We explain what kinematics is and what is the origin of this branch of physics. Elements of kinematics and application examples.
What is kinematics?
Kinematics is a branch of physics that studies the movement of solid objects and their trajectory as a function of time without taking into account the origin of the forces that motivate it. For this, the speed (the change in displacement per unit of time) and the acceleration (change in speed) of the moving object are taken into consideration.
The origins of kinematics date back to ancient astronomy, when astronomers and philosophers such as Galileo Galilei observed the movement of spheres on inclined planes and in free fall to understand the movement of the celestial stars. These studies, together with those of Nicholas Copernicus, Tycho Brahe and Johannes Kepler, served as a reference for Isaac Newton to formulate his three Laws of Motion, and all of this together founded modern kinematics at the beginning of the 18th century.
The contributions of the French Jean Le Rond d'Alembert, Leonhard Euler and André-Marie Ampère were key in the establishment of this discipline, baptized by Ampère himself as kinematics (from Greek kinéinmove, move).
The much later postulation of relativity by Albert Einstein would turn this discipline upside down and found relativistic kinematics, in which time and space are not absolute dimensions, as the speed of light is.
Elements of kinematics
The basic elements of kinematics are three: space, time and a mobile. We must take into consideration that in classical mechanics the first two (time and space) are absolute dimensions, independent of the mobile and prior to its existence.
Space is described by Euclidean geometry, time is considered unique in any region of the universe, and a mobile can be any body in motion. The simplest mobiles are particles (and their study opens the field of particle kinematics), but more frequently rigid solids are considered (analogous to a system of particles and corresponding to what we know as bodies or objects).
In that sense, classical kinematics contemplates the following types of movement:
- Uniform rectilinear movement A body moves at a constant speed v, with zero acceleration in a straight line.
- Uniformly accelerated rectilinear motion A body moves at a speed that varies linearly (since its acceleration is constant) as time progresses.
- Simple harmonic motion It is a periodic back-and-forth motion in which a body oscillates around an equilibrium point in a given direction and in regular units of time.
- Parabolic movement It is the composition of two different rectilinear movements: one horizontal and of constant speed, and another vertical and uniformly accelerated.
- Uniform circular motion As its name indicates, it is the movement that traces perfect circles along its path, keeping the module of its speed unchanged over time.
- Uniformly accelerated circular motion It is the movement that traces perfect circles along its path, but with a speed that varies in module over time.
- Complex harmonic motion. It is the combination of various simple harmonic movements, in different directions.
Examples of kinematics
Most of the known movements on the face of the earth are good examples of kinematics studies. The fall of a body for example, is a movement uniformly accelerated by the force of gravity that the Earth exerts on all objects. This force is what we call weight and it points towards the center of the planet.
Another example is a body suspended from an elastic like a spring, whose movement will be simple or complex harmonic depending on the forces we exert on it.
Finally, the movement of the clock hands or a loose object inside a centrifuge (clothes in the washing machine, for example), illustrates uniform or accelerated circular motion, respectively.
