If distance is expressed in meters and time in seconds, speed is expressed in m/s (meters per second). It is the measure in the international system of units that is used mainly in physics! The heavier an object is, the more energy must be consumed for it to gain speed and then lose speed (kinetic energy). This has a significant impact on car traffic, the pollution it causes and the severity of the accidents it causes. In 2002, when Rotterdam limited and monitored the speed on the section of the A13 motorway through the Overschie district (from 120 km/h to 80 km/h at 3.5 km), NOx levels dropped by 15 to 20%, PM10 by 25 to 30% and carbon monoxide (CO) by 21%. CO2 emissions have decreased by 15% and the number of accidents by 60% (-90% for fatalities), with noise divided by 2. The definition of “average speed” corresponds to the definition of “speed” given above. It is calculated by the relation v = d / t, where v is the velocity, d is the distance and t is the time. The kinetic energy of an object moving in translation is linear with its mass and the square of its velocity: acceleration is the derivative of velocity, and velocity is the derivative of distance from time. Acceleration is the rate of change in the speed of an object over time. The mean acceleration a of an object whose velocity changes from vi to vf during a period t is given by: a = v f − v i t {displaystyle a={frac {v_{f}-v_{i}}{t}}}.

We recognize in 1 2 r 2 d θ d t = d A ( t ) d t {displaystyle {frac {1}{2}},r^{2},{frac {mathrm {mathrm {d} theta }{mathrm {d} t}}={frac {mathrm {d} A(t)}{mathrm {d} t}};} , the surface velocity. In physics, the variation of velocity as a function of time is acceleration. A downward speed (a deceleration) is a negative acceleration. If we take into account the direction of motion, an object that takes a curve also undergoes an acceleration that changes the direction of its speed, even if its value in number remains the same. As Aristotle`s heiress, velocity estimation undoubtedly made great strides in the Middle Ages, thanks to the conceptualization of speed as an intense quantity and the resulting precision for the idea of velocity variation. These are the works of the Oxford schools (the Oxford calculators) and the University of Paris (Nicole Oresme), in which some authors such as Pierre Duhem, Anneliese Maier or Marshall Clagett saw the precursors of Galileo 3. According to Aristotle, every body that falls has a certain speed, determined by nature, which can neither be increased nor decreased, except by the use of force or resistance to it. Aristotle assumes that a mobile phone ten times heavier than another moves ten times faster and therefore falls ten times faster. According to him, all bodies in the universe derive the origin of their movement from a first engine, the movements being transmitted by contact. In addition, there is the idea that objects move to reach the right place intended for them, where they find silence: movement involves the action of a driving force, a motor connected to the mobile: separated from the first, the second stops.

The instantaneous velocity is a vector in physics, i.e. analogous to an arrow and its orientation is essential, because if the direction of the velocity changes by turning on a circle at constant speed on the meter, by bicycle or car, this vector speed has a variation, speed speed or centripetal acceleration perpendicular to the speed at the center of the circle. This multiplies by the mass gives the centripetal force, which makes it possible to turn and suffer by countering it with centrifugal force to avoid leaving the turn in the injured trench if you drive too fast through the skidding. The instantaneous velocity vector v → {displaystyle {vec {v}}} of an object whose position at time t is given by r → ( t ) {displaystyle {vec {r}}(t)} is defined by the derivative v → = d r → d t {displaystyle {vec {v}}={frac {mathrm {d} {vec {r}}}{mathrm {d} t}}}. This centripetal acceleration A can be calculated by considering the velocity of the end of the velocity vector V rotating one turn at the same time T as that fixed by the bicycle or mobile to rotate a turn on the circle of radius R, which gives A = V^2/R with a centripetal force multiplied by 4, if the speed V doubles or if the radius is divided by 4. Also, do not make too many speed mistakes before a turn, otherwise you slip, 10% speed error causes a 20% higher force to get off the road. The law of the fall of bodies, pronounced in Galileo`s De motu (1564-1642), stipulates that bodies fall after a uniformly accelerated movement and, on the other hand, that all bodies, large and small, heavy and light, that is, whatever their size and nature, fall (at least in complete vacuum) at the same rate; in other words, and since Galileo had no knowledge of the gravity of the earth, that the acceleration of the fall was a universal constant.  Galileo thus marks the end of Aristotelianism.

On the other hand, velocity can correspond to very different use cases, depending on whether it is a single vector or a vector field: velocity is an intense quantity: it is defined for a point in space, and a composite system does not add up the speed of its different parts. 