Is it ensure, insure, or assure?
1817 as a term in philosophy; 1827 in the sense "pertaining to force producing motion" (the opposite of static), from French dynamique introduced by German mathematician Gottfried Leibnitz (1646-1716) in 1691 from Greek dynamikos "powerful," from dynamis "power," from dynasthai "to be able, to have power, be strong enough," of unknown origin. The figurative sense of "active, potent, energetic" is from 1856 (in Emerson). Related: Dynamically.
"energetic force; motive force," 1894, from dynamic (adj.).
dynamics dy·nam·ics (dī-nām'ĭks)
The branch of physics that deals with the effects of forces on the motions of bodies. Also called kinetics. Compare kinematics.