impetus vs propulsion

impetus

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

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

  • Anything that impels; a stimulating factor. 

  • An activity in response to a stimulus. 

  • 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. 

propulsion

noun
  • The action of driving or pushing, typically forward or onward; a propulsive force or impulse. 

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