a spring===
If a mass is hanging from a spring, the same considerations apply as before. However, if this system is then perturbed (e.g., the mass is given a slight kick upwards or downwards, say), the mass starts to oscillate up and down. Because of these accelerations (and subsequent decelerations), we conclude from Newton's second law that a net force is responsible for the observed change in velocity. The gravitational force pulling down on the mass is no longer equal to the upward elastic force of the spring. In the terminology of the previous section, F<sub>1</sub> and F<sub>3</sub> are no longer equal.

However, it is still true that F<sub>1</sub> = F<sub>2</sub> and F<sub>3</sub> = F<sub>4</sub>, as this is required by Newton's third law.