The branch of mechanics concerned with motion of objects, as well as the reason i.e. the forces acting on such bodies. This, along with kinematics constitute dynamics, which is concerned purely with the effects of forces on moving bodies.
"The professor explained that while kinematics describes how objects move, kinetics investigates the forces causing that motion within the broader field of dynamics."
In plain English: Kinetics is the study of how forces cause objects to move and change their speed.
"The kinetics of this chemical reaction were too fast to measure accurately with our current equipment."
The word kinetics comes from the Ancient Greek term kinētikós, which originally meant "pertaining to motion." It entered English through scientific usage to describe the branch of mechanics concerned with forces and movement.