Who do you think moves faster agronomist Vasechkin, Renault car Or a Boeing plane? Which of them will get from Moscow to Krasnodar faster? The answer is obvious. Renault is faster than Vasechkin, but slower than Boeing.
That is, we not only know how different objects move, but we can also compare their speeds. What is speed in physics? How to find the speed of a body, and what are the units of speed?
Speed in physics: how to find speed?
In the 7th grade, the concept of speed is introduced in physics lessons. Without a doubt, all schoolchildren by this moment are already familiar with this word and imagine what it means. They also know that speed is measured in km/h. But it is unlikely that they can coherently explain what speed is in physics, what are the units of speed. That is why this seemingly simple concept requires explanation and analysis.
In physics, the speed of movement of Vasechkin, Renault and Boeing is called the speed of their movement. And this speed characterizes which path each of the participants of this journey overcomes per unit of time. And if in flight we will overcome the distance of 1350 kilometers between Moscow and Krasnodar in two hours, by car we will need no less than 15 hours, then on foot the reckless Vasechkin will be able to walk his entire vacation at a brisk pace and arrive at the place only in order to to kiss my mother-in-law, to eat pancakes and to catch a plane to Moscow in order to be in time for work on Monday. Accordingly, per unit of time per hour, the plane will fly 670 kilometers, the car will travel 90 kilometers, and the tourist Vasechkin will wave as many as five kilometers of the road. And then they say that the speed of an airplane is 670 kilometers per hour, a car is 90 kilometers per hour, and a pedestrian is 5 kilometers per hour. That is, the speed is determined by dividing the distance traveled per unit of time by an hour, a minute, or a second.
Speed units
In practice, units such as km / h, m / s and some others are used. They denote speed with the letter v, distance with the letter s, and time with the letter t. Formula for finding speed in physics looks like this: v=(s)/(t).
And if we need to recalculate the speed not in kilometers per hour, but in meters per second, then the recalculation takes place as follows. Since 1 km = 1000 m, and 1 h = 60 min = 3600 s, we can write: 1 km / h = (1000 m) / (3600 s). And then the speed of the aircraft will be equal to: 670 km / h \u003d 670 × (1000 m) / (3600 s) \u003d 186 m / s
In addition to its numerical value, the speed also has a direction, therefore, in the figures, the speed is indicated by an arrow and is called a vector quantity.
Average speed in physics
Let's note one more point. In our example, the driver of the car was driving at a speed of 90 km/h. On the highway, he could drive evenly at that speed for a long time. But passing different cities along the way, he either stopped at traffic lights, then crawled in traffic jams, then dialed in short fits good speed. Those. his speed on different parts of the path was uneven. In this case, the concept of average speed is introduced. Average speed in physics is denoted v_av and is considered in the same way as the speed at uniform motion. Just take the total travel distance and divide by the total time.
Example 1
For example, a car is moving along a road and there are people in it. They carry out the movement along with the transport on the highway. That is, people move in space relative to the road, but relative to the car itself, people do not move.
From this example it can be seen that, initially, it is necessary to determine the body considered in motion, which in the sciences is called the reference point. The coordinate system is closely interconnected with the method of measuring time, which as a result creates the concept of reference.
Basically, the location of the body is given by a coordinate. Let's analyze one example: the size of a station in orbit near the Earth can be ignored, and only the trajectory of movement can be calculated spaceship while docking at the station. Thus, the dimensions of physical elements can be neglected, and sometimes the body is considered a material point. The line along which the given value moves is called the trajectory, the length of which is called the path. The unit of the path is the meter (m). mechanical movement characterized by three physiological quantities: speed, displacement, and acceleration.
The concept of the speed of mechanical movement
Definition 2
Velocity is a physical quantity that is equal to the movement of the body to the time interval during which this interaction occurred.
Mechanical movement is also evaluated by how quickly the body (point) moves. This is the speed of movement. Speed is a concept of a vector quantity. In order to fully set it, it is necessary to establish directly the direction and magnitude of the speed along which it was originally measured. As a rule, the speed of the elements is considered along the trajectory of motion. In this case, the value of the object under study is determined as the path traveled in one unit of time. In other words, to find the correct coefficient of the trajectory of motion, the path of the body must be divided by the time during which it was passed.
Definition 3
Instantaneous speed is the speed of a point at a particular moment in time or at a particular point in the trajectory.
This is a vector physical quantity numerically equal to the limit to which average speed for a very short period of time. The indicated trajectory is the first derivative of the vector with respect to time. The instantaneous velocity vector is determined tangentially to the line of motion of the body in the direction of its further movement.
This value gives an accurate representation of the movement of an object in this moment time.
For example, during a trip in a car at a certain point in time, the driver looks at the speedometer and sees that the display shows 100 km / h. Then the arrow points to 90 km / h, and after a couple of minutes - 110 km / h.
Remark 1
The value of the instantaneous speed of transport at certain points in time is the received instrument readings.
Is there a physical meaning in the concept of "instantaneous velocity"? This term is characterized by a change in the movement of elements in space. But in order to find out how its location has changed, one should observe the movement over a certain period of time.
Even the most advanced speed measuring devices measure movement over a specific period of time - a finite time interval. The definition of "the speed of the body at the moment" is not considered correct from the point of view of physics. However, it is this thesis that is very convenient in mathematical calculations, so it is used all the time.
The law of addition of speeds
The speed of any physical body relative to a fixed concept of reference is always equal to the vector sum of the movement of elements relative to the moving system. This theory helps to accurately determine the location of an object in a particular period of time.
To understand this law, it is necessary to consider two frames of reference, one of which is associated with a fixed reference point $O$. We denote this concept by $K$, which will be called fixed.
The second system, denoted by $K'$ and moving relative to the body $O$ with the speed $ \bar(u)$ - will be considered as moving.
It must be understood that speed is a vector quantity. It is possible to determine only the direction of the velocity of the vector from the trajectory of motion. The velocity vector is directed tangentially to the trajectory along which the body passes, which is currently moving.
Negative speed
Remark 2
The speed of the body can be negative in the case when the body moves in the opposite direction from the coordinate axis in the selected frame of reference.
A British scientist, Roberta Boyd, was able to assign a "negative" speed to the beam of light, at which the peak of the pulse moved towards the source, and not away from it. Interestingly, if you change the medium in a special way and pass light through it, it is possible to easily control the speed of the light pulse - "freezing" or slowing it down tens of thousands of times, or even stopping it altogether.
In this aspect, we are talking about the group velocity, which determines the propagation speed of one beam of light pulses. Due to scattering, this element can move several orders of magnitude slower than each individual photon, and vice versa - faster than the speed of light in vacuum.
IN this case we are not talking about a violation of the laws of nature, because the very first photons in the impulse reach the end, not " faster than light". In the case of stopping the light beam, it is necessary to speak about the absorption of the pulse by the prepared medium with re-emission. This saves all important parameters the original object, "to the last photon."
This topic will be useful not only for high school students, but even for adults. In addition, the article will be of interest to parents who want to explain simple things from the natural sciences to their children. Among the very important topics is speed in physics.
Quite often, students cannot figure out how to solve problems, distinguish between the available types of speeds, and it is even more difficult to understand scientific definitions. Here we will consider everything in a more accessible language, so that everything is not only clear, but even interesting. But you still have to remember some things, because Technical science(physics and mathematics) require memorizing formulas, units of measurement and, of course, the meanings of symbols in each formula.
Where is it found?
To begin with, let's recall that this topic refers to such a section of physics as mechanics, subsection "Kinematics". In addition, the study of speed does not end here, it will be in the following sections:
- optics,
- vibrations and waves
- thermodynamics,
- quantum physics and so on.
Also, the concept of speed is found in chemistry, biology, geography, computer science. In physics, the topic "velocity" occurs most often and is studied in depth.
In addition, this word is used in everyday life by all of us, especially among motorists, drivers transport technology. Even experienced cooks sometimes use a phrase such as "beat the egg whites with a mixer at medium speed."
What is speed?
Velocity in physics is a kinematic quantity. It means the distance traveled by a body in a certain period of time. Let's say a young man moves from home to a store, covering two hundred meters in one minute. On the contrary, his old grandmother will pass by the same route in six minutes in small steps. That is, the guy moves much faster than his elderly relative, as he develops speed much more, taking very fast long steps.
The same is true for a car: one car goes faster and the other slower because the speeds are different. Later we will consider numerous examples related to this concept.
Formula
At the lesson at school, the speed formula in physics is necessarily considered in order to make it convenient to solve problems.
- V is, respectively, the speed of movement;
- S is the distance covered by the body when moving from one point in space to another;
- t - travel time.
You should remember the formula, because it will come in handy in the future when solving many problems and not only. For example, you might be wondering how fast you can get from home to work or school. But you can find out the distance in advance using a map on your smartphone or computer, or using a paper version, knowing the scale and having a ruler with you. Next, you note the time before you start moving. Arriving at your destination, see how many minutes or hours it took to pass without stopping.
What is measured?
Speed is most often measured using the SI system of units. Below are not only units, but also examples of where they are applied:
- km/h (kilometer per hour) - transport;
- m/s (meter per second) - wind;
- km/s (kilometer per second) - space objects, rockets;
- mm/h (millimeter per hour) - liquids.
Let's first figure out where the fractional line came from and why the unit of measurement is just that. Pay attention to the physics formula for speed. What do you see? The numerator is S (distance, path). How is distance measured? In kilometers, meters, millimeters. In the denominator, respectively, t (time) - hours, minutes, seconds. Hence, the units of measurement of the quantity are exactly the same as presented at the beginning of this section.
Let's consolidate with you the study of the velocity formula in physics as follows: what distance will the body overcome in a specific period of time? For example, a person walks 5 kilometers in 1 hour. Total: the speed of a person is 5 km / h.
What does it depend on?
Often teachers ask students the question: "What determines the speed?". Students often get lost and don't know what to say. In fact, everything is very simple. Just look at the formula for a hint to pop up. The speed of a body in physics depends on the time of motion and distance. If at least one of these parameters is unknown, it will be impossible to solve the problem. In addition, other types of speeds can be found in the example, which will be discussed in the following sections of this article.
In many tasks in kinematics, you have to build dependency graphs, where the X-axis is time, and the Y-axis is distance, path. From such images, one can easily assess the nature of the speed of movement. It is worth noting that in many professions related to transport, electric machines often use graphics. For example, on the railroad.
At the right time, measure the speed
There is another topic that scares middle school students - instantaneous speed. In physics, this concept occurs as a definition of the magnitude of speed in an instantaneous period of time.
Let's look at a simple example: the driver is driving a train, his assistant is watching the speed from time to time. You can see it in the distance. You should check how fast the train is moving right now. The driver's assistant reports at 4:00 pm that the speed is 117 km/h. This is the instantaneous speed recorded exactly at 4 pm. Three minutes later, the speed was 98 km/h. This is also the instantaneous speed relative to 16 hours 03 minutes.
Start of movement
Without the initial speed, physics does not represent almost any movement of transport equipment. What is this parameter? This is the speed at which the object starts moving. Let's say a car cannot start moving instantly at a speed of 50 km/h. She needs to speed up. When the driver presses the pedal, the car begins to move smoothly, for example, at a speed of 5 km/h first, then gradually 10 km/h, 20 km/h and so on (5 km/h is the initial speed).
Of course, you can make abrupt start, which happens to runners-athletes when hitting a tennis ball with a racket, but still there is always an initial speed. By our standards, only the stars, planets and satellites of our Galaxy do not have it, since we do not know when the movement began and how. Indeed, until death, space objects cannot stop, they are always in motion.
uniform speed
Speed in physics is a combination of individual phenomena and characteristics. There are also uniform and non-uniform motion, curvilinear and rectilinear. Let's give an example: a person walks along a straight road with the same speed, overcoming a distance of 100 meters from point A to point B.
On the one hand, this can be called rectilinear and uniform speed. But if you attach very accurate speed and route sensors to a person, you can see that there is still a difference. Uneven speed is when the speed changes regularly or constantly.
In everyday life and technology
The speed of motion in physics exists everywhere. Even microorganisms move, albeit at a very slow speed. It is worth noting that there is rotation, which is also characterized by speed, but has a unit of measurement - rpm (revolutions per minute). For example, the speed of rotation of the drum in washing machine. This unit measurement is used wherever there are mechanisms and machines (engines, motors).
In geography and chemistry
Even water has a speed of movement. Physics is just a subsidiary science in the field of processes occurring in nature. For example, wind speed, waves in the sea - all this is measured by the usual physical parameters, quantities.
Surely, many of you are familiar with the phrase "the rate of a chemical reaction." Only in chemistry does it have a different meaning, since it means how long this or that process will take place. For example, potassium permanganate will dissolve faster in water if you shake the vessel.
Stealth Speed
There are invisible things. For example, we cannot see how particles of light, various radiations move, how sound propagates. But if there were no movement of their particles, then none of these phenomena would exist in nature.
Computer science
Almost every modern person is faced with the concept of "speed" while working on a computer:
- Internet speed;
- page loading speed;
- processor loading speed and so on.
There are a huge number of examples of the speed of movement in physics.
Reading the article carefully, you got acquainted with the concept of speed, learned what it is. Let this material help you study the "Mechanics" section in depth, show interest in it and overcome fear when answering in the lessons. After all, speed in physics is a common concept that is easy to remember.
Who do you think moves faster, the agronomist Vasechkin, a Renault car or a Boeing plane? Which of them will get from Moscow to Krasnodar faster? The answer is obvious. Renault is faster than Vasechkin, but slower than Boeing.
That is, we not only know how different objects move, but we can also compare their speeds. What is speed in physics? How to find the speed of a body, and what are the units of speed?
Speed in physics: how to find speed?
In the 7th grade, the concept of speed is introduced in physics lessons. Without a doubt, all schoolchildren by this moment are already familiar with this word and imagine what it means.
- They also know that speed is measured in km/h and is denoted by the letter V.
But it is unlikely that they can coherently explain what speed is in physics, what are the units of speed. That is why this seemingly simple concept requires explanation and analysis.
In physics, the speed of movement of Vasechkin, Renault and Boeing called their speed. And this speed characterizes which path each of the participants of this journey overcomes per unit of time. And if in flight we will overcome the distance of 1350 kilometers between Moscow and Krasnodar in two hours, by car we will need no less than 15 hours, then on foot the reckless Vasechkin will be able to walk his entire vacation at a brisk pace and arrive at the place only in order to to kiss my mother-in-law, to eat pancakes and to catch a plane to Moscow in order to be in time for work on Monday.
Accordingly, per unit of time per hour, the plane will fly 670 kilometers, the car will travel 90 kilometers, and the tourist Vasechkin will wave as many as five kilometers of the road. And then they say that the speed of an airplane is 670 kilometers per hour, a car is 90 kilometers per hour, and a pedestrian is 5 kilometers per hour. That is, the speed is determined by dividing the distance traveled per unit of time by an hour, a minute, or a second.
Speed units
In practice, units such as km / h, m / s and some others are used. They denote speed with the letter v, distance with the letter s, and time with the letter t. Formula for finding speed in physics looks like that:
- V = s / t,
Where s is the distance traveled
t is the time taken to overcome this path
And if we need to recalculate the speed not in kilometers per hour, but in meters per second, then the recalculation takes place as follows. Since 1 km = 1000 m, and 1 h = 60 min = 3600 s, we can write: 1 km / h = (1000 m) / (3600 s). And then the speed of the aircraft will be equal to: 670 km / h \u003d 670 × (1000 m) / (3600 s) \u003d 186 m / s
In addition to its numerical value, the speed also has a direction, therefore, in the figures, the speed is indicated by an arrow and is called a vector quantity.
Average speed in physics
Let's note one more point. In our example, the driver of the car was driving at a speed of 90 km/h. On the highway, he could drive evenly at that speed for a long time. But passing through different cities along the way, he either stopped at traffic lights, crawled in traffic jams, or picked up a good speed in short bursts.
Those. his speed on different parts of the path was uneven. In this case, the concept of average speed is introduced. The average speed in physics is denoted by V _av and is considered the same as the speed with uniform motion. Just take the total travel distance and divide by the total time.
Speed is a quantitative characteristic of the movement of the body.
average speed is a physical quantity equal to the ratio of the point displacement vector to the time interval Δt during which this displacement occurred. The direction of the average speed vector coincides with the direction of the displacement vector The average speed is determined by the formula:
Instant Speed, that is, the speed at a given moment of time is a physical quantity equal to the limit to which the average speed tends with an infinite decrease in the time interval Δt:
In other words, the instantaneous speed at a given moment of time is the ratio of a very small movement to a very small period of time during which this movement occurred.
The instantaneous velocity vector is directed tangentially to the trajectory of the body (Fig. 1.6).
Rice. 1.6. Instantaneous velocity vector.
In the SI system, speed is measured in meters per second, that is, the unit of speed is considered to be the speed of such uniform rectilinear motion, in which in one second the body travels a distance of one meter. The unit of speed is denoted m/s. Often speed is measured in other units. For example, when measuring the speed of a car, train, etc. The commonly used unit of measure is kilometers per hour:
1 km/h = 1000 m / 3600 s = 1 m / 3.6 s
1 m/s = 3600 km / 1000 h = 3.6 km/h
Addition of speeds
The speed of the body in various systems reference connects the classical law of addition of speeds.
body speed relative to fixed frame of reference is equal to the sum of the velocities of the body in moving frame of reference and the most mobile frame of reference relative to the fixed one.
For example, passenger train is moving along the railroad at a speed of 60 km/h. A person is walking along the carriage of this train at a speed of 5 km/h. If we consider the railway to be motionless and take it as a frame of reference, then the speed of a person relative to the frame of reference (that is, relative to railway), will be equal to the addition of the speeds of the train and the person, that is
60 + 5 = 65 if the person is walking in the same direction as the train
60 - 5 = 55 if the person and the train are moving in different directions
However, this is only true if the person and the train are moving along the same line. If a person moves at an angle, then this angle will have to be taken into account, remembering that speed is vector quantity.
Now let's look at the example described above in more detail - with details and pictures.
So, in our case, the railway is fixed frame of reference. The train that is moving along this road is moving frame of reference. The car on which the person is walking is part of the train.
Let's associate the XOY coordinate system with the fixed reference system (Fig. 1.7), and the X P O P Y P coordinate system with the moving reference system (see also the Reference System section). And now let's try to find the speed of a person relative to a fixed frame of reference, that is, relative to the railway.
This displacement addition law. In our example, the movement of a person relative to the railway is equal to the sum of the movements of a person relative to the wagon and the wagon relative to the railway.
Rice. 1.7. The law of addition of displacements.
This is the law speed addition:
The speed of the body relative to the fixed frame of reference is equal to the sum of the velocities of the body in the moving frame of reference and the speed of the moving frame itself relative to the fixed one.
