What does this sign mean in physics. School program: What is N in physics

Turning to physical applications of the derivative, we will use several other symbols for those adopted in physics.

First, the designation of functions is changing. In fact, what functions are we going to differentiate? These functions serve physical quantities depending on time. For example, the coordinate of the body X (T) and its speed V (T) can be given by formulas:

(Reads ¾ isx with a point¿).

There is another designation derivative, very common both in mathematics and physics:

(Reads ¾DE XE for DE TE¿).

Let us dwell on the sense of designation (1.16). Mathematician understands his bicon or like a limit:

either as a fraction, in the denominator which is the increment of time DT, and in the numerator the so-called DX differential function x (t). The concept of differential is not difficult, but we will not discuss it now; It is waiting for you in the first year.

The physicist, not cited by mathematical rigor, understands the designation (1.16) more informally. Let DX be a change in coordinates during DT. Take the DT interval as small as the DX \u003d DT ratio close to its limit (1.17) with the accuracy.

And then, the physicist will say, the derivative coordinate in time is simply a fraction, in the numerator of which it costs a sufficiently small change in the coordinate of the DX, and in the denominator there is a sufficiently small period of time DT, during which this change of coordinate occurred.

Such a nestor understanding of the derivative is characteristic of reasoning in physics. Next, we will adhere to this particular physical level of rigor.

The X (T) derivative of the physical value x (t) is again a time function, and this function can again be indifferentiated to find the derivative derivative, or the second derivative function x (t). Here is one designation of the second derivative:

the second derivative of the function X (T) is indicated by (t)

(Reads ¾ isx with two points¿), but another:

the second derivative of the function X (T) is indicated by 2

(It is read by two ix on DE TE Square¿ or ¾ DE two X-in-father for DE TE twice¿).

Let's return to the original example (1.13) and consider the derivative of the coordinates, and at the same time we will look at the joint use of the designation (1.15) and (1.16):

x (t) \u003d 1 + 12t 3T2)

x (t) \u003d dt d (1 + 12t 3t2) \u003d 12 6t:

(DT D Differentiation Symbol in front of the bracket is all the same as the barcode from the bracket in the former designations.)

Please note that the coordinate derivatives turned out to be equal to speed (1.14). This is not a random coincidence. The connection of the derivative coordinate with the velocity of the body will be found out in the following section ¾ Meaning movement.

1.1.7 Limit of vector quantity

Physical quantities are not only scalar, but also vector. Accordingly, often we are interested in the rate of change of vector value that is, the derivative of the vector. However, before talking about the derivative, it is necessary to deal with the concept of the limit of vector value.

Consider the sequence of vectors ~ U1; ~ U2; ~ U3; ::: Having done, if necessary, parallel transfer, we started up to one point O (Fig.1.5):

Suppose that the sequence of points A1; A2; A3; :::: ¾Things¿2 to point B:

lim An \u003d B:

Denote ~ V \u003d OB. We will say then that the sequence of the blue vectors ~ UN tends to the red vector ~ V, or that the vector ~ V is the limit of the sequence of vectors ~ un:

~ V \u003d Lim ~ un:

2 It is quite a fairly intuitive understanding of this to the flow, but you may be interested in a more strict explanation? Then here it is.

Let it happen on the plane. ¾Things of the A1 sequence; A2; A3; ::: To the point B means the following: Some small circle with the center at the point B we took, all points of the sequence, starting at some, will fall inside this circle. In other words, outside of any circle with the center B there is only a finite number of points of our sequence.

And if it happens in space? The definition of ¾ is modified slightly: you only need to replace the word ¾ SKUND for the word ¾SHAR¿.

Suppose now that the ends of the blue vectors in fig. 1.5 Run not a discrete set of values, but a continuous curve (for example, specified by the dotted line). Thus, we are dealing not with the sequence of vectors ~ un, and with a vector ~ U (T), which changes with time. This is exactly what we need in physics!

Further explanation is almost the same. Let t strive for a certain value of T0. If a

at the same time, the ends of the vectors ~ U (T) are the target¿ in some point b, then we say that vector

~ V \u003d OB is the limit of vector value ~ U (T):

t! T0.

1.1.8 Differentiation vectors

Finding out what is the limit of vector magnitude, we are ready to make the next step enter the concept of the vector derivative.

Suppose that there is some vector ~ U (t), depending on time. This means that the length of this vector and its direction can vary over time.

By analogy with the usual (scalar) function, the concept of change (or increment) of the vector is introduced. Changing the vector ~ U per time T is vector:

~ U \u003d ~ U (T + T) ~ U (T):

Please note that the difference in vectors is standing on the right side of this ratio. The change in the vector ~ U is shown in Fig. 1.6 (Recall that when subtracting vectors, we will begin to start them at one point, connect the ends and the one that the vector from which subtracts is performed) by the arrow.

~ U (T) ~ U

Fig. 1.6. Changing vector

If the time lapse T is sufficiently small, then the vector ~ U during this time changes little (in physics, at least it is always considered). Accordingly, if with T! 0 ratio ~ U \u003d T tends to a certain limit, then this limit is called the derivative of the vector ~ U:

With the designation of the vector derivative, we will not use the point from above (since the symbol ~ u_ does not look too good) and limited to the designation (1.18). But for the derivative of Scalar, we, of course, we freely use both symbols.

Recall that D ~ U \u003d DT is a symbol of the derivative. It can be understood as a fraction, in the numerator of which it is worth the differential of the vector ~ U, the corresponding period of time DT. Above, we did not discuss the concept of differential, since it does not pass it at school; We will not discuss differential and here.

However, at the physical level of strictness, D ~ U \u003d DT derivative can be considered a fraction, in the denominator which is a very small time interval DT, and in the numerator, the corresponding small change d ~ U vector ~ U. With a sufficiently small DT, the value of this fraction is different from

the limit in the right-hand side (1.18) is so little that, taking into account the existing measurement accuracy, these differences can be neglected.

This (not quite strict) physical understanding of the derivative will be quite enough.

The rules of differentiation of vector expressions are largely similar to the scalar differentiation rules. We will need only the simplest rules.

1. A permanent scalar multiplier is submitted for the sign of the derivative: if C \u003d const, then

d (C ~ U) \u003d C D ~ U: DT DT

We use this rule in the ¾ Impulse section, when the second law of Newton

2. A constant vector multiplier is carried out for a sign of the derivative: if ~ c \u003d const, then dt d (x (t) ~ c) \u003d x (t) ~ C:

3. The derivative of the vectors is equal to the sum of their derivatives:

dT D (~ U + ~ V) \u003d D ~ U DT + D ~ V DT:

We will use repeatedly two rules. Let's see how they work in the most important situation of the differentiation of the vector in the presence of the rectangular coordinate system Oxy Z (Fig. 1.7).

Fig. 1.7. Baseline decomposition

As is known, any vector ~ u is the only way unfolded on the basis of single

vectors ~, ~, ~: i j k

~ U \u003d ux i + uy j + uz k:

Here UX, UY, Uz projections of the vector ~ U on the coordinate axes. They are the coordinates of the vector ~ U in this basis.

The vector ~ u in our case depends on time, which means that its coordinates UX, UY, UZ are time functions:

Differentiate is equality. First, we use the range of differentiation of the amount:

Thus, if the vector ~ U has coordinates (UX; UY; Uz), the coordinates of the derivative D ~ U \u003d DT are the coordinates of the vector ~ U, namely (UX; UZ).

Due to the special importance of formula (1.20) we will give a more direct conclusion. At the time of time T + T according to (1.19) we have:

~ U (T + T) \u003d UX (T + T) I + UY (T + T) J + Uz (T + T) K:

Write a change in the vector ~ U:

~ U \u003d ~ U (T + T) ~ U (T) \u003d

UX (T + T) I + UY (T + T) J + Uz (T + T) K UX (T) I + UY (T) J + Uz (T) K \u003d

In the limit at T! 0 fractions UX \u003d T, UY \u003d T, Uz \u003d T transitions accordingly in derivatives UX, UY, UZ, and we again get the relation (1.20):

UX i + UY J + Uz K.

It is no secret to anyone that there are special designations for values \u200b\u200bin any science. Letter notation in physics prove that this science is no exception in terms of identifying values \u200b\u200busing special characters. The main values, as well as their derivatives, quite a lot, each of which has its own character. So, alphabetic designations in physics are discussed in detail in this article.

Physics and basic physical quantities

Thanks to Aristotle, the word physics is beginning to be used, since it was he who first used this term that was considered to be synonymous with the term philosophy. This is due to the common object of study - the laws of the Universe, more specifically, how it functions. As you know, the first scientific revolution occurred in the XVI-XVII centuries, it was thanks to her physics was highlighted in independent science.

Mikhail Vasilyevich Lomonosov introduced the word physics into Russian through the publication of the textbook in translated from the German - the first physics textbook in Russia.

So, physics is a section of natural science dedicated to the study of the general laws of nature, as well as matter, its movement and structure. The main physical quantities are not so much, as it may seem at first glance - they are only 7:

• length,
• weight,
• time,
• current force
• temperature,
• amount of substance
• the power of light.

Of course, they have their letter notation in physics. For example, the M symbol is selected for the mass, and for temperature - T. also all values \u200b\u200bhave its own unit of measure: in the power of the light - candela (CD), and the amount of the substance is a unit of measurement.

Derivatives physical quantities

Derivatives of physical quantities are much larger than the main. They are numbered 26, and often some of them are attributed to the main.

So, the area is derived from length, the volume is also from length, speed - from time, length, and acceleration, in turn, characterizes the speed of change of speed. The pulse is expressed through a mass and speed, force - the product of mass and acceleration, mechanical work depends on force and length, the energy is proportional to the mass. Power, pressure, density, surface density, linear density, heat, voltage, electrical resistance, magnetic stream, moment of inertia, moment of impulse, moment of force - they all depend on the mass. Frequency, angular speed, angular acceleration is inversely proportional to time, and the electrical charge has a direct dependence on time. The angle and body angle are derived values \u200b\u200bof length.

What letter is indicated in physics? The voltage that is a scalar value is denoted by the letter U. For speed, the designation has the form of the letter V, for mechanical work - a, and for energy - E. The electrical charge is taken to denote the letter Q, and the magnetic flow - F.

C: General Information

The international system of units (SI) is a system of physical units, which is based on the international system of quantities, including the names and designations of physical quantities. She was adopted by the General Conference on Measures and Sighs. It is this system that regulates alphabetic designations in physics, as well as their dimension and units of measurement. The letters of the Latin alphabet are used, in some cases Greek. It is also possible as notation to use special characters.

Conclusion

So, in any scientific discipline there are special designations for various kinds of quantities. Naturally, physics is no exception. There are many letter designations: force, area, mass, acceleration, voltage, etc. They have their own designations. There is a special system that is called an international system of units. It is believed that the main units cannot be mathematically derived from others. The derivatives of the same values \u200b\u200bare obtained by multiplying and division from the main.

The construction of the drawings is not easy, but without it in the modern world. After all, to make even the most common object (a tiny bolt or nut, the shelf for books, the design of a new dress and the like), initially need to carry out the corresponding calculations and draw the drawing of the future product. However, it often makes it one person, but is engaged in the manufacture of something according to this scheme.

In order not to be confused in the understanding of the depicted object and its parameters, the conventions of length, widths, heights and other values \u200b\u200bapplied during the design are taken worldwide. What are they? Let's find out.

Values

The area, height and other designations of this nature are not only physical, but also mathematical values.

The one of their letter denotation (used by all countries) was settled in the middle of the twentieth century by the international system of units (SI) and applied to this day. It is for this reason that all such parameters are denoted by Latin, and not by Cyrillic letters or Arabic rizu. In order not to create separate difficulties, when developing standards of design documentation in most modern countries, it was decided to use practically the same conditional designations that are used in physics or geometry.

Any graduate of the school remembers that depending on whether the two-dimensional or three-dimensional figure (product) is depicted in the drawing, it has a set of basic parameters. If two dimensions are present - this is a width and length, if there are three of them - the height is added.

So, for starters, let's find out how correctly the length is width, the height indicate in the drawings.

Width

As mentioned above, in mathematics considered the value is one of the three spatial dimensions of any object, provided that its measurements are made in the transverse direction. So what is famous width? The designation of the letter "in" it has. This is known throughout the world. Moreover, according to GOST, it is permissible to use both the title and lower case Latin liter. Often the question arises about why this letter is chosen. After all, the reduction is usually made on the first Greek or English name of the magnitude. In this case, the width in English will look like "width".

This is probably the fact that this parameter was most widely used in geometry. In this science, describing the figures, often length, width, the height is denoted by the letters "A", "B", "C". According to this tradition, when choosing a letter "B" (or "B") was borrowed by the SI system (although for other two measurements began to be used different from geometric characters).

The majority believes that this was done, in order not to confuse the width (designation of the letter "B" / "B") with weight. The fact is that the latter is sometimes referred to as "W" (abbreviation from the English name Weight), although it is permissible to use other liter ("G" and "P"). According to the international standards of the SI system, the width is measured in meters or multiple (dolly) units. It is worth noting that in geometry is sometimes also permissible to use "W" to designate the width, however, in physics and other accurate sciences, such a designation is usually not applied.

Length

As already indicated, in mathematics length, height, width is three spatial dimensions. In this case, if the width is a linear size in the transverse direction, then the length is in the longitudinal one. Considering it as the magnitude of physics, it can be understood that under this word is meant the numerical characteristic of the length of the lines.

In English, this term is referred to as Length. It is because of this that this value is denoted by the title or lowercase initial literary of this word - "L". As the width, the length is measured in meters or their multiple (dolly) units.

Height

The presence of this magnitude indicates that it is necessary to deal with a more complex - three-dimensional space. In contrast to the length and width, the height numerically characterizes the size of the object in the vertical direction.

In English, she is written as "height". Therefore, according to international standards, it is denoted by the Latin Liter (H "/" H ". In addition to height, in the drawings, this letter sometimes acts as the depth designation. Height, width and length - all these parameters are measured in meters and their multiple and dolly units (kilometers, centimeters, millimeters, etc.).

Radius and diameter

In addition to the considered parameters, the drawings have to deal with others.

For example, when working with circles, it becomes necessary to determine their radius. This is called a segment that connects two points. The first of them is the center. The second is directly on the circumference itself. On Latin, this word looks like "Radius". Hence the lowercase or title "R" / "R".

Drawing circumference, in addition to the radius, often have to face with a close phenomenon - with a diameter. It is also a segment connecting two points on the circle. At the same time, it necessarily passes through the center.

Numerically diameter is equal to two radius. In English, this word is written like this: "Diameter". Hence the reduction - a large or small Latin letter "D" / "D". Often the diameter in the drawings are denoted by the crooked circle - "Ø".

Although this is a common reduction, it is worth it in mind that GOST provides for the use of only Latin "D" / "D".

Thickness

Most of us remember the school lessons of mathematics. Even then, the teachers were told that the Latin Litera "S" is made to designate such a magnitude as the area. However, according to generally accepted standards, in the drawings in this way a completely different parameter is written - the thickness.

Why is that? It is known that in the case of a height, width, length, the designation letters could be explained by their writing or tradition. That's just the thickness in English looks like "Thickness", and in the Latin version - "crassities". It is also not clear why, in contrast to other values, the thickness can be denoted only the lowercase literary. The designation "S" also applies when describing the thickness of pages, walls, ribs, and so on.

Perimeter and Square

Unlike all the magnitude listed above, the word "perimeter" came from Latin or English, but from the Greek. It is formed from "περιμετρέο" ("Measure the circle"). And today, this term has retained its value (the total length of the boundaries of the figure). Subsequently, the word fell into English ("Perimeter") and fixed in the SI system in the form of a reduction in the letter "P".

The area is a value showing the quantitative characteristic of a geometric shape with two dimensions (length and width). Unlike the previously listed earlier, it is measured in square meters (as well as in the dollars and multiple units). As for the subject notation of the square, it differs in different areas. For example, in mathematics it is familiar with everyone since childhood the Latin letter "S". Why so - no information.

Some of ignorance think that this is due to English writing the word "Square". However, in it, mathematical area is "Area", and "Square" is an area in an architectural understanding. By the way, it is worth remembering that "Square" is the name of the geometric figure "Square". So it is worth being attentive when studying the drawings in English. Due to the translation of "AREA" in separate disciplines, the letter "A" is used as the designation. In rare cases, "F" is also used, however, in physics, this letter means the value called "Power" ("Fortis").

Other common abbreviations

Designations of height, widths, lengths, thickness, radius, diameters are the most used in drawing up drawings. However, there are other values \u200b\u200bthat are also often present in them. For example, the lower case "T". In physics, this means "temperature", however, according to GOST, a unified system of design documentation, this letter is a step (screw springs, and the like). However, it is not used when it comes to gears and threads.

The title and lowercase letter "A" / "A" (according to all the same standards) in the drawings is used to indicate not the area, but the intercentrose and the mid-scene distance. In addition to different values, in the drawings often have to denote the angles of different sizes. This is customary to use the lower case listers of the Greek alphabet. The most used - "α", "β", "γ" and "δ". However, it is permissible to use others.

What standard determines the letter notation of the length, width, height, area and other values?

As already mentioned above, so that there is no misunderstanding when reading the drawing, representatives of different nations adopted general standards of alphabetic designation. In other words, if you doubt the interpretation of one or another reduction, look at the GOST. Thus, you will learn how it is correctly indicated by the height, width, length, diameter, radius, and so on.

The study of physics at school lasts for several years. At the same time, students face the problem that the same letters denote completely different values. Most often, this fact concerns Latin letters. How then to solve the tasks?

It is not necessary to scare this repetition. Scientists have tried to introduce them to the designation so that the same letters did not meet in the same formula. Most often, the disciples face Latin N. It can be line or capital. Therefore, it logically arises the question of what N is in physics, that is, in a certain student who met the formula.

What indicates the capital letter n in physics?

Most often in the school year, it meets when studying mechanics. After all, there it can immediately be in the spirit of the values \u200b\u200b- the power and strength of the normal reaction of the support. Naturally, these concepts do not intersect, because used in different sections of mechanics and are measured in different units. Therefore, you always need to determine exactly what N is in physics.

Power is the rate of energy change. This is a scalar value, that is just a number. The unit of its measurement serves Watt (W).

The power of the normal reaction of the support is the force that has an action on the body from the support or suspension. In addition to the numerical value, it has a direction, that is, this is a vector magnitude. Moreover, it is always perpendicular to the surface on which external impact is performed. Unit of measurement of this n is Newton (H).

What is N in physics, in addition to already specified values? It may be:

constant avogadro;

an increase in the optical device;

concentration of substance;

debye number;

full radiation power.

What can denote the lowercase letter n in physics?

The list of items that may be hidden behind it are quite extensive. The designation N in physics is used for such concepts:

refractive index, and it can be absolute or relative;

neutron is a neutral elementary particle with a slightly greater than the proton;

rotation frequency (used to replace the Greek letter "NU", as it is very similar to the Latin "WE") - the number of revolts per unit of time is measured in Hertz (Hz).

What does N in physics mean, except for the specified values? It turns out that it is hidden the main quantum number (quantum physics), concentration and constant of the huge (molecular physics). By the way, when calculating the concentration of the substance, it is necessary to know the value, which is also recorded by the Latin "En". It will be discussed below.

What physical value can be denoted by n and n?

Her name comes from the Latin word numerus, in translation it sounds like a "number", "quantity". Therefore, the answer to the question of what n means in physics is quite simple. This is the number of any items, bodies, particles - everything about what is in question in a certain task.

Moreover, "quantity" is one of the few physical quantities that do not have a unit of measurement. This is just a number, without name. For example, if we are talking about 10 particles in the problem, then n will be simply 10. But if it turns out that the line "EN" is already busy, then use the uppercase letter.

The formulas in which the capital n appears

The first of them determines the power, which is equal to the ratio of work by time:

In molecular physics there is such a concept as a chemical amount of matter. Denotes the Greek letter "NU". To count it, you should split the number of particles on number of Avogadro :

By the way, the last value is also denoted by such a popular letter N. Only she always has a lower index - A.

To determine electric charge, The formula will be required:

Another formula with N in physics - Frequency of oscillations. To count it, you need to divide their number for a while:

The letter "En" appears in the formula for the appeal period:

Formulas in which the line N is found

In the school year of physics, this letter is most often associated with the refractive index of the substance. Therefore, it is important to know the knowledge of the formulas with its application.

So, for the absolute refractive index of the formula is written as follows:

Here C is the speed of light in vacuum, V is its speed in the refracting medium.

The formula for the relative refractive index is somewhat more complicated:

n 21 \u003d V 1: V 2 \u003d N 2: N 1,

where N 1 and N 2 are absolute refractive indices of the first and second medium, V 1 and V 2 - the velocity of the light wave in these substances.

How to find N in physics? This will help us the formula in which you want to know the angles of the fall and refractive to the beam, that is, N 21 \u003d sin α: sin γ.

What is N in physics, if this is the refractive index?

Typically, the tables are given values \u200b\u200bfor absolute refractive indices various substances. Do not forget that this value depends not only on the properties of the medium, but also on the wavelength. Table values \u200b\u200bof the refractive index are given for the optical range.

So, it became clear what N is in physics. In order not to remain any questions, it is worth considering some examples.

Task on power

№1. During plowing, the tractor pulls a plow evenly. At the same time, it makes the power of 10 kN. With this movement for 10 minutes it overcomes 1.2 km. It is required to determine the power developing them.

Translation of units in si. It is possible to start with the strength, 10 n are equal to 10,000 N. Then the distance: 1.2 × 1000 \u003d 1200 m. Time remains - 10 × 60 \u003d 600 s.

Choosing a formula. As mentioned above, N \u003d A: T. But the task is no value for work. For its calculation, another formula is useful: a \u003d F × S. The final formula for the power formula looks like this: n \u003d (F × S): t.

Decision. Calculate first work, and then - power. Then in the first action it turns out 10,000 × 1 200 \u003d 12,000,000 j. The second action gives 12,000,000: 600 \u003d 20,000 W.

Answer. The power of the tractor is 20,000 W.

Tasks for refractive index

№2. The absolute refractive index in the glass is 1.5. The speed of propagation of light in glass is less than in vacuum. It is required to determine how many times.

In si translate data is not required.

When choosing the formula, you need to stop on this: n \u003d s: v.

Decision. From this formula, it can be seen that v \u003d s: n. This means that the speed of light propagation in the glass is equal to the speed of light in a vacuum divided into refractive index. That is, it decreases one and a half times.

Answer. The speed of propagation of light in glass is less than in vacuum, 1.5 times.

№3. There are two transparent environments. The speed of light in the first of them is equal to 225,000 km / s, in the second - by 25,000 km / s less. The beam of light goes from the first environment in the second. The angle of the fall α is 30º. Calculate the value of the refractive angle.

Do I need to translate into si? Speeds are given in generated units. However, when substituting in the formula, they will reduce. Therefore, you do not need to translate speeds in m / s.

Choosing the formulas necessary to solve the problem. It will be necessary to use the law of the refraction of light: N 21 \u003d sin α: sin γ. And also: n \u003d s: v.

Decision. In the first formula, N 21 is the ratio of two refractive indices of the substances under consideration, that is, N 2 and N 1. If you write down the second specified formula for the proposed environments, then such: N 1 \u003d C: V 1 and N 2 \u003d C: V 2. If you draw up the ratio of the last two expressions, it turns out that N 21 \u003d V 1: V 2. Substituting it in the formula of the law of refraction, one can derive such an expression for the sinus of the refractive angle: sin γ \u003d sin α × (v 2: v 1).

We substitute in the formula of the values \u200b\u200bof the specified speeds and sine 30º (equal to 0.5), it turns out that the sine of the refractive angle is 0.44. According to the Bradys table, it turns out that the angle γ is equal to 26º.

Answer. The value of the refractive angle is 26º.

Tasks for the treatment period

№4. Blades windmill Rotate with a period of 5 seconds. Calculate the number of revolutions of these blades in 1 hour.

Translate into units of SI is needed only time 1 hour. It will be equal to 3,600 seconds.

Selection of formulas. The rotational period and the number of revolutions are associated with the formula T \u003d T: N.

Decision. From the specified formula, the number of revolutions is determined by the ratio of time to the period. Thus, n \u003d 3600: 5 \u003d 720.

Answer. The number of revolutions of the blades of the mill is 720.

№5. The screw of the aircraft rotates with a frequency of 25 Hz. What time will required the screw to make 3,000 revolutions?

All data are given with C, so nothing needed to translate.

Necessary formula: frequency ν \u003d n: t. It only needs to withdraw the formula for an unknown time. It is a divider, so it is assumed to be divided by n on ν.

Decision. As a result of the division of 3,000 on 25, the number 120 is obtained. It will be measured in seconds.

Answer. The screw of the aircraft performs 3000 revolutions for 120 s.

Let's summarize

When the student in the task in physics is found a formula containing N or N, he needs deal with two moments. The first - from which section of the physics is equality. This can be clear from the header in the textbook, the directory or words of the teacher. Then it should be decided on what is hidden behind the multicalen "En". Moreover, this helps the name of the units of measurement, unless, of course its value is given.Another option is also allowed: look carefully for the remaining letters in the formula. Perhaps they will be familiar and will give a prompt in the question.