The main feature of mechanical movement
The branch of physics, called mechanics, studies motion, describes and characterizes it, and clarifies the causes of motion and rest.
A change in the position of some bodies relative to others is called mechanical motion. Children moved around residential buildings, schools, and trees. But the schoolchildren’s clothes remained as they were. What if the children got to the stadium by bus? They moved relative to houses, poles, pedestrians, but did not move in relation to the driver, seats, or the bus itself.
The main feature of mechanical motion is that it is relative. How far does a seventh grade student walk during a school lesson? “Nothing,” say those who count the distance from the school desk. “81,000 km,” others will answer, taking into account that the student, together with the school and the Earth, moves around the Sun at a speed of 108,000 km/h. This means that the student does not move relative to his desk during the lesson, but travels a long distance relative to the Sun.
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The body moves, leaves a trace, or this trace can be mentally imagined. In physics, the line along which a body moves is called a trajectory. When a basketball player passes a pass to a teammate, the ball flies in a straight line - the trajectory is straight. When a goalkeeper throws a ball into the field, it flies along a curved line - a curved trajectory.
The most complex trajectory for study is divided into straight and curved sections.
Movement is characterized by the distance traveled or the length of the trajectory. A ski track is a trajectory, and the length of a ski track is the distance or path traveled by the skier. The trajectory of the tip of the clock hand is a circle, the distance traveled is the length of the circle.
The distance traveled is denoted by the letter s. It is a distance or length and is therefore measured in meters.
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Movement, in addition to trajectory, is described by a path per unit of time, i.e. in one second. If this path remains unchanged, then the movement is uniform (the same in one second), otherwise it is uneven (unequal in one second).
Of the many different types of movements, the simplest is rectilinear movement. In reality, such movements are extremely rare. A car, an athlete, a motorcycle, an airplane and other moving bodies participate in rectilinear uniform motion only for short periods of time.
So, to remember:
Lesson summary on the topic: “Mechanical movement. Relativity of motion. Material point"
Municipal budgetary educational institution
secondary school with. Starobashirovo
Lesson summary on the topic:
«Mechanical movement. Relativity of motion. Material point »
Completed by: physics teacher
first qualification category
Safin D.R.
Lesson 1
“Mechanical movement. Relativity of motion. Material point".
Lesson objectives:
explain the need to study mechanics. Show the possibilities of its practical application. To formulate students’ idea of a material point; Give students an idea of the relativity of motion.
During the classes
- Organizing time.
In the introductory part of the lesson, the teacher explains what the students will learn this school year. Reminds them of safety precautions in physics lessons and during laboratory work.
Next, you need to remember what physics is.
Physics is a science that studies the most general properties of the world around us.
Main sections of physics: mechanics, dynamics, statics, thermodynamics, molecular physics, electrodynamics, atomic and nuclear physics, quantum physics, optics, elementary particles, structure and evolution of the Universe, etc.
- Learning new material.
In this lesson we will get acquainted with the definition of a material point, consider determining the position of different bodies using coordinates. In addition, we will consider what a reference system is and why it is needed.
Mechanics is the science of movement and interaction of macroscopic (from the Greek “ makros ” - large, long) bodies.
The name “Mechanics” comes from the Greek word “mechanike” (the science of machines, the construction of machines). The first simple machines - lever, wedge, wheel, inclined plane, etc. Today they are called the simplest mechanisms.
The first works on mechanics that have reached us, in which the simplest mechanisms are described, belong to scientists of Ancient Greece.
Essays:
- “Physics” by Aristotle (IV century BC). In this work, the term “mechanics” was first introduced into science;
- The ancient Greek scientist Archimedes formulated the law of equilibrium of the lever and the law of floating bodies (III century BC);
- G. Galileo formulated the law of inertia, and also established the laws of falling bodies and oscillations of a pendulum. Further, the English physicist Isaac Newton, relying on the work of Galileo, created the doctrine of mechanical motion and interaction of bodies. Today this branch of physics is called “Classical mechanics”.
Classical mechanics (mechanics) is divided into three sections: kinematics, dynamics and statics.
The word “kinematics” comes from the Greek word “kinematos” - movement. Kinematics studies the movement of bodies, but does not find out why this body moves in one way and not another (there is no reason for the movement).
The main task of kinematics is to determine the location of bodies at any time (find coordinates). To describe motion, special concepts (material point, reference system, trajectory) and quantities (path, displacement, acceleration and speed) are introduced.
Now let's study mechanical motion.
We are interested in the fact that the body was in one place, and after some time it ended up in another. How would you describe it? For example, a car was in the parking lot in the morning, and then drove up to the house. Looking out the window, you will point with your finger where he was in the morning, and then show where he is standing now.
But at the same time, it should be noted that the same body can simultaneously move and not move (you are in the classroom at rest relative to the Earth, but at the same time you are moving around the Sun together with the Earth).
Mechanical movement is a change in the position of a body in space relative to other bodies over time.
Sometimes even the simplest body movement is difficult to learn. In this case, a number of simplifications are introduced. For example, if we consider the movement of a 4 m long car traveling 150 km, then the distance it covers is 37,500 times its own length. It is in this case that the car is considered as a (material) point.
A material point is a body whose dimensions can be neglected in the conditions of this problem.
Any model has its limits of application and not all bodies can be considered material points and not in all cases. If we consider the movement of a car from a parking lot to a house, it can be considered a material point; its dimensions are not important.
But if we're considering how it will fit in a parking lot between two adjacent cars, its size and shape need to be taken into account.
Relativity of motion
manifests itself in the fact that speed, trajectory, path and some other characteristics of movement are relative (different in different reference systems).
Let's say a person sitting motionless on a moving platform watches a watermelon lying on the same platform. For him, the watermelon is at rest. At the same time, a person located near the railway track, mentally connecting the frame of reference with the ground, will see that the watermelon is moving.
- Consolidation of the studied material.
Exercise.
In which of the listed cases can bodies be considered material points, and in which cannot?
- After the athlete throws the disc, it flies to a distance of 40 m (material point).
- A skier runs the competition distance. (Material point, but not always: don’t forget about the photo finish).
- A skater performs free program exercises (Not a material point).
- The Earth moves in a circular orbit around the Sun (Material point).
- The earth rotates around its axis. (Not a material point).
- Homework.
- Read paragraph 1 (1-2).
- Complete exercises 1.1.- 1.6. according to Gendenstein's problem book.
Movement speed. First problems in physics
Physical education lesson. A school running competition is underway. No matter how hard Artem tried (he ran 60 m in 12 seconds), the winner’s place went to Denis, who ran the distance in 10 seconds. This means that Denis ran faster than his opponent. It took him less time to cover the same path.
What characterizes the speed of movement? Seventh-graders are familiar with the word “speed,” which precisely defines the speed of movement. The speed of an airplane is greater than the speed of a car, but less than the speed of a rocket.
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The highest speed in nature is light (300,000 km/s); nothing can move faster than light.
How to find speed? In the example given, Denis ran 60 meters in 10 seconds, which means he ran 6 meters in a second, and Artem spent 12 seconds in 60 meters, i.e., he ran 5 meters in 1 second. Denis was ahead of Artem by 1 m for one second, which means he ran faster.
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Speed is the distance traveled by a body per unit time. When solving problems, it is irrational to write down a rule each time showing how to find the desired value, for example, “To calculate the speed, you need to divide the distance traveled by the time of this journey,” then compose a numerical expression and count. Therefore, in physics and other sciences the concept of “formula” is used. A formula is a rule written using letters.
If you enter the following symbols: speed – v, distance traveled – s, time – t, then the rule for calculating speed will be written briefly and clearly:
v = s/t
This is the very first formula of seventh grade physics. It's called the speed formula. It is important to remember formulas and be able to apply them to specific tasks.
Speed is measured in m/s (SI), i.e. the unit of distance is divided by the unit of time, following the formula. Different units are used. For example, traffic movement is often measured in km/h.
For example, is a driver breaking the rules if a passenger car is moving at a speed of 20 m/s, and there is a sign on the side of the road with a speed limit of “60”? On road signs, speed is taken in km/h. This means that the speed of the car must also be converted to km/h. 1 m = 0.001 km, 1 h = 3600 s.
20 m/s = 20 ∙ 0.001 ∙ 3600 km/h = 72 km/h.
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The conclusion is that the driver violated traffic rules.
Now you need to learn how to correctly solve and formulate physical problems. There is a certain order of solving:
- Write down the conditions of the problem in abbreviated form;
- Express the given quantities in SI;
- Write down the required formula;
- Do mathematical calculations;
- Write the resulting result in the answer.
Problem one: A jumping dragonfly flies at a speed of 36 km/h, and a starling flies at a speed of 12 m/s. Will the starling catch up with the dragonfly?
Sample task format:
Task two: The cheetah is considered the fastest animal in the world. In pursuit of prey, he runs 36 km in 20 minutes. What is the speed of a cheetah?
Sample task format:
Knowing the speed, it is easy to determine the distance traveled. For example, a shark has a speed of 10 m/s, which means that in 1 s it will swim 10 m, in 2 s - 20 m, in 3 s - 30 m, etc. And in 15 seconds? You need to multiply the speed by the time. The result is 150 m. There is also a rule for the distance traveled, which can be written as a path formula:
s = v ∙ t
and the time formula is:
t = s/v
Problem three: Old Man Hottabych with Volka and Mishka went out of town in a Volga car moving at a speed of 108 km/h. They drove 2 hours to the rest stop. At what distance from the city did you stop?
Solution:
In the case of this problem, there is no need to convert units to SI. They correspond to each other (time is given in hours and speed in kilometers per hour, not meters per second) and give a realistic idea of time and distance.
In the examples given, it was assumed that the speed did not change throughout the entire path, i.e., the movement was rectilinear and uniform. What about uneven movement? From its definition it turns out that the speed of a body is different on individual sections of the path.
Uneven movement is characterized by another value - average speed. To find it, you need to divide the path (even if it consists of separate sections) by the total time of movement.
vav = s/t
The average speed of a wolf when running is 16 m/s, this does not mean that it runs at this speed all the time. He runs one section of the path at a speed of 18 m/s, another at a speed of 14 m/s, and on average 16 m/s.
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Sometimes the average speed is considered the speed of uniform motion. For example, a bus moves at a speed of 60 km/h. But this is his average speed. At stops the bus slows down and then picks up speed; on smooth sections of the road it goes a little faster, on rough sections slower. So the speed that is obtained on average is taken.
Speed means fast or slow.
§ 9. Mechanical motion and its types
Chapter 1. Mechanical phenomena- Give examples of the movement of physical bodies.
1. Look carefully around you. You will notice that the bodies and objects around you behave differently. Some are at rest: the table at which you are sitting, the blackboard, portraits of scientists on the wall of the classroom. Others are moving: a teacher demonstrating an experiment, the hands of a clock on the wall, a book falling from a desk. Looking out the window, you will see even more moving bodies: cars, bicycles, pedestrians, etc. Imagine that you are in the forest. Everything seems to move here: leaves on the trees and even their trunks, grass, bugs, dragonflies, clouds in the sky. All these movements are very different. They are united by one common property: all bodies change their position in space
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| The change in the position of a body in space relative to other bodies over time is called mechanical motion. |
2. Consider Figure 22. The same ball on a thread attached to point D makes different movements. Let us mark points A and C on the surface of the ball and observe how the position of these points changes during movement. In case a) all points move equally, any straight line drawn in the body moves parallel to itself. This movement is called translational . In case b) the ball moves in a circle, and in case c) it oscillates. These are examples of other types of mechanical motion - rotational and oscillatory, respectively.
There are a lot of examples of mechanical movement. A car moving on the highway; sleds rolling down the mountain; an airplane during takeoff and landing are all examples of forward motion. Rotational motion is performed by the minute and hour hands of a clock, a child on a carousel, and the Moon around the Earth. Examples of oscillatory motion are the movements of a pendulum on a wall clock, a boy on a swing, or the strings of a playing guitar.
Mechanical movement is the simplest type of movement. You will become familiar with other, more complex types of movement in physics lessons later.
- Self-test questions
1. What is called mechanical movement?
2. What types of mechanical movement do you know?
3. What type of mechanical motion - translational, rotational or oscillatory - are the movements of the following bodies:
- — train in the subway;
- aspen leaf in the wind;
— Earth around the Sun;
— fan blades;
- wings of a flying butterfly?
When answering the question, make a table of three columns corresponding to three different types of movement and fill it out. Complete the table with your own examples.
4. What instruments are needed to study movement?
5*. Give examples of movements other than mechanical.
Vector – number and direction
If an arrow sign is used somewhere, then it is clear that it shows the direction in which to move. Does speed have a direction? Where will the bus be if it moves from the stop at a speed of 70 km/h? The location of the bus cannot be named, since it is unknown where the bus goes from the stop and in what direction. Speed also has a numerical value. It can be small and large. A turtle moves slowly, but a cheetah runs fast.
It turns out that speed has a numerical value and a direction. Such quantities are called vector quantities (simply vectors).
A vector quantity is indicated by an arrow above the letter, for example, . The numerical value of a vector quantity is written with two vertical bars and is called the vector modulus. For example, the modulus of the bus speed vector | | = 70 km/h. Knowing the absolute value of the velocity vector, it is possible to calculate the distance the bus has traveled from the stop, and with the direction of the vector known (i.e. which direction from the stop) it is already determined by the location of the bus.
Quantities that have no direction are called scalars. Examples of scalar quantities: temperature, time, volume, area, length.
A vector is depicted as a directed segment, that is, having a beginning and an end. The end of the segment is marked with an arrow.
Basic properties of vectors
Two or more vectors with the same magnitude and direction and are equal.
- Vectors that differ in direction are not equal, even if their numerical values are equal.
- The sum of identically directed vectors in modulus is equal to the sum of the moduli of these vectors and has the same direction.
Using this rule, the speed with the current is found. The motor ship has its own speed, which can be created by the motor ship's engines. The river flow has its own speed. When moving with the flow, the river seems to help the ship. The speeds add up. The numerical result is greater. For example, the ship's own speed is 60 km/h, and the speed of the river is 2 km/h. The resulting absolute speed is 62 km/h, but the direction remains the same.
- If the vectors are directed in opposite directions, then the resulting vector is directed towards the larger of them, and its numerical value is equal to the difference in the numerical values of these vectors.
Using this rule, the speed upstream is found. When moving against the current, the river seems to interfere with the movement of the ship, pushing it backwards with the current. This means that the modulus of the current velocity must be subtracted from the modulus of the speed of the ship. It is important here that the vector of its own speed is greater than the vector of the current speed, otherwise the ship will move backward, even with constant operation of the engines. For example, a motor ship with its own speed of 60 km/h moves in the opposite direction against the flow of a river whose speed is 2 km/h. As a result, the motor ship will have a numerical speed of 58 km/h, the direction of the speed vector will be against the current.
- Vectors can be multiplied, divided, added. How this is done is studied in mathematics lessons.
In physics, the properties of vectors are used in the study of quantities that have magnitude and direction. The first of these is speed.
