It is necessary to know the point of application and the direction of each force. It is important to be able to determine exactly what forces act on the body and in what direction. Force is denoted as , measured in Newtons. In order to distinguish between forces, they are designated as follows
Below are the main forces acting in nature. It is impossible to invent non-existent forces when solving problems!
There are many forces in nature. Here we consider the forces that are considered in the school physics course when studying dynamics. Other forces are also mentioned, which will be discussed in other sections.
Gravity
Every body on the planet is affected by the Earth's gravity. The force with which the Earth attracts each body is determined by the formula
The point of application is at the center of gravity of the body. Gravity always pointing vertically down.
Friction force
Let's get acquainted with the force of friction. This force arises when bodies move and two surfaces come into contact. The force arises as a result of the fact that the surfaces, when viewed under a microscope, are not smooth as they seem. The friction force is determined by the formula:
A force is applied at the point of contact between two surfaces. Directed in the direction opposite to the movement.
Support reaction force
Imagine a very heavy object lying on a table. The table bends under the weight of the object. But according to Newton's third law, the table acts on the object with exactly the same force as the object on the table. The force is directed opposite to the force with which the object presses on the table. That is up. This force is called the support reaction. The name of the force "speaks" react support. This force arises whenever there is an impact on the support. The nature of its occurrence at the molecular level. The object, as it were, deformed the usual position and connections of the molecules (inside the table), they, in turn, tend to return to their original state, "resist".
Absolutely any body, even a very light one (for example, a pencil lying on a table), deforms the support at the micro level. Therefore, a support reaction occurs.
There is no special formula for finding this force. They designate it with the letter, but this force is just a separate type of elastic force, so it can also be denoted as
The force is applied at the point of contact of the object with the support. Directed perpendicular to the support.
Since the body is represented as a material point, the force can be depicted from the center
Elastic force
This force arises as a result of deformation (changes in the initial state of matter). For example, when we stretch a spring, we increase the distance between the molecules of the spring material. When we compress the spring, we decrease it. When we twist or shift. In all these examples, a force arises that prevents deformation - the elastic force.
Hooke's law
The elastic force is directed opposite to the deformation.
Since the body is represented as a material point, the force can be depicted from the center
When connected in series, for example, springs, the stiffness is calculated by the formula
When connected in parallel, the stiffness
Sample stiffness. Young's modulus.
Young's modulus characterizes the elastic properties of a substance. This is a constant value that depends only on the material, its physical state. Characterizes the ability of a material to resist tensile or compressive deformation. The value of Young's modulus is tabular.
Learn more about the properties of solids.
Body weight
Body weight is the force with which an object acts on a support. You say it's gravity! The confusion occurs in the following: indeed, often the weight of the body is equal to the force of gravity, but these forces are completely different. Gravity is the force that results from interaction with the Earth. Weight is the result of interaction with the support. The force of gravity is applied at the center of gravity of the object, while the weight is the force that is applied to the support (not to the object)!
There is no formula for determining weight. This force is denoted by the letter .
The support reaction force or elastic force arises in response to the impact of an object on a suspension or support, therefore the body weight is always numerically the same as the elastic force, but has the opposite direction.
The reaction force of the support and the weight are forces of the same nature, according to Newton's 3rd law they are equal and oppositely directed. Weight is a force that acts on a support, not on a body. The force of gravity acts on the body.
Body weight may not be equal to gravity. It can be either more or less, or it can be such that the weight is zero. This state is called weightlessness. Weightlessness is a state when an object does not interact with a support, for example, a state of flight: there is gravity, but the weight is zero!
It is possible to determine the direction of acceleration if you determine where the resultant force is directed
Note that weight is a force, measured in Newtons. How to correctly answer the question: "How much do you weigh"? We answer 50 kg, naming not weight, but our mass! In this example, our weight is equal to gravity, which is approximately 500N!
Overload- the ratio of weight to gravity
Strength of Archimedes
Force arises as a result of the interaction of a body with a liquid (gas), when it is immersed in a liquid (or gas). This force pushes the body out of the water (gas). Therefore, it is directed vertically upwards (pushes). Determined by the formula:
In the air, we neglect the force of Archimedes.
If the Archimedes force is equal to the force of gravity, the body floats. If the Archimedes force is greater, then it rises to the surface of the liquid, if it is less, it sinks.
electrical forces
There are forces of electrical origin. Occur in the presence of an electric charge. These forces, such as the Coulomb force, Ampère force, Lorentz force, are discussed in detail in the Electricity section.
Schematic designation of the forces acting on the body
Often the body is modeled by a material point. Therefore, in the diagrams, various points of application are transferred to one point - to the center, and the body is schematically depicted as a circle or rectangle.
In order to correctly designate the forces, it is necessary to list all the bodies with which the body under study interacts. Determine what happens as a result of interaction with each: friction, deformation, attraction, or maybe repulsion. Determine the type of force, correctly indicate the direction. Attention! The number of forces will coincide with the number of bodies with which the interaction takes place.
The main thing to remember
1) Forces and their nature;
2) Direction of forces;
3) Be able to identify the acting forces
Distinguish between external (dry) and internal (viscous) friction. External friction occurs between solid surfaces in contact, internal friction occurs between layers of liquid or gas during their relative motion. There are three types of external friction: static friction, sliding friction and rolling friction.
Rolling friction is determined by the formula
The resistance force arises when a body moves in a liquid or gas. The magnitude of the resistance force depends on the size and shape of the body, the speed of its movement and the properties of the liquid or gas. At low speeds, the resistance force is proportional to the speed of the body
At high speeds it is proportional to the square of the speed
Consider the mutual attraction of an object and the Earth. Between them, according to the law of gravity, a force arises 
Now let's compare the law of gravity and the force of gravity 
The value of free fall acceleration depends on the mass of the Earth and its radius! Thus, it is possible to calculate with what acceleration objects on the Moon or on any other planet will fall, using the mass and radius of that planet.
The distance from the center of the Earth to the poles is less than to the equator. Therefore, the acceleration of free fall at the equator is slightly less than at the poles. At the same time, it should be noted that the main reason for the dependence of the acceleration of free fall on the latitude of the area is the fact that the Earth rotates around its axis.
When moving away from the surface of the Earth, the force of gravity and the acceleration of free fall change inversely with the square of the distance to the center of the Earth.
Today we will talk about the unit of measurement of luminous intensity. This article will reveal to readers the properties of photons, which will allow them to determine why light comes in different brightnesses.
Particle or wave?
At the beginning of the twentieth century, scientists were puzzled by the behavior of light quanta - photons. On the one hand, interference and diffraction spoke of their wave nature. Therefore, light was characterized by properties such as frequency, wavelength, and amplitude. On the other hand, they convinced the scientific community that photons transfer momentum to surfaces. This would be impossible if the particles did not have mass. Thus, physicists had to admit: electromagnetic radiation is both a wave and a material object.
Photon energy
As Einstein proved, mass is energy. This fact proves our central luminary, the Sun. A thermonuclear reaction turns a mass of highly compressed gas into pure energy. But how to determine the power of the emitted radiation? Why in the morning, for example, is the luminous intensity of the sun lower than at noon? The characteristics described in the previous paragraph are interconnected by specific relationships. And they all point to the energy that electromagnetic radiation carries. This value changes upwards when:
- decrease in wavelength;
- increasing frequency.
What is the energy of electromagnetic radiation?
A photon is different from other particles. Its mass, and therefore its energy, exists only as long as it moves through space. When colliding with an obstacle, a quantum of light increases its internal energy or gives it a kinetic moment. But the photon itself ceases to exist. Depending on what exactly acts as an obstacle, various changes occur.
- If the obstacle is a solid body, then most often the light heats it up. The following scenarios are also possible: a photon changes direction, stimulates a chemical reaction, or causes one of the electrons to leave its orbit and go to another state (photoelectric effect).
- If the obstacle is a single molecule, for example, from a rarefied gas cloud in outer space, then a photon makes all its bonds vibrate more strongly.
- If the obstacle is a massive body (for example, a star or even a galaxy), then the light is distorted and changes the direction of motion. This effect is based on the ability to "look" into the distant past of the cosmos.
Science and Humanity
Scientific data often seem to be something abstract, inapplicable to life. This also happens with the characteristics of light. When it comes to experimenting or measuring the radiation of stars, scientists need to know the absolute values (they are called photometric). These concepts are usually expressed in terms of energy and power. Recall that power refers to the rate of change of energy per unit time, and in general it shows the amount of work that the system can produce. But man is limited in his ability to perceive reality. For example, the skin feels heat, but the eye does not see the photon of infrared radiation. The same problem with units of luminous intensity: the power that radiation actually shows is different from the power that the human eye can perceive.
Spectral sensitivity of the human eye
We remind you that the discussion below will focus on average indicators. All people are different. Some do not perceive individual colors at all (colorblind). For others, the culture of color does not coincide with the accepted scientific point of view. For example, the Japanese do not distinguish between green and blue, and the British - blue and blue. In these languages, different colors are denoted by one word.
The unit of luminous intensity depends on the spectral sensitivity of the average human eye. The maximum daylight falls on a photon with a wavelength of 555 nanometers. This means that in the light of the sun, a person sees the green color best. Night vision maximum is a photon with a wavelength of 507 nanometers. Therefore, under the moon, people see blue objects better. At dusk, everything depends on the lighting: the better it is, the more “green” the maximum color that a person perceives becomes.
The structure of the human eye
Almost always, when it comes to vision, we say what the eye sees. This is an incorrect statement, because the brain perceives first of all. The eye is only an instrument that transmits information about the light output to the main computer. And, like any tool, the entire color perception system has its limitations.
There are two different types of cells in the human retina - cones and rods. The former are responsible for daytime vision and perceive colors better. The latter provide night vision, thanks to the sticks, a person distinguishes between light and shadow. But they do not perceive colors well. The sticks are also more sensitive to movement. That is why, if a person walks through a moonlit park or forest, he notices every swaying of the branches, every breath of the wind.
The evolutionary reason for this separation is simple: we have one sun. The moon shines by reflected light, which means that its spectrum does not differ much from the spectrum of the central luminary. Therefore, the day is divided into two parts - illuminated and dark. If people lived in a system of two or three stars, then our vision would probably have more components, each of which was adapted to the spectrum of one luminary.
I must say, on our planet there are creatures whose eyesight is different from human. Desert dwellers, for example, detect infrared light with their eyes. Some fish can see near ultraviolet, as this radiation penetrates the deepest into the water column. Our pet cats and dogs perceive colors differently, and their spectrum is reduced: they are better adapted to chiaroscuro.
But people are all different, as we mentioned above. Some representatives of mankind see near infrared light. This is not to say that they would not need thermal cameras, but they are able to perceive slightly redder shades than most. Others have developed the ultraviolet part of the spectrum. Such a case is described, for example, in the film "Planet Ka-Pax". The protagonist claims that he came from another star system. The examination revealed that he had the ability to see ultraviolet radiation.
Does this prove that Prot is an alien? No. Some people can do it. In addition, the near ultraviolet is closely adjacent to the visible spectrum. No wonder some people take a little more. But Superman is definitely not from Earth: the X-ray spectrum is too far from the visible for such vision to be explained from a human point of view.
Absolute and relative units for determining the luminous flux
A quantity independent of the spectral sensitivity, which indicates the flow of light in a known direction, is called a "candela". already with a more "human" attitude is pronounced the same way. The difference is only in the mathematical designation of these concepts: the absolute value has a subscript "e", relative to the human eye - "υ". But do not forget that the sizes of these categories will vary greatly. This must be taken into account when solving real problems.
Enumeration and comparison of absolute and relative values
To understand what the power of light is measured in, it is necessary to compare the "absolute" and "human" values. On the right are purely physical concepts. On the left are the values into which they turn when passing through the system of the human eye.
- The power of radiation becomes the power of light. Concepts are measured in candela.
- Energy brightness turns into brightness. The values are expressed in candela per square metre.
Surely the reader saw familiar words here. Many times in their lives, people say: "Very bright sun, let's go into the shade" or "Make the monitor brighter, the movie is too gloomy and dark." We hope the article will slightly clarify where this concept came from, as well as what the unit of luminous intensity is called.
Features of the concept of "candela"
We have already mentioned this term above. We also explained why the same word is used to refer to completely different concepts of physics related to the power of electromagnetic radiation. So, the unit of measure for the intensity of light is called the candela. But what is it equal to? One candela is the intensity of light in a known direction from a source that emits strictly monochromatic radiation with a frequency of 5.4 * 10 14, and the energy force of the source in this direction is 1/683 watts per unit solid angle. The reader can easily convert frequency into wavelength, the formula is very easy. We will prompt: the result lies in visible area.
The unit of measurement for the intensity of light is called the "candela" for a reason. Those who know English remember that a candle is a candle. Previously, many areas of human activity were measured in natural parameters, for example, horsepower, millimeters of mercury. So it is not surprising that the unit of measurement for the intensity of light is the candela, one candle. Only a candle is very peculiar: with a strictly specified wavelength, and producing a specific number of photons per second.
We are all accustomed in life to use the word strength in comparative terms, saying men are stronger than women, a tractor is stronger than a car, a lion is stronger than an antelope.
Force in physics is defined as a measure of the change in the speed of a body that occurs when bodies interact. If force is a measure, and we can compare the application of different forces, then it is a physical quantity that can be measured. In what units is force measured?
Force units
In honor of the English physicist Isaac Newton, who did enormous research in the nature of the existence and use of various types of force, 1 newton (1 N) is accepted as a unit of force in physics. What is a force of 1 N? In physics, one does not simply choose units of measurement, but makes a special agreement with those units that have already been adopted.
We know from experience and experiments that if a body is at rest and a force acts on it, then the body under the influence of this force changes its speed. Accordingly, to measure the force, a unit was chosen that would characterize the change in the speed of the body. And do not forget that there is also the mass of the body, since it is known that with the same force the impact on different objects will be different. We can throw the ball far, but the cobblestone will fly away a much shorter distance. That is, taking into account all the factors, we come to the definition that a force of 1 N will be applied to the body if a body with a mass of 1 kg under the influence of this force changes its speed by 1 m / s in 1 second.
Gravity unit
We are also interested in the unit of gravity. Since we know that the Earth attracts to itself all the bodies on its surface, then there is a force of attraction and it can be measured. And again, we know that the force of attraction depends on the mass of the body. The greater the mass of the body, the stronger the Earth attracts it. It has been experimentally established that The force of gravity acting on a body of mass 102 grams is 1 N. And 102 grams is approximately one tenth of a kilogram. And to be more precise, if 1 kg is divided into 9.8 parts, then we will just get approximately 102 grams.
If a force of 1 N acts on a body weighing 102 grams, then a force of 9.8 N acts on a body weighing 1 kg. The acceleration of free fall is denoted by the letter g. And g is 9.8 N/kg. This is the force that acts on a body of mass 1 kg, accelerating it every second by 1 m / s. It turns out that a body falling from a great height picks up a very high speed during the flight. Why then do snowflakes and raindrops fall quite calmly? They have a very small mass, and the earth pulls them towards itself very weakly. And the air resistance for them is quite large, so they fly to the Earth with not very high, rather the same speed. But meteorites, for example, when approaching the Earth, gain a very high speed and upon landing, a decent explosion is formed, which depends on the size and mass of the meteorite, respectively.
We already know that a physical quantity called force is used to describe the interaction of bodies. In this lesson, we will take a closer look at the properties of this quantity, the units of force and the device that is used to measure it - with a dynamometer.
Topic: Interaction of bodies
Lesson: Units of force. Dynamometer
First of all, let's remember what power is. When another body acts on a body, physicists say that a force acts on this body from the other body.
Force is a physical quantity that characterizes the action of one body on another.
Strength is denoted by a Latin letter F, and the unit of force in honor of the English physicist Isaac Newton is called newton(we write with a small letter!) and is designated H (we write a capital letter, since the unit is named after the scientist). So,
Along with the newton, multiple and submultiple units of force are used:
kilonewton 1 kN = 1000 N;
meganewton 1 MN = 1000000 N;
millinewton 1 mN = 0.001 N;
micronewton 1 µN = 0.000001 N, etc.
Under the action of a force, the speed of the body changes. In other words, the body begins to move not uniformly, but accelerated. More precisely, uniformly accelerated: for equal intervals of time, the speed of the body changes equally. Exactly speed change physicists use bodies under the influence of a force to determine the unit of force in 1 N.
Units of measurement of new physical quantities are expressed through the so-called basic units - units of mass, length, time. In the SI system, this is the kilogram, meter and second.
Let, under the action of some force, the speed of the body weighing 1 kg changes its speed 1 m/s for every second. It is this force that is taken for 1 newton.
one newton (1 N) is the force under which the body mass 1 kg changes its speed to 1 m/s every second.
It has been experimentally established that the force of gravity acting near the surface of the Earth on a body of mass 102 g is 1 N. The mass of 102 g is approximately 1/10 kg, or, to be more precise,
But this means that a 9.8 N gravity force will act on a body with a mass of 1 kg, that is, a body 9.8 times larger, near the Earth's surface. Thus, in order to find the gravity force acting on a body of any mass, you need multiply the value of the mass (in kg) by the coefficient, which is usually denoted by the letter g:
We see that this coefficient is numerically equal to the force of gravity, which acts on a body with a mass of 1 kg. It bears the name acceleration of gravity . The origin of the name is closely related to the definition of a force of 1 Newton. After all, if a force of 9.8 N rather than 1 N acts on a body with a mass of 1 kg, then under the influence of this force the body will change its speed (accelerate) not by 1 m / s, but by 9.8 m / s every second. In high school, this issue will be considered in more detail.
Now you can write a formula that allows you to calculate the force of gravity acting on a body of arbitrary mass m(Fig. 1).
Rice. 1. Formula for calculating gravity
You should know that the free fall acceleration is equal to 9.8 N/kg only at the Earth's surface and decreases with height. For example, at an altitude of 6400 km above the Earth, it is 4 times less. However, when solving problems, we will neglect this dependence. In addition, gravity also acts on the Moon and other celestial bodies, and on each celestial body, the acceleration of free fall has its own value.
In practice, it is often necessary to measure force. For this, a device called a dynamometer is used. The basis of a dynamometer is a spring to which a measurable force is applied. Each dynamometer, in addition to the spring, has a scale on which the force values \u200b\u200bare plotted. One of the ends of the spring is equipped with an arrow, which indicates on the scale what force is applied to the dynamometer (Fig. 2).
Rice. 2. Dynamometer device
Depending on the elastic properties of the spring used in the dynamometer (on its stiffness), under the action of the same force, the spring may elongate more or less. This allows the manufacture of dynamometers with different measurement limits (Fig. 3).
Rice. 3. Dynamometers with measurement limits of 2 N and 1 N
There are dynamometers with a measurement limit of several kilonewtons and more. They use a spring with a very high stiffness (Fig. 4).
Rice. 4. Dynamometer with a measurement limit of 2 kN
If a load is suspended from a dynamometer, then the mass of the load can be determined from the dynamometer readings. For example, if a dynamometer with a load suspended from it shows a force of 1 N, then the mass of the load is 102 g.
Let us pay attention to the fact that the force has not only a numerical value, but also a direction. Such quantities are called vector quantities. For example, speed is a vector quantity. Force is also a vector quantity (they also say that force is a vector).
Consider the following example:
A body of mass 2 kg is suspended from a spring. It is necessary to depict the force of gravity with which the Earth attracts this body, and the weight of the body.
Recall that gravity acts on the body, and weight is the force with which the body acts on the suspension. If the suspension is stationary, then the numerical value and direction of the weight is the same as that of gravity. Weight, like gravity, is calculated using the formula shown in fig. 1. A mass of 2 kg must be multiplied by the free fall acceleration of 9.8 N/kg. With not too accurate calculations, the acceleration of free fall is often assumed to be 10 N / kg. Then the force of gravity and weight will be approximately equal to 20 N.
To display the vectors of gravity and weight in the figure, it is necessary to select and show in the figure the scale in the form of a segment corresponding to a certain force value (for example, 10 N).
The body in the figure is depicted as a ball. The point of application of gravity is the center of this ball. We depict the force as an arrow, the beginning of which is located at the point of application of the force. Let's point the arrow vertically down, since gravity is directed towards the center of the Earth. The length of the arrow, in accordance with the selected scale, is equal to two segments. Next to the arrow we depict the letter , which denotes the force of gravity. Since we indicated the direction of the force in the drawing, a small arrow is placed above the letter to emphasize what we are depicting. vector size.
Since the weight of the body is applied to the gimbal, we place the beginning of the arrow representing the weight at the bottom of the gimbal. When drawing, we also observe the scale. Next we place the letter denoting the weight, not forgetting to place a small arrow above the letter.
The complete solution of the problem will look like this (Fig. 5).
Rice. 5. A formalized solution to the problem
Once again, pay attention to the fact that in the problem considered above, the numerical values and directions of gravity and weight turned out to be the same, but the points of application were different.
There are three factors to consider when calculating and displaying any force:
the numerical value (modulus) of the force;
the direction of the force
point of application of force.
Force is a physical quantity that describes the action of one body on another. It is usually denoted by the letter F. The unit of force is newton. In order to calculate the value of gravity, it is necessary to know the free fall acceleration, which at the Earth's surface is 9.8 N/kg. With such a force, the Earth attracts a body with a mass of 1 kg. When depicting a force, it is necessary to take into account its numerical value, direction and point of application.
Bibliography
- Peryshkin A. V. Physics. 7 cells - 14th ed., stereotype. - M.: Bustard, 2010.
- Peryshkin A. V. Collection of problems in physics, 7-9 cells: 5th ed., stereotype. - M: Exam Publishing House, 2010.
- Lukashik V. I., Ivanova E. V. Collection of problems in physics for grades 7-9 of educational institutions. - 17th ed. - M.: Enlightenment, 2004.
- A single collection of digital educational resources ().
- A single collection of digital educational resources ().
- A single collection of digital educational resources ().
Homework
- Lukashik V. I., Ivanova E. V. Collection of problems in physics for grades 7-9 No. 327, 335-338, 351.
Force is one of the key concepts of physics. With its help, the degree of external influence of one body on another is measured. The concept of force was used by scientists of antiquity in their works on statics and movement. So, he studied forces in the process of designing simple mechanisms in the 3rd century. BC e. Archimedes. The first ideas about strength were formulated by Aristotle and existed for many centuries. In the 17th century, Isaac Newton formulated the three basic laws of dynamics, which describe the interaction of any forces.
The first law is that a body at rest remains at rest, and a moving body continues to move in a straight line at a constant speed, unless an external force acts on it. So, the soccer ball remains at rest until the player kicks it.
The second law is that the motion of a body changes in proportion to the force applied to it. So, the stronger the blow, the faster the flight of the soccer ball.
The third law - the action of any force causes an equal and opposite reaction to it. So, when a gymnast performs a flip or pushes off from a stationary object, the direction of his movement is determined by the force of counteraction (reaction).
However, by the beginning of the 20th century, Albert Einstein formulated the theory of relativity, where he showed that Newtonian mechanics is correct only at relatively low speeds and masses of bodies.
Force units
Newton
In the international system of units (SI system), force is measured in newtons (N, N). The unit is named after the English physicist Isaac Newton. One newton is the force that causes an acceleration of 1 m/s² of a body with a mass of 1 kg.
1 N = 105 dyn.
1 N ≈ 0.10197162 kgf.
Kilogram-force
A unit of force that is not part of the SI system. The kilogram-force is approximately equal to the force acting on a body of mass 1 kilogram under the influence of the standard acceleration of free fall (the acceleration of the fall of bodies under the influence of the Earth's gravity in airless space is approximately equal to 9.8 m / s²).
1 kgf \u003d 9.80665 newtons (exactly) ≈ 10 N
1 N ≈ 0.10197162 kgf ≈ 0.1 kgf
In a number of European states, the name kilopond (denoted kp) is officially adopted for the kilogram-force.
Multiple units are used less often: a ton-force equal to 103 kgf, or a gram-force equal to 10 -3 kgf.
Dina
A unit of force in the CGS system of units that was widely used before the adoption of the International System of Units (SI). Its designation: dyn, dyn. 1 dyne is equal to the force that, acting on a mass of 1 g, imparts to it an acceleration of 1 cm / s².
1 dyne \u003d g cm / s² \u003d 10 −5 N.
Pound-force
The SI system is not used in all countries. So, in England there is a traditional system of measures, according to which the unit of force is the pound-force. Its designation is lbf (short for English pound force).
1 lbf = 4.44822 newtons
Kip (kilo-pound force)
In the United States, force is measured in kips (or kilopounds). Formed from the merger of the English words "kilo" + "pound".
1 kip = 4448.2216152605 newtons
In order to quickly and accurately convert one unit to another, use our website.
Instruments for measuring force
Force is measured by means of dynamometers, gravimeters, force-measuring machines and presses. Dynamometers - devices that measure the force of elasticity. They come in three types: spring, hydraulic, electric. The dynamometer is also used in medicine. With its help, doctors measure the strength of various muscle groups of a person.
