Examples of mechanical movement are known to us from everyday life. These are passing cars, flying planes, sailing ships. The simplest examples of mechanical movement we create ourselves, passing by other people. Every second, our planet moves in two planes: around the Sun and its axis. And these are also examples of mechanical motion. So let's talk more specifically about this today.
What happens mechanics
Before we say what examples of mechanical motion exist, let's look at what is called mechanics. We will not go into scientific jungle and operate with a huge number of terms. Speaking quite simply, then mechanics is a branch of physics that studies the motion of bodies. And what kind of mechanics can it be? Students in physics lessons get acquainted with its subsections. These are kinematics, dynamics and statics.
Each of the subsections also studies the motion of bodies, but has features characteristic only of it. 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 movement of a mechanical system in kinematics is considered without taking into account the reasons leading to the movement. At the same time, we know that the reason for the acceleration that sets the body in motion is precisely the force.
What if forces need to be considered
But the next section, called dynamics, is already considering the interactions of bodies during movement. Mechanical motion, the speed of which is one of the important parameters, in dynamics is inextricably linked with this concept. The last section is static. She is studying the equilibrium conditions of mechanical systems. The simplest static example is balancing hour weights. Note to teachers: physics lesson “Mechanical motion” at school should begin with just that. First give examples, then divide the mechanics into three parts, and only then proceed with the rest.
What are the challenges
Even if we turn to just one section, suppose it will be kinematics, a huge number of different problems awaits us here. 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 kinematic motion problems can be reduced to cases of free fall. We’ll talk about this now.
What is free fall in kinematics
This process can be given several definitions. However, all of them will inevitably come down to one point. In a free fall, only gravity acts on the body. It is directed from the center of mass of the body along the radius to the center of the earth. Otherwise, you can “twist” the wording and definitions as you like. However, the presence of gravity alone in the process of such movement is a prerequisite.
How to solve the problems of free fall in kinematics
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 to the problem. A quarter is devoted to understanding the process, and another quarter to the process of computing. But formulas, formulas and, once again, formulas - that’s what helps.
We can call free fall a special case of uniformly accelerated motion. Why? Yes, because we have everything that is needed for this. The acceleration does not change, it is equal to 9.8 meters per second squared. On this basis, we can move on. The formula of the distance traveled by the body during uniformly accelerated motion is: S = Vot + (-) at ^ 2/2. Here S is the distance, Vo is the initial velocity, t is time, a is acceleration. Now let's try to bring this formula to the case of free fall.
As we said earlier, this is a special case of uniformly accelerated motion. And if a is a conventional conventional designation for acceleration, then g in the formula (replaces a) will have a very definite numerical value, also called tabular. Let's rewrite the formula of the distance traveled by the body for the case of free fall: S = Vot + (-) gt ^ 2/2.
It is clear that in this case, the movement will occur in a vertical plane. We draw the attention of readers to the fact that none of the parameters that we can express from the formula written above does not depend on body weight. Whether you throw a box or a stone, for example, from the roof, or two stones of different masses, these objects will simultaneously land and fall at the same time they begin to fall.
Free fall. Mechanical movement. Tasks
By the way, there is such a thing as instant speed. It denotes speed at any given moment in time. And with a free fall, we can easily determine it, knowing only the initial speed. And if it is equal to zero, then the matter is generally trifling. The formula for the instantaneous velocity for free fall in kinematics is: V = Vo + gt. Notice that the “-" sign is gone. After all, it is set when the body slows down. But how can a body slow down during a fall? Thus, if the initial speed was not reported, the instantaneous will be equal to simply the product of the acceleration of gravity g and time t elapsed from the moment the motion started.
Physics. Free-fall mechanical movement
Let's move on to specific tasks on this topic. Suppose the following condition. The children decided to have fun and throw a tennis ball from the roof of the house. Find out what the speed of a tennis ball was when it hit the ground, if the house has twelve floors. The height of one floor is taken equal to three meters. The ball is released from the hands.
The solution to this problem will not be one-step, as you might think at first. Everything seems to be very simple, just substitute the necessary numbers in the formula for instantaneous speed and that’s all. But when we try to do this, we may run into a problem: we do not know the time the ball fell. Let's look at the rest of the details of the task.
Tricks in the conditions
Firstly, we are given the number of floors, and we know the height of each of them. It is equal to three meters. Thus, we can immediately calculate the normal distance from the roof to the ground. Secondly, we are told that the ball is released from the hands. As usual, in the problems of mechanical motion (and indeed in problems in general) there are small details that at first glance may seem insignificant. However, here this expression suggests that the tennis ball does not have an initial speed. Well, one of the terms in the formula then disappears. Now we need to find out the time that the ball spent in the air before it hit the ground.
For this, we need the distance formula for mechanical motion. First of all, we remove the product of the initial velocity by the time of movement, since it is equal to zero, which means that the product will be equal to zero. Next, we 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. We can only extract the square root of this expression to find out how much time has passed before the ball hits the ground. We substitute the numbers, extract the root and get about 2.71 seconds. Now we substitute this number into the formula of instantaneous speed. We get about 26.5 meters per second.
Note to teachers and students: one could go a little different way. In order not to get confused in these numbers, the final formula should be simplified as much as possible. This will be useful in that there is less risk of getting confused in your own calculations and making a mistake in them. In this case, we could do the following: express the time from the distance formula, but not substitute the numbers, but substitute this expression into the instantaneous velocity formula. Then it would look like this: V = g * sqrt (2S / g). But after all, the acceleration of gravity can be introduced into the radical expression. To do this, imagine it in a square. We get V = sqrt (2S * g ^ 2 / g). Now we will reduce the acceleration of gravity in the denominator, and in the numerator we will erase its degree. As a result, we get V = sqrt (2gS). The answer will be the same, only the calculations will be less.
Summary and Conclusion
So what did we learn today? There are several sections that physics studies. The mechanical movement in it is divided into statics, dynamics and kinematics. Each of these mini-sciences has its own characteristic features, which are taken into account when solving problems. However, we can give a general description 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. Mechanics also include cases of free fall, since they are particular types of uniformly accelerated motion. And with these situations, it is kinematics that works for us.