During a good period of studies carried out in mechanics, the considered movements are rectilinear, that is, they occur along a straight line. However, the movements of bodies do not occur that way. Planets, satellites and other bodies have movements along curvilinear paths. The transition from the study of rectilinear movements to the study of curvilinear movements requires more care and the inclusion of peculiar characteristics of curvilinear movements.
Talking about direction and direction in a movement is very important, as many people believe that they are the same thing, which in fact they are not. To understand the difference between direction and sense, look at the following picture:
In this engraving we have two pairs of straight lines. In one of the pairs, as can be seen, the lines are opposite each other. By observing these lines we can conclude which direction is linked with respect to the horizontal, vertical, north, south, east and west position. The direction is the orientation of the mobile. By joining direction and sense we can determine the position of any body or object that is describing a curvilinear trajectory. Returning to the picture, we can say that the lines a and b have the same direction and direction, whereas the lines c and d have the same direction, but opposite directions.
In the study of physics it is very important to differentiate between two types of quantities: scalar quantities and vector quantities. Scalar quantities are those that need only a number to be represented. Mass, energy, time, and temperature are examples of this type of quantity. Vector quantities, unlike scalar quantities, need something more so that they can be represented correctly. In addition to the numerical part, also called the module, it needs a direction and a sense for its perfect determination. Force, acceleration, impulse, momentum, among others are examples of vector quantities.