The branch of mechanics concerned with the forces that cause motions of bodies; SYN. kinetics.
Or kinetics In mechanics, the mathematical and physical study of the behavior of bodies under the action of forces that produce changes of motion in them.

In music, symbols indicating relative loudness, changes in loudness such as crescendo and diminuendo, or loudness in accentuation such as rinforzando.
Dynamic markings from Monteverdi to J S Bach were primarily objective indicators of tonal change, occasionally spatial and directional in significance. Dynamic expression emerged slowly during the 18th century as instruments of the orchestra evolved away from fixed registers toward individual freedom of expression, coinciding with the coming of age of the fortepiano, named to advertise its expressive advantages over the fixed-register harpsichord. After 1950 many composers attempted to bring dynamics under serial control, but with only limited success.