Examples of mechanical motion are known to us fromEveryday life. These are cars passing by, planes flying by, ships sailing by. We create the simplest examples of mechanical movement ourselves, passing by other people. Every second our planet makes movement in two planes: around the Sun and its axis. And these are also examples of mechanical movement. So let's talk about this more specifically today.
Before saying what examples existmechanical motion, let's understand in general what is called mechanics. We will not go into the scientific jungle and operate with a huge number of terms. Speaking quite simply, mechanics is a branch of physics that studies the movement of bodies. And what could it be, this mechanic? Schoolchildren in physics classes get acquainted with its subsections. This is kinematics, dynamics and statics.
Each of the subsections also studies the movement of bodiesbut it is characteristic only for him. Which, by the way, is universally used in solving relevant problems. Let's start with kinematics. Any modern school textbook or electronic resource will make it clear that the motion of a mechanical system in kinematics is considered without taking into account the reasons leading to the motion. At the same time, we know that the cause of acceleration, which will cause the body to move, is precisely force.
But considering the already interactions of bodies withThe next section, called dynamics, deals with the movement. Mechanical motion, the speed in which is one of the important parameters, in the dynamics is inextricably linked with this concept. The last of the sections is static. She is studying the equilibrium conditions of mechanical systems. The simplest static example is balancing an hour of a scale. Note to teachers: a lesson in physics “Mechanical Movement” at school should begin with this. First, give examples, then divide the mechanics into three parts, and only then proceed to the rest.
Even if we turn to just onesection, suppose it will be kinematics, we are waiting for a huge number of very different tasks. The thing is that there are several conditions on the basis of which the same task can be presented in a different light. Moreover, the problem of kinematic motion can be reduced to cases of free fall. We will talk about this now.
This process can be given several definitions.However, all of them will inevitably be reduced to one moment. With a free fall, only gravity acts on the body. It is directed from the center of body mass along the radius to the center of the Earth. Otherwise, you can "twist" the wording and definitions as soon as you wish However, the presence of only one force of gravity in the process of such movement is imperative.
First we need to “get hold of” the formulas.If you ask a modern teacher in physics, he will answer you that knowledge of formulas is already half the solution of the problem. A quarter is devoted to understanding the process and another quarter to the calculation process. But formulas, formulas, and once again formulas — this is what constitutes support.
We can call free fall privatecase of uniformly accelerated motion. Why? Yes, because we have everything that is needed for this. Acceleration does not change, it is equal to 9.8 meters per second squared. On this basis, we can move on. The formula for the distance traveled by the body in uniformly accelerated motion is: S = Vot + (-) at ^ 2/2. Here S is the distance, Vo is the initial velocity, t is the time, a is the acceleration. Now let's try to bring this formula under the case of free fall.
As we said earlier, this is a special case.uniformly accelerated motion. And if a is a conventional conventional notation for acceleration, then g in the formula (replaces a) will have a well-defined numerical value, also called a tabular one. Let's rewrite the formula for the distance covered by the body for the free fall case: S = Vot + (-) gt ^ 2/2.
It is clear that in such a case the movement will beoccur in a vertical plane. We draw readers' attention to the fact that none of the parameters that we can express from the formula written above does not depend on body weight. If you throw a box or a stone, for example, from the roof, or two different masses of stone, these objects will simultaneously fall and start landing at the same time.
By the way, there is such a thing as instantaneousspeed. It denotes the speed at any time of movement. And with a free fall, we can easily determine it, knowing only the initial velocity. And if it is zero, then the matter is generally trifling. The formula of instantaneous velocity in free fall in kinematics is: V = Vo + gt. Notice that the “-” sign is missing. After all, it is set when the body slows down. And how can a body slow down when falling? Thus, if the initial speed was not communicated, the instantaneous speed will be equal to simply the product of the acceleration due to gravity and the time t elapsed since the start of the movement.
Let's move on to specific tasks on this topic.Suppose the condition is as follows. The children decided to have fun and throw a tennis ball from the roof of the house. Find out what was the speed of a tennis ball at the time of impact with the ground, if the house has twelve floors. The height of one floor is equal to three meters. The ball is released from the hands.
The solution to this problem will not be one-step, asyou might think first. It seems that everything seems utterly simple, just substitute the necessary numbers in the formula of instant speed and that's it. But when we try to do this, we may encounter a problem: we do not know when the ball falls. Let's analyze the rest of the task.
First, we are given the number of floors, and we knowthe height of each one. It is three meters. Thus, we can immediately calculate the normal distance from the roof to the ground. Secondly, we are told that the ball is being released from the hands. As usual, there are small details in the tasks for mechanical motion (and indeed in problems in general), which at first glance may seem to be meaningless. However, this expression suggests that the tennis ball does not have an initial speed. Great, one of the terms in the formula then disappears. Now we need to know the time that the ball spent in the air before the impact with the ground.
To do this, we need a distance formula formechanical movement. First of all, we remove the product of the initial velocity for the time of movement, since it is equal to zero, and therefore, the product will be equal to zero. Next, multiply both sides of the equation by two to get rid of the fraction. Now we can express the square of time. To do this, double the distance divided by the acceleration of gravity. It remains only to extract the square root of this expression to find out how much time has passed before the ball hit the ground. Substituting the numbers, extracting the root and get about 2.71 seconds. Now this number is substituted into the formula of instant speed. We get about 26.5 meters per second.
На заметку учителям и ученикам:could go a little different way. In order not to be confused in these numbers, one should maximally simplify the final formula. This will be useful in that there is less risk of becoming entangled in their own calculations and to make a mistake in them. In this case, we could do the following: express time from the distance formula, but not substitute numbers, but substitute this expression into the formula of instantaneous velocity. Then it would look like this: V = g * sqrt (2S / g). But after all, the acceleration of free fall can be made in the radical expression. For this we represent it in the square. We get V = sqrt (2S * g ^ 2 / g). Now we reduce the acceleration of free fall in the denominator, and in the numerator we delete its degree. As a result, we get V = sqrt (2gS). The answer will be the same, only the calculations will be less.
So, what have we learned today?There are several sections that studies physics. Mechanical motion in it is divided in it into static, dynamic and kinematic. Each of these mini-sciences has its own characteristic features, which are taken into account when solving problems. However, we can give a general characteristic of such a concept as mechanical motion. Grade 10 is the time of the most active study of this section of physics, according to the school curriculum. The mechanics also include free fall cases, since they are particular types of uniformly accelerated motion. And with these situations it works exactly kinematics.
