From the seventh grade physics course, we remember that the mechanical motion of a body is its movement in time relative to other bodies. Based on such information, we can assume the necessary set of tools for calculating the movement of the body.

First, we need something in relation to which we will make our calculations. Next, we need to agree on how we will determine the position of the body relative to this "something". And finally, you will need to fix the time somehow. Thus, in order to calculate where the body will be at a particular moment, we need a frame of reference.

Frame of reference in physics

In physics, a reference system is a set of a reference body, a coordinate system associated with a reference body, and a clock or other device for measuring time. At the same time, one should always remember that any frame of reference is conditional and relative. It is always possible to adopt another frame of reference, relative to which any movement will have completely different characteristics.

Relativity is generally an important aspect that should be taken into account in almost any calculation in physics. For example, in many cases we are far from being able to determine the exact coordinates of a moving body at any time.

In particular, we cannot place observers with clocks every hundred meters along railway track from Moscow to Vladivostok. In this case, we calculate the speed and location of the body approximately for some period of time.

We do not care about the accuracy of up to one meter when determining the location of a train on a route of several hundred or thousands of kilometers. For this, there are approximations in physics. One of such approximations is the concept of "material point".

Material point in physics

A material point in physics denotes a body, in cases where its size and shape can be neglected. It is assumed that the material point has the mass of the original body.

For example, when calculating the time it will take an airplane to fly from Novosibirsk to Novopolotsk, we do not care about the size and shape of the aircraft. It is enough to know what speed it develops and the distance between cities. In the case when we need to calculate the wind resistance at a certain height and at a certain speed, then we can’t do without an exact knowledge of the shape and dimensions of the same aircraft.

Almost any body can be considered a material point, either when the distance covered by the body is large in comparison with its size, or when all points of the body move in the same way. For example, a car that traveled a few meters from the store to the intersection is quite comparable to this distance. But even in such a situation, it can be considered a material point, because all parts of the car moved the same way and at the same distance.

But in the case when we need to place the same car in the garage, it can no longer be considered a material point. You have to take into account its size and shape. These are also examples when it is necessary to take into account relativity, that is, with respect to what we make specific calculations.

Description of the video lesson

The objects and objects around us (in the language of physics, they are called physical bodies) occupy a certain position in space relative to each other. If over time the position of one body relative to another does not change, this means that the first body is at rest relative to the second. For example, a road sign and a tree are at rest relative to each other. If over time the position of one body relative to another changes, this means that the first body performs a mechanical movement relative to the second body. For example, a tram and a tree. The tram makes a mechanical movement relative to the tree. The mechanical movement of a body is a change in its position in space relative to others, which occurs over time. We know how to describe the movement and calculate the main parameters for the simplest case from the course of mathematics and physics of the seventh grade. We can set the position of the body using the coordinate line. To find the speed of a body, it is necessary to divide the path by time ... However, in practical life, more complex types of mechanical movement are more common. And to describe them, we need new tools. Consider the following types of movement:
- forward movement(for example, descent from the mountain on a sleigh);
- rotational motion (for example, the daily rotation of the Earth);
- oscillating motion(for example, the movement of a pendulum).

How, then, or with what help, can we describe more complex types of motion? First, you need to choose an object, relative to which we will consider the movement of bodies of interest to us. Secondly, from the course of mathematics, we know that you can set the position of a point using a coordinate system (for example, rectangular). Thirdly, you will need to count the time. That is, in order to calculate where the body will be at a particular moment, we need a frame of reference. In physics, a reference system is a set of a reference body, a coordinate system associated with the reference body, and a fixed device for measuring time. It is important to remember that any frame of reference is conditional and relative. By choosing a different frame of reference, we will get motion with completely different parameters. Bodies in physics are real, they often have significant dimensions, in contrast to an abstract point in mathematical system coordinates. So can we use the coordinate system to find the location physical body? If the dimensions of the body itself are many times smaller than other dimensions that have to be dealt with under the conditions of a particular problem, then the dimensions of the body itself can be neglected under these particular conditions. Then such a body in physics is taken as a material point.
For example, we need to calculate the time it takes for a plane to fly from Minsk to Burgas. In this condition of the problem, the dimensions and shape of the transport itself are not important to us. You need to know the speed that it develops and the distance between cities. These data will be sufficient to solve the problem. In this problem, it is legitimate to take an airplane as a material point. If we need to calculate the wind resistance at a certain height and at a specific speed, then when solving this problem, we cannot do without the exact knowledge of the shape and dimensions of the same aircraft, because the drag force depends on the shape and speed of the aircraft. This means that the body (plane) cannot be taken as a material point. A body can also be taken as a material point if all points of the body move in the same way (such a movement is called translational). For example, if a subway train travels even just one stop, but along a straight section, it can be considered a material point, because all parts of the train moved in the same way and for an equal distance.
Choose from the proposed conditions of the problems the case when the body can be considered a material point:
1. Calculate the pressure the tank exerts on the surface.
2. Determine the volume of the ball using measuring device beaker.
3. Determine the height to which the space shuttle has risen.
When lifting a Space Shuttle, one can neglect the size of the rocket itself compared to the distances it ascends. Hence, it can be taken as a material point.
In other cases, the dimensions of the bodies themselves must be taken into account when solving the problem.

Molchanova Larisa Anatolievna

physics and science teacher

MBOU secondary school №150, Krasnoyarsk

Topic 1 "Laws of motion and interaction of bodies"

Lesson 1.1 Material point. Reference system

The purpose of the lesson: Get to know the basic characteristics of the movement. Consider the concept of a material point and the principles of its application. Explain the components of a reference system.

During the classes

1. Organizational moment

Marking those present. Announcement of the topic and the name of the lesson (slide 1).

2. Learning new material

1) Introductory speech of the teacher:

We begin to consider with you the issues of the interaction of bodies, a material point, a reference system from the main question -traffic . In general, any changes that occur in nature can be called movement.

In the 9th grade physics course, we will begin the study of movement with the simplest type of movement - mechanical movement (slide 2).

Consider several types of movement. This is, first of all, progressive, the simplest, quite rare; rotational and oscillatory (slide 3).

In addition, we can talk about the shape of the trajectory of the body, that is, the line along which the body moves (slide 4).

Please note that when studying movement, we should consider issues related to the body. To characterize the movement, the following quantities are used: first of all, the distance traveled, speed, trajectory, as well as very important things - these are the coordinates of the body (slide 5).

Let's see where all these quantities meet. First of all, it is necessary to note the value familiar from the 7th grade - the distance traveled. It is denoted by the letter S and is expressed in meters. The distance traveled is the length of the trajectory, and the trajectory is the line along which the body moves (slides 6, 7).

The next characteristic is speed. From the 7th grade course, we can recall that speed is a value that characterizes the speed of movement. It is denoted by the Latin letter V and is measured in m / s (slide 8).

And the next characteristic is the coordinates. I draw your attention to the fact that from mathematics you remember that in order to determine the location of the body, you must specify its coordinates. Until the middle of the last millennium, only one coordinate was used - we call it the OX axis or the abscissa axis. But today you know that our space is three-dimensional, and therefore the coordinates along three axes are used: x, y andz(slide 9). By setting these points (coordinates), we can determine the location of the body.

A very important issue that should be noted here is how to consider the body in a given coordinate system. The fact is that bodies can be very different in size - large, super-large (some cosmic bodies), small, microscopic, therefore, in order to make it more convenient to use the characteristics of the movement of a body, it is necessary to introduce such a thing as a material point (slide 10).

If we consider, for example, a train that approaches the station, then in this case we cannot consider it as a material point compared to the station, since their sizes are comparable (slide 11). In this case, if the sizes of the bodies are comparable, then they cannot be considered MT. But if we consider the movement of the train, for example, from Moscow to Krasnoyarsk, then in this case the distance that the train overcomes is much larger (incommensurable) with its size. Therefore, in this case, we can talk about the train as a MT (slide 12). In this case, it is convenient to set the coordinates of the body (points) and talk about how it moves.

Consider how to determine the location of the body in space. In order to do this, you need to know the coordinates. If we are dealing with one-dimensional motion, that is, the body moves along one straight line along the OX axis, then there will be only one coordinate - x. If the movement is two-dimensional, then there will be two coordinates - x and y, in this case we will use a rectangular coordinate system. And if three-dimensional, then three coordinates. This coordinate system is calledcartesian system coordinates.

It should also be noted that the body is moving in time, so we need to say what coordinates the body has at a given time. And here we come to a very important conclusion: to characterize the movement, it is necessary to introducereference system.

We call the reference system three components - this is the coordinate system, the reference body and the reference time device - the clock (slide 13). I draw your attention to the fact that another MT can also be the reference body, in this case we can say that the origin of coordinates coincides with the reference point. In general, this will be the reference system used throughout the world. In addition to such a reference system, others are also used - containing not only a rectangular coordinate system, but also the so-called radius vector. Using such a system, you can also set the location of the body, for example, this is widely used in navigation.

3. Consolidation of the studied

Performing exercise 1 page 9 of the textbook (frontal analysis).

4. Summary of the lesson

In conclusion of the lesson, it should be noted that we characterize any movement by a reference system, coordinates. It must be remembered that we can use such a concept as MT. You also need to remember that sometimes we can consider not the whole body as a whole, but some part of it and take this part as MT. In the next lessons, we will get acquainted with other characteristics of movement.

Homework: §1.

Municipal educational institution

"Razumenska average comprehensive school№2"

Belgorodsky district of the Belgorod region

Physics lesson summary
in 9th grade

« »

prepared

math and physics teacher

Elsukova Olga Andreevna

Belgorod

2013

Topic: Laws of interaction and motion of bodies.

Lesson topic: Material point. Reference system.

Form of the lesson:lesson

Type of: I + II(a lesson in the study of knowledge and methods of activity)

Place of the lesson in the section:1

Targets and goals:

to ensure the perception, comprehension and primary memorization by students of the concepts of a material point, translational movement, reference system;

organize the activities of students to reproduce the studied material;

generalize knowledge about the concept of "material point";

check the practical application of the studied material;

develop cognitive independence and creativity students;

develop the skills of creative assimilation and application of knowledge;

develop the communication skills of students;

develop the oral speech of students;

Lesson equipment: board, chalk, textbook.

During the classes:

Organization of the beginning of the training session:

Greet students;

Check the sanitary and hygienic condition of the class ( is the class ventilated, the board is washed, the presence of chalk), if there are discrepancies with sanitary and hygienic standards, ask the students to correct them together with the teacher.

Get to know the students, mark those absent from the lesson;

Preparation for the active work of students:

Today in the lesson we have to return to the study of mechanical phenomena. In the 7th grade, you have already encountered mechanical phenomena and before starting to study new material, let's remember:

What is mechanical movement?

Mechanical movement- called the change in the position of the body in space over time.

What is uniform mechanical motion?

Uniform mechanical movement is movement at a constant speed.

What is speed?

Speed- this is physical quantity, which characterizes the speed of movement of the body, numerically equal to the ratio of movement in a small period of time to the value of this gap.

What average speed?

average speed is the ratio of the total distance traveled to the total time.

How to determine the speed if we know the distance and time?

In the 7th grade, you solved fairly simple problems to find the path, time or speed of movement. This year we will take a closer look at what types of mechanical motion exist, how to describe mechanical motion of any kind, what to do if the speed changes during the motion, etc.

Already today we will get acquainted with the basic concepts that help to describe both quantitatively and qualitatively mechanical movement. These concepts are very handy tools when considering any kind of mechanical motion.

Learning new material:

Everything in the world around us is in constant motion. What is meant by the word "movement"?

Movement is any change that occurs in the environment.

The simplest type of motion is the mechanical motion already known to us.

When solving any problems related to mechanical movement, it is necessary to be able to describe this movement. And this means that you need to determine: the trajectory of movement; movement speed; the path traveled by the body; the position of the body in space at any given time.

For example, during exercises in the Republic of Armenia, in order to launch a projectile, you need to know the flight path, how far it will fall.

From the course of mathematics, we know that the position of a point in space is specified using a coordinate system. Suppose we need to describe the position not of a point, but of the whole body, which, as we know, consists of many points, and each point has its own set of coordinates.

When describing the motion of a body that has dimensions, other questions arise. For example, how to describe the movement of a body if, during movement, the body also rotates around its own axis. In such a case, in addition to its own coordinate, each point of the given body has its own direction of motion and its own modulus of speed.

An example is any of the planets. When the planet rotates, opposite points on the surface have the opposite direction of motion. Moreover, the closer to the center of the planet, the lower the speed of the points.

How then to be? How to describe the movement of a body that has a size?

To do this, you can use the concept, which implies that the size the body seems to disappear, but the mass of the body remains. This concept is called a material point.

Let's write the definition:

The material point is called a body whose dimensions can be neglected under the conditions of the problem being solved.

Material points do not exist in nature. A material point is a model of a physical body. With the help of a material point, a fairly large number of problems are solved. But it is not always possible to apply the replacement of a body by a material point.

If, under the conditions of the problem being solved, the size of the body does not have a special effect on the movement, then such a replacement can be made. But if the size of the body begins to affect the movement of the body, then the replacement is impossible.

For example, a soccer ball. If it flies and moves quickly across the football field, then it is a material point, and if it lies on the shelves of a sports store, then this body is not a material point. The plane flies in the sky - a material point, landed - its size can no longer be neglected.

Sometimes it can be taken as a material point of the body, the dimensions of which are comparable. For example, a person is going up an escalator. He just stands, but each of his points moves in the same direction and at the same speed as a person.

Such a movement is called progressive. Let's write down the definition.

translational movement – This is the movement of a body in which all its points move in the same way. For example, the same car makes forward movement along the road. More precisely, only the body of the car performs translational motion, while its wheels perform rotational motion.

But with the help of one material point, we will not be able to describe the motion of the body. Therefore, we introduce the concept of a reference system.

Any reference system consists of three elements:

1) The very definition of mechanical motion implies the first element of any frame of reference. "The motion of a body relative to other bodies". The key phrase is about other bodies. Count body - this body, relative to which the movement is considered

2) Again, the second element of the reference system follows from the definition of mechanical motion. The key phrase is over time. This means that in order to describe the movement, we need to determine the time of movement from the beginning at each point of the trajectory. And for counting time we need watch.

3) And we already voiced the third element at the very beginning of the lesson. In order to set the position of the body in space, we need coordinate system.

In this way, A reference system is a system that consists of a reference body, a coordinate system associated with it, and a clock.

Reference systems We will use two types of Cartesian system: one-dimensional and two-dimensional.

The mechanical movement of a body is a change in its position in space relative to other bodies over time. To judge whether a given body is moving or not, one must first select a reference body, and then see if its position of the considered body changes relative to the selected reference body. In this case, the body can move relative to one reference body and rest relative to another.

It is possible to take a body as a material point even if its dimensions are commensurate with the distance it has traveled. For example, a person stands motionless on the step of an escalator. At any moment in time, all points of his body move in the same way. Such a movement is called progressive.

But if it is necessary to determine the path that the body has traveled for a certain period of time, then we will need more instruments for measuring time - watches. The coordinate system associated with the reference body and the clock for timing form a reference system that allows you to determine the position of a moving body at any time.