study of how objects move under the action of forces (contrast with statics).
Merriam-Webster Online Dictionary
dynamics (noun plural but singular or plural in construction)1.
a branch of mechanics that deals with forces and their relation primarily to the motion but sometimes also to the equilibrium of bodies
2.
a pattern or process of change, growth, or activity - population dynamics
3.
variation and contrast in force or intensity (as in music)
dynamics (Wikipedia)