They are called scalar quantities, those which, as for example the temperature or the time, are completely described by a number, which represents their value. To univocally define a scalar quantity, it is therefore sufficient to indicate a numerical value accompanied by the relative unit of measurement (the length of a time interval is equal to 5 seconds, the temperature of a room is 20 ° C etc.). The number that defines the measure of a scalar is indicated with the term modulo, or more frequently intensity.

They are called vector quantities that, in order to be defined, need not only an intensity but also a direction and a direction. The vector quantities are represented by means of geometric figures called vectors, which are oriented segments, symbolized by an arrow: the module (the intensity) is identified by the length of the arrow segment, the direction from the straight line on which it lies and the direction from the tip of the arrow (see fig. 2.1). The point from which the oriented segment originates is called origin. Speed and force are examples of vector quantities. For complete information on the speed of a car, for example, it can be said that it travels at 120 km / h, along the A1 motorway (its direction), in the direction of Florence (the direction).

Two vectors are defined equal which have the same modulus, the same direction and the same direction; two vectors are defined opposite that have the same module and the same direction, but in the opposite direction.

A vector is indicated, according to the vector notation, with a letter surmounted by an arrow, for example, or with a **bold** letter , for example **A** (this is the notation that will be followed in this volume). Later, when you want to take into consideration only the intensity of a vector, it will show as a scalar, using *italics* , for example *A*