Do you think you are moving or not when you read this text? Almost each of you will immediately answer: no, I’m not moving. And he will be wrong. Some might say: moving. And they will also be wrong. Because in physics, some things are not quite what they seem at first glance.
For example, the concept of mechanical motion in physics always depends on a reference point (or body). Thus, a person flying on an airplane moves relative to his relatives remaining at home, but is at rest relative to his friend sitting next to him. So, bored relatives or a friend sleeping on a shoulder are, in this case, bodies of reference for determining whether our aforementioned person is moving or not.
Definition of mechanical movement
In physics, the definition of mechanical motion studied in the seventh grade is as follows: the change in the position of a body relative to other bodies over time is called mechanical motion. Examples of mechanical motion in everyday life include the movement of cars, people and ships. Comets and cats. Air bubbles in a boiling kettle and textbooks in a heavy schoolboy’s backpack. And every time a statement about the movement or rest of one of these objects (bodies) will be meaningless without indicating the body of reference. Therefore, in life, most often, when we talk about movement, we mean movement relative to the Earth or static objects - houses, roads, and so on.
Mechanical motion path
It is also impossible not to mention such a characteristic of mechanical movement as trajectory. A trajectory is a line along which a body moves. For example, boot prints in the snow, the trail of an airplane in the sky, and the trail of a tear on a cheek are all trajectories. They can be straight, curved or broken. But the length of the trajectory, or the sum of the lengths, is the path traveled by the body. The path is designated by the letter s. And it is measured in meters, centimeters and kilometers, or in inches, yards and feet, depending on what units of measurement are accepted in this country.
Types of mechanical movement: uniform and uneven movement
What are the types of mechanical movement? For example, when driving a car, the driver moves at different speeds when driving around the city and at almost the same speed when driving on the highway outside the city. That is, it moves either unevenly or evenly. So the movement, depending on the distance traveled in equal periods of time, is called uniform or uneven.
Examples of uniform and uneven movement
There are very few examples of uniform motion in nature. The Earth moves almost uniformly around the Sun, raindrops drip, bubbles float in the soda. Even a bullet fired from a pistol moves straight and evenly only at first glance. Due to friction with the air and the gravity of the Earth, its flight gradually becomes slower and its trajectory decreases. In space, a bullet can move really straight and evenly until it collides with some other body. But with uneven movement the situation is much better - there are many examples. The flight of a ball while playing football, the movement of a lion hunting prey, the travel of chewing gum in the mouth of a seventh grader, and a butterfly fluttering over a flower are all examples of uneven mechanical movement of bodies.
95. Give examples of uniform motion.
It occurs very rarely, for example, the movement of the Earth around the Sun.
96. Give examples of uneven movement.
Movement of a car, plane.
97. A boy slides down a mountain on a sled. Can this movement be considered uniform?
No.
98. Sitting in the carriage of a moving passenger train and observing the movement of an oncoming freight train, it seems to us that the freight train is going much faster than our passenger train was going before it met. Why is this happening?
Relative passenger train, freight train moves with the total speed of passenger and freight trains.
99. The driver of a moving car is in motion or at rest relative to:
a) roads;
b) car seats;
c) gas stations;
d) the Sun;
e) trees along the road?
In motion: a, c, d, d
At rest: b
100. Sitting in the carriage of a moving train, we watch through the window a car that goes forward, then seems motionless, and finally moves backward. How to explain what we see?
Initially, the speed of the car is higher than the speed of the train. Then the speed of the car becomes equal to the speed of the train. After this, the speed of the car decreases compared to the speed of the train.
101. The plane performs a “dead loop”. What trajectory do observers on the ground see?
A circular path.
102. Give examples of the movement of bodies along curved trajectories relative to the ground.
The movement of planets around the Sun; boat movement on the river; Flight of bird.
103. Give examples of the motion of bodies that have a rectilinear trajectory relative to the ground.
Moving train; man walking straight.
104. What types of movement do we observe when writing with a ballpoint pen? Chalk?
Uniform and uneven.
105. Which parts of a bicycle, when moving in a straight line, describe rectilinear trajectories relative to the ground, and which parts – curved ones?
Straight-line: handlebar, saddle, frame.
Curvilinear: pedals, wheels.
106. Why do they say that the Sun rises and sets? What is the reference body in this case?
The reference body is considered to be the Earth.
107. Two cars are moving along a highway so that some distance between them does not change. Indicate relative to which bodies each of them is at rest and relative to which bodies they are moving during this period of time.
The cars are at rest relative to each other. Cars move relative to surrounding objects.
108. The sled is rolling down the mountain; the ball rolls down an inclined chute; The stone released from the hands falls. Which of these bodies are moving forward?
A sled moving forward from the mountain and a stone released from the hands.
109. A book placed on a table in a vertical position (Fig. 11, position I) falls from a push and takes position II. Two points A and B on the binding of the book described the trajectories AA1 and BB1. Can we say that the book moved forward? Why?
Do you think you are moving or not when you read this text? Almost each of you will immediately answer: no, I’m not moving. And he will be wrong. Some might say: moving. And they will also be wrong. Because in physics, some things are not quite what they seem at first glance.
For example, the concept of mechanical motion in physics always depends on a reference point (or body). Thus, a person flying on an airplane moves relative to his relatives remaining at home, but is at rest relative to his friend sitting next to him. So, bored relatives or a friend sleeping on a shoulder are, in this case, bodies of reference for determining whether our aforementioned person is moving or not.
Definition of mechanical movement
In physics, the definition of mechanical motion studied in the seventh grade is as follows: the change in the position of a body relative to other bodies over time is called mechanical motion. Examples of mechanical motion in everyday life include the movement of cars, people and ships. Comets and cats. Air bubbles in a boiling kettle and textbooks in a heavy schoolboy’s backpack. And every time a statement about the movement or rest of one of these objects (bodies) will be meaningless without indicating the body of reference. Therefore, in life, most often, when we talk about movement, we mean movement relative to the Earth or static objects - houses, roads, and so on.
Mechanical motion path
It is also impossible not to mention such a characteristic of mechanical movement as trajectory. A trajectory is a line along which a body moves. For example, boot prints in the snow, the trail of an airplane in the sky, and the trail of a tear on a cheek are all trajectories. They can be straight, curved or broken. But the length of the trajectory, or the sum of the lengths, is the path traveled by the body. The path is designated by the letter s. And it is measured in meters, centimeters and kilometers, or in inches, yards and feet, depending on what units of measurement are accepted in this country.
Types of mechanical movement: uniform and uneven movement
What are the types of mechanical movement? For example, when driving a car, the driver moves at different speeds when driving around the city and at almost the same speed when driving on the highway outside the city. That is, it moves either unevenly or evenly. So the movement, depending on the distance traveled in equal periods of time, is called uniform or uneven.
Examples of uniform and uneven movement
There are very few examples of uniform motion in nature. The Earth moves almost uniformly around the Sun, raindrops drip, bubbles float in the soda. Even a bullet fired from a pistol moves straight and evenly only at first glance. Due to friction with the air and the gravity of the Earth, its flight gradually becomes slower and its trajectory decreases. In space, a bullet can move really straight and evenly until it collides with some other body. But with uneven movement the situation is much better - there are many examples. The flight of a ball while playing football, the movement of a lion hunting prey, the travel of chewing gum in the mouth of a seventh grader, and a butterfly fluttering over a flower are all examples of uneven mechanical movement of bodies.
The simplest type of mechanical motion is the movement of a body along a straight line with constant speed in magnitude and direction. This movement is called uniform . With uniform motion, a body travels equal distances in any equal periods of time. For a kinematic description of uniform rectilinear motion, the coordinate axis OX conveniently positioned along the line of movement. The position of the body during uniform motion is determined by specifying one coordinate x. The displacement vector and the velocity vector are always directed parallel to the coordinate axis OX.
Therefore, the displacement and speed during linear motion can be projected onto the axis OX and consider their projections as algebraic quantities.
If at some point in time t 1 body was at a point with coordinate x 1, and at a later moment t 2 - at the point with coordinate x 2, then the displacement projection Δ s per axis OX in time Δ t = t 2 - t 1 is equal
This value can be both positive and negative depending on the direction in which the body moved. With uniform motion along a straight line, the displacement module coincides with the distance traveled. The speed of uniform rectilinear motion is called the ratio 
If υ > 0, then the body moves towards the positive direction of the axis OX; at v< 0 тело движется в противоположном направлении.
Coordinate dependence x from time t (law of motion) is expressed for uniform linear motion linear mathematical equation :
In this equation, υ = const is the speed of the body, x 0 - coordinate of the point where the body was at the moment of time t= 0. Graph of the law of motion x(t) is a straight line. Examples of such graphs are shown in Fig. 1.3.1.
For the law of motion shown in graph I (Fig. 1.3.1), with t= 0 the body was at the point with coordinate x 0 = -3. Between moments in time t 1 = 4 s and t 2 = 6 s the body moved from the point x 1 = 3 m to point x 2 = 6 m. Thus, for Δ t = t 2 - t 1 = 2 s the body moved by Δ s = x 2 - x 1 = 3 m. Therefore, the speed of the body is
The speed value turned out to be positive. This means that the body moved in the positive direction of the axis OX. Let us note that in the motion graph the speed of a body can be geometrically defined as the aspect ratio B.C. And A.C. triangle ABC(see Fig. 1.3.1)
The greater the angle α that the straight line forms with the time axis, i.e., the greater the slope of the graph ( steepness), the greater the speed of the body. Sometimes they say that the speed of a body is equal to the tangent of the angle α of the inclination of the straight line x (t). From a mathematical point of view, this statement is not entirely correct, since the sides B.C. And A.C. triangle ABC have different dimensions: side B.C. measured in meters, and the side A.C.- in seconds.
Similarly, for the movement shown in Fig. 1.3.1 direct II, we find x 0 = 4 m, υ = -1 m/s.
In Fig. 1.3.2 law of motion x (t) of the body is depicted using straight line segments. In mathematics, such graphs are called piecewise linear. This movement of a body along a straight line not uniform. In different sections of this graph, the body moves at different speeds, which can also be determined by the slope of the corresponding segment to the time axis. At the break points of the graph, the body instantly changes its speed. On the graph (Fig. 1.3.2) this occurs at points in time t 1 = -3 s, t 2 = 4 s, t 3 = 7 s and t 4 = 9 s. From the motion schedule it is easy to find that on the interval ( t 2 ; t 1) the body moved at a speed υ 12 = 1 m/s, over the interval ( t 3 ; t 2) - at a speed υ 23 = -4/3 m/s and at the interval ( t 4 ; t 3) - at a speed υ 34 = 4 m/s.
It should be noted that with a piecewise linear law of rectilinear motion of a body, the distance traveled l does not match the movement s. For example, for the law of motion shown in Fig. 1.3.2, the movement of the body in the time interval from 0 s to 7 s is zero ( s= 0). During this time the body has traveled l= 8 m.
As kinematics, there is a situation in which a body, in any arbitrarily taken equal periods of time, travels along segments of a path of equal length. This is a uniform movement. An example would be the movement of a speed skater in the middle of a distance or a train on a flat stretch.
Theoretically, a body can move along any trajectory, including a curved one. At the same time, there is the concept of path - this is the name of the distance traveled by a body along its trajectory. Path is a scalar quantity and should not be confused with displacement. The last term we denote the segment between the starting point of the path and the final point, which, during curvilinear movement, obviously does not coincide with the trajectory. Displacement is a vector quantity that has a numerical value equal to the length of the vector.
A natural question arises: in what cases are we talking about uniform motion? Will the movement of, for example, a carousel in a circle at the same speed be considered uniform? No, because with such movement the velocity vector changes its direction every second.
Another example is a car traveling in a straight line at the same speed. Such movement will be considered uniform as long as the car does not turn anywhere and its speedometer shows the same number. It is obvious that uniform motion always occurs in a straight line, and the velocity vector does not change. The path and movement in this case will coincide.
Uniform motion is motion along a straight path at a constant speed, in which the lengths of the distance traveled over any equal periods of time are the same. A special case of uniform motion can be considered a state of rest, when the speed and distance traveled are equal to zero.
Speed is a qualitative characteristic of uniform motion. Obviously, different objects travel the same path in different times (pedestrian and car). The ratio of the path traveled by a uniformly moving body to the period of time during which this path was traveled is called the speed of movement.
Thus, the formula describing uniform motion looks like this:
V = S/t; where V is the speed of movement (is a vector quantity);
S - path or movement;
Knowing the speed of movement, which is constant, we can calculate the path traveled by the body in any arbitrary period of time.
Sometimes uniform and uniformly accelerated motion are mistakenly confused. These are completely different concepts. - one of the variants of uneven movement (i.e. one in which the speed is not a constant value), which has an important distinguishing feature - the speed in this case changes by the same amount over the same periods of time. This quantity, equal to the ratio of the difference in speed to the period of time during which the speed changed, is called acceleration. This number, indicating how much the speed has increased or decreased per unit time, can be large (then the body is said to quickly gain or lose speed) or insignificant when the object accelerates or decelerates more smoothly.
Acceleration, like speed, is physical. The direction of the acceleration vector always coincides with the velocity vector. An example of uniformly accelerated motion is the case of an object, in which the attraction of the object by the earth's surface changes per unit time by a certain amount, called the acceleration of gravity.
Uniform motion can theoretically be considered as a special case of uniformly accelerated motion. Obviously, since the speed does not change during such movement, then acceleration or deceleration does not occur, therefore, the magnitude of acceleration during uniform movement is always equal to zero.