impetus

Impetus is a noun that denotes a driving force or stimulus that initiates or sustains an action, process, or change. It can refer to a tangible momentum in physical terms or to a more abstract motivation that prompts people or institutions to act. In everyday language, one might speak of an idea or event giving impetus to reform or progress.

In philosophy and medieval physics, impetus described an internal quantity imparted to a moving body, which would persist after the initial cause ceased. Thinkers such as John Buridan and Nicole Oresme developed theories in which motion was sustained by this internal impulse, proportional to certain properties of the body and its prior motion. The concept aimed to explain why objects continued moving without continuous external contact. Impetus, as a theory of motion, preceded the modern understanding of inertia and momentum.

In modern physics, impetus is largely a historical term. The standard concepts are momentum, the product of mass and velocity, and impulse, the change in momentum due to a force. These quantities form the basis of Newtonian mechanics and subsequent physical theories. The word remains in historical discussions of the development of mechanics and in nontechnical, figurative uses to describe a catalyst for change.

Etymology and usage: impetus comes from the Latin impetūs, from impellere, meaning to drive on. The term has retained a broad sense related to motivation and momentum, though its technical meaning in physics has largely been replaced by more precise terminology.