There are several types of motion known to modern mechanics . They differ from each other in the trajectory, the presence or absence of acceleration, and a number of other parameters. However, they have common features: in particular, each type of kinematic action can be expressed using the coordinate line. So, the rectilinear movement has a straight line trajectory, which is clear from its name. We consider this case in more detail.
Uniform rectilinear movement - a movement characterized by movement at equal distances over equal intervals of time. Simply put, for each selected time interval (whether it be 1, 2, 3 seconds or 10 minutes), the body will cover a distance absolutely equal to that which will be covered in the next same time period. Strictly speaking, an ideal rectilinear movement is rather an abstract model, since it is almost impossible to achieve such a movement under normal conditions: there are always obstacles. Among them - air resistance, features of a moving body and others.
The formula for such a movement is as simple and well known since high school:
s = v / t,
where s is the distance traveled by the body for a certain length of time, v is the speed during this movement, t is the actual time taken to cover the distance. It is logical that the speed during this movement is also constant and unchanged.
Equally alternating rectilinear motion is a special case of the above described with the only difference that here we have accelerated motion. The formula for it is as follows:
s = Vo + - at2 / 2
It reads as: es (distance) equals ve zero (initial speed) plus or minus a (acceleration) te (time) squared divided by two. The "+ -" sign is not accidental. The fact is that rectilinear motion with acceleration can be either equally accelerated or equally slow. The first case corresponds to the plus formula, the second to the minus. Examples: acceleration and braking of a train. In the case when the train has gained the necessary speed and does not need acceleration anymore, it moves with a relatively stable speed, that is, it makes a rectilinear movement.
It is also worth considering such a type of motion as uneven rectilinear. A body moving in this way has a speed that can change without any system, that is, at an arbitrary point in time. Therefore, it is customary to distinguish it from the so-called average speed. Uneven PD differs from uniform in that the speed along the entire path can vary (as a rule, in real life we deal with this kind of movement). However, it is impossible to measure speed at any given moment in time, and it is not required. For this, a concept such as average speed was introduced . Its formula is exactly the same as the speed formula with a uniform AP:
v = s / t,
with the only difference being that it is impossible to find a specific position at one time or another, however, it is possible to determine how much distance has been covered during a given time.
In contrast to the average, the instantaneous speed with uneven AP is the speed that the body has at a particular moment in time.
Now that the specifics of motion with equal speed are more or less disclosed, it is important to clarify and distinguish between straight and curved motion. Given the fact that linear motion in mechanics almost always means rectilinear, it is worth noting: scientists believe that such a motion is a special case of curvilinear. The definition of curvilinear motion is as follows: a motion whose trajectory does not correspond to a straight line, but to a curved line. Probably, due to the fact that an ideal straight path does not exist (only as an abstraction), and what we have adopted as a direct path is only an approximate value of this, we can really consider straight-line motion to be a curved shape.