What are the basic concepts of kinematics? What kind of science is this and what is it studying? Today we will talk about what kinematics is, what basic concepts of kinematics are in the problems and what they mean. In addition, we will talk about the quantities with which we most often have to deal.
Kinematics. Basic concepts and definitions
First, let's talk about what it is. One of the most studied branches of physics in a school course is mechanics. Molecular physics, electricity, optics, and some other sections, such as, for example, nuclear and atomic physics, follow in an unspecified order. But let's take a closer look at the mechanics. This branch of physics studies the mechanical motion of bodies. It establishes some laws and studies its methods.
Kinematics as part of mechanics
The latter is divided into three parts: kinematics, dynamics and statics. These three sub-sciences, if you can call them that, have some features. For example, statics studies the equilibrium rules of mechanical systems. Immediately an association with the scales comes to mind. Dynamics studies the laws of motion of bodies, but at the same time draws attention to the forces acting on them. But kinematics does the same, only forces are not taken into account. Consequently, the mass of those very bodies is not taken into account in tasks.
The basic concepts of kinematics. Mechanical movement
The subject in this science is the material point. By it is meant a body whose dimensions, in comparison with a certain mechanical system, can be neglected. This so-called idealized body is akin to the ideal gas, which is considered in the section of molecular physics. In general, the concept of a material point, both in mechanics in general and in kinematics in particular, plays a rather important role. The so-called translational movement is most often considered .
What does this mean and how can it be?
Typically, movements are divided into rotational and translational. The basic concepts of kinematics of translational motion are mainly associated with the quantities used in the formulas. We will talk about them later, but for now let's get back to the type of movement. It is clear that when it comes to rotational, the body spins. Accordingly, translational motion will be called the movement of the body in a plane or linearly.
The theoretical basis for solving problems
Kinematics, the basic concepts and formulas of which we are considering now, has a huge number of tasks. This is achieved through conventional combinatorics. One of the methods of diversity here is to change unknown conditions. One and the same task can be presented in a different light, simply changing the purpose of its solution. It is required to find distance, speed, time, acceleration. As you can see, the options are the whole sea. If we connect the conditions of free fall here, the space becomes simply unimaginable.
Values and formulas
First of all, we make one reservation. As is known, quantities can have a twofold nature. On the one hand, a certain numerical value can correspond to a certain value. But on the other, it may have a direction of distribution. For example, a wave. In optics, we are faced with a concept such as wavelength. But if there is a coherent light source (the same laser), then we are dealing with a beam of plane-polarized waves. Thus, the wave will correspond not only to a numerical value denoting its length, but also to a given direction of propagation.
Classic example
Similar cases are an analogy in mechanics. Let's say a trolley rolls in front of us. By the nature of the movement, we can determine the vector characteristics of its speed and acceleration. It will be a little more difficult to do this with forward movement (for example, on a level floor), so we will consider two cases: when the trolley rolls up and when it rolls down.
So, let's imagine that the trolley goes up a small slope. In this case, it will slow down if external forces do not act on it. But in the opposite situation, namely, when the trolley rolls down from top to bottom, it will accelerate. Speed in two cases is directed to where the object is moving. This should be taken as a rule. But acceleration can change the vector. When decelerating, it is directed in the opposite direction for the velocity vector. This explains the slowdown. A similar logical chain can be applied to the second situation.
Other values
We just talked about the fact that in kinematics they operate not only with scalar quantities, but also with vector ones. Now let's take it one step further. In addition to speed and acceleration, in solving problems, such characteristics as distance and time are used. By the way, speed is divided into initial and instantaneous. The first of them is a special case of the second. Instantaneous speed - this is the speed that can be found at any given time. And from the initial, probably, everything is clear.
Task
A considerable part of the theory was studied by us earlier in the previous paragraphs. Now it remains only to give the basic formulas. But we will do even better: not just consider the formulas, but also apply them in solving the problem to finally consolidate the knowledge gained. In kinematics, a whole set of formulas is used, combining which, you can achieve everything you need to solve. We present a problem with two conditions in order to understand this completely.
A cyclist brakes after crossing the finish line. It took him five seconds to stop completely. Find out with what acceleration it braked, and also what braking distance it managed to go. The braking distance is assumed to be linear; the final speed should be set equal to zero. At the moment of crossing the finish line, the speed was 4 meters per second.
In fact, the task is quite interesting and not as simple as it might seem at first glance. If we try to take the distance formula in kinematics (S = Vot + (-) (at ^ 2/2)), then nothing will come of us, since we will have an equation with two variables. What to do in this case? We can go in two ways: first, calculate the acceleration by substituting the data in the formula V = Vo - at, or express the acceleration from there and substitute it into the distance formula. Let's use the first method.
So, the final speed is zero. Initial - 4 meters per second. By transferring the corresponding quantities to the left and right sides of the equation, we obtain the expression of acceleration. Here it is: a = Vo / t. Thus, it will be equal to 0.8 meters per second squared and will be inhibitory in nature.
We pass to the distance formula. We simply substitute data into it. We get the answer: the braking distance is 10 meters.