impetus vs incentive

impetus

noun
  • Anything that impels; a stimulating factor. 

  • The force or energy associated with a moving body; a stimulus. 

  • An activity in response to a stimulus. 

  • A force, either internal or external, that impels; an impulse. 

  • A principle of motive force, held as exquivalent to weight times velocity by John Buridan, in an auxiliary theory of Aristotelian dynamics introduced by John Philoponus, describing projectile motion against gravity as linear until it transitions to a vertical drop and the intellectual precursor to the concepts of inertia, momentum and acceleration in classical mechanics. 

incentive

noun
  • Something that motivates, rouses, or encourages. 

  • A bonus or reward, often monetary, to work harder. 

adj
  • Inciting; encouraging or moving; rousing to action; stimulating. 

  • Serving to kindle or set on fire. 

How often have the words impetus and incentive occurred in a corpus of books? (source: Google Ngram Viewer )