Kinematics (from the French term cinématique, coined by the physicist André-Marie Ampère from the Greek κίνημα -ατος, kinema -atos = “motion”, itself derived from the verb κινέω, kineo = “to move”) is the branch of Newtonian mechanics that deals with the quantitative description of the motion of bodies using only the notions of space and time, independent of the causes (forces) of the motion itself, task instead of dynamics.
Modern kinematics was born with the studies of Galileo Galilei, but its modern definition, using the principles of calculus, can be dated to the speech of Pierre Varignon on January 20, 1700 before the Royal Academy of Sciences in Paris. The second half of the 18th century was enriched by the contributions of Jean Le Rond d’Alembert and André-Marie Ampère. With Albert Einstein’s theory of relativity in 1905, relativistic kinematics began.
The definition of kinematics as “geometry of motion” is correct for a description of motion in the context of relativity, but can also be accepted for a classical description, since the relativistic and classical schemes coincide in almost all common cases. Depending on whether the body in motion is a material point, a rigid system, a deformable system, etc., one speaks of the kinematics of the point, the kinematics of rigid systems, the kinematics of deformable systems, and so on.
To study the motion of a body means to determine, instant by instant, the position of each of its points with respect to a given reference system, and in many cases this can be done with simple rules if we know the motion of one or more points of the body.
The study of relative motion, i.e. the laws of motion defined by two observers moving with respect to each other, is the subject of relative kinematics; it is based on the two postulates that it is possible to determine an absolute time independent of the observer, and that the distance between two fixed points is also independent of the relative motion of the observers.
These postulates are valid in the most common cases, as experience shows, but are no longer valid when the relative velocity of the observers is close to that of light. The description of motion is then the subject of relativistic kinematics, of which relative kinematics is a special case corresponding to the infinite speed of light.