What is the weight of the body length. Academy of Entertaining Sciences

01.10.2019 -

We often use phrases like: "A pack of sweets weighs 250 grams" or "I weigh 52 kilograms." The use of such offers is automatic. But what is weight? What is it made up of and how is it calculated?

First you need to understand that it is wrong to say: "This object weighs X kilograms." In physics there is two different concepts - mass and weight. Mass is measured in kilograms, grams, tons, and so on, and body weight is calculated in newtons. Therefore, when we say, for example, that we weigh 52 kilograms, we actually mean mass, not weight.

Weight – is a measure of the body's inertia. The more inertia the body has, the more time it will take to give it speed. Roughly speaking, the higher the mass value, the harder it is to move the object. In the international system of units, mass is measured in kilograms. But it is also measured in other units, for example;

ounce;
lb;
stone;
US ton;
English ton;
gram;
milligram and so on.

When we say one, two, three kilograms, we compare the mass with the reference mass (the prototype of which is in France at the BIPM). Mass is denoted by m.

The weight – is the force that acts on the suspension or support due to an object attracted by gravity. This is a vector quantity, which means it has a direction (like all forces), unlike mass ( scalar). The direction always goes to the center of the Earth (due to gravity). For example, if we are sitting on a chair, the seat of which is parallel to the Earth, then the force vector is directed straight down. Weight is denoted P and is calculated in newtons [N].

If the body is in motion or at rest, then the force of gravity (Ftyazh) acting on the body is equal to the weight. This is true if the motion is in a straight line with respect to the earth and it has a constant speed. Weight acts on the support, and gravity acts on the body itself (which is located on the support). These are different values, and regardless of the fact that they are equal in most cases, you should not confuse them.

Gravity is the result of the attraction of the body to the ground, weight is the effect of the body on the support. Since the body bends (deforms) the support with its weight, another force arises, it is called the elastic force (Fupr). Newton's third law states that bodies interact with each other with forces of the same modulus, but different in vector. From this it follows that for the elastic force there must be an opposite force, and this one is called the reaction force of the support and is denoted by N.

Modulo |N|=|P|. But since these forces are multidirectional, then, by opening the module, we get N = - P. That is why the weight can be measured with a dynamometer, which consists of a spring and a scale. If you hang a load on this device, the spring will stretch to a certain point on the scale.

How to measure body weight

Newton's second law states that acceleration is equal to force divided by mass. Thus, F=m*a. Since Fstrand is equal to P (if the body is at rest or moves in a straight line (relative to the Earth) with the same speed), then the P of the body will be equal to the product of mass and acceleration (P=m*a).

We know how to find the mass, and we know what the weight of the body is, it remains to figure out the acceleration. Acceleration is a physical vector quantity that denotes the change in the speed of a body per unit of time. For example, an object moves for the first second at a speed of 4 m / s, and in the second second its speed increases to 8 m / s, which means that its acceleration is 2. According to the international system of units, acceleration is calculated in meters per second squared [m / s 2 ].

If you place the body in a special environment where there will be no air resistance force - vacuum, and remove the support, then the object will begin to fly with uniform acceleration. The name of this phenomenon is acceleration of gravity, which is denoted by g and is calculated in meters per second squared [m/s 2 ].

Interestingly, the acceleration does not depend on the mass of the body, which means that if we throw a piece of paper and a weight on Earth at special conditions, at which there is no air (vacuum), then these objects will land at the same time. Since the sheet has a large surface area and relatively small mass, in order to fall, it has to face a lot of air resistance. . This doesn't happen in a vacuum., and so a feather, a piece of paper, a weight, a cannonball and other objects will fly at the same speed and fall at the same time (assuming they start flying at the same time and their initial speed is zero ).

Since the Earth has the shape of a geoid (or in other words, an ellipsoid), and not an ideal ball, then the acceleration of free fall in different areas Lands are different. For example, at the equator it is 9.832 m/s 2 , and at the poles 9.780 m/s 2 . This is because in some parts of the Earth the distance to the core is greater, and in some it is less. The closer an object is to the center, the more it is attracted. The further the object, the less gravity. Usually, at school, this value is rounded up to 10, this is done for the convenience of calculations. If it is necessary to measure more accurately (in engineering or military affairs, and so on), then specific values \u200b\u200bare taken.

Thus, the formula for calculating the weight of the body will look like this: P=m*g.

Examples of tasks for calculating body weight

First task. A weight of 2 kg is placed on the table. What is the weight of the cargo?

To solve this problem, we need a formula for calculating the weight P=m*g. We know the mass of the body, and the free fall acceleration is approximately 9.8 m/s 2 . We substitute these data into the formula and get P \u003d 2 * 9.8 \u003d 19.6 N. Answer: 19.6 N.

Second task. A paraffin ball with a volume of 0.1 m 3 was placed on the table. What is the weight of the ball?

This task must be solved in the following sequence;

First we need to remember the weight formula P=m*g. We know the acceleration - 9.8 m / s 2. It remains to find the mass.
Mass is calculated using the formula m=p*V, where p is the density and V is the volume. The density of paraffin can be seen in the table, the volume is known to us.
It is necessary to substitute the values \u200b\u200bin the formula to find the mass. m=900*0.1=90 kg.
Now we substitute the values in the first formula to find the weight. P=90*9.9=882 N.

Answer: 882 N.

Video

This video lesson deals with the topic - gravity and body weight.

Didn't get an answer to your question? Suggest a topic to the authors.

The concept with which we are familiar from early childhood is the mass. And yet, in the course of physics, some difficulties are associated with its study. Therefore, it is necessary to clearly define how it can be recognized? And why is it not equal to weight?

Determination of mass

The natural scientific meaning of this quantity is that it determines the amount of matter that is contained in the body. To denote it, it is customary to use latin letter m. The unit of measurement in the standard system is the kilogram. in tasks and Everyday life off-system ones are also often used: grams and tons.

In a school physics course, the answer to the question: “What is mass?” given in the study of the phenomenon of inertia. Then it is defined as the ability of a body to resist a change in the speed of its movement. Therefore, the mass is also called inert.

What is weight?

First, it is a force, that is, a vector. Mass, on the other hand, is a scalar weight always attached to a support or suspension and directed in the same direction as gravity, that is, vertically downwards.

The formula for calculating the weight depends on whether this support (suspension) is moving. When the system is at rest, the following expression is used:

P \u003d m * g, where P (in English sources the letter W is used) is the weight of the body, g is the acceleration of free fall. For the earth, g is usually taken equal to 9.8 m / s 2.

The mass formula can be derived from it: m = P / g.

When moving down, that is, in the direction of the weight, its value decreases. So the formula takes the form:

P \u003d m (g - a). Here "a" is the acceleration of the system.

That is, when these two accelerations are equal, a state of weightlessness is observed when the weight of the body is zero.

When the body begins to move upwards, they speak of an increase in weight. In this situation, an overload condition occurs. Because body weight increases, and its formula will look like this:

P \u003d m (g + a).

How is mass related to density?

Decision. 800 kg/m 3 . In order to use the already known formula, you need to know the volume of the spot. It is easy to calculate if we take the spot for a cylinder. Then the volume formula will be:

V = π * r 2 * h.

Moreover, r is the radius, and h is the height of the cylinder. Then the volume will be equal to 668794.88 m 3. Now you can calculate the mass. It will turn out like this: 535034904 kg.

Answer: the mass of oil is approximately equal to 535036 tons.

Task number 5. Condition: The length of the longest telephone cable is 15151 km. What is the mass of copper that went into its manufacture, if the cross section of the wires is 7.3 cm 2?

Decision. The density of copper is 8900 kg/m 3 . The volume is found by a formula that contains the product of the area of the base and the height (here, the length of the cable) of the cylinder. But first you need to convert this area to square meters. That is, divide this number by 10000. After calculations, it turns out that the volume of the entire cable is approximately equal to 11000 m 3.

Now we need to multiply the density and volume values to find out what the mass is equal to. The result is the number 97900000 kg.

Answer: the mass of copper is 97900 tons.

Another issue related to mass

Task number 6. Condition: The largest candle weighing 89867 kg was 2.59 m in diameter. What was its height?

Decision. Wax density - 700 kg / m 3. The height will need to be found from That is, V must be divided by the product of π and the square of the radius.

And the volume itself is calculated by mass and density. It turns out to be equal to 128.38 m 3. The height was 24.38 m.

Answer: the height of the candle is 24.38 m.

BODY WEIGHT

force with which the body acts due to gravity to the Earth on a support (or suspension) that keeps it from free fall. If the body and support are motionless relative to the Earth, then V. t. is equal to its gravity. The unit of W. t. (in SI) is newton (N).

Big encyclopedic polytechnic dictionary. 2004 .

See what "BODY WEIGHT" is in other dictionaries:

body weight- The modulus of the resultant forces of gravity acting on the particles of this body. [Collection of recommended terms. Issue 102. Theoretical Mechanics. USSR Academy of Sciences. Committee of Scientific and Technical Terminology. 1984] Themes theoretical mechanics… … Technical Translator's Handbook

body weight- The modulus of the resultant forces of gravity acting on the particles of this body ... Polytechnic terminological explanatory dictionary

See Human body weight. * * * HUMAN BODY WEIGHT HUMAN BODY WEIGHT, see Human body weight (see HUMAN BODY WEIGHT) … encyclopedic Dictionary

See Human Body Mass... Big Encyclopedic Dictionary

See Human Body Mass... Great Soviet Encyclopedia

The same as the mass of a human body ... Natural science. encyclopedic Dictionary

REDUCED BODY WEIGHT- In general, a condition in which body weight is 10% or more below normal for a person's type of institution and age. Like obesity, this term is used quite loosely, since it is impossible to give a definition that applies equally to ... ... Dictionary in psychology

Weight: In physics: Weight is the force exerted by a body on a support or suspension. Specific gravity is the ratio of the weight of a substance to its volume. Statistical weight, in quantum mechanics and quantum statistics, the number of different quantum states with a given ... ... Wikipedia

WEIGHT, the force of GRAVITATIONAL attraction of the body. The weight of a body is equal to the product of the mass of the body and the acceleration of free fall. The mass remains constant, but the weight depends on the location of the object on the Earth's surface. As height increases, weight decreases... Scientific and technical encyclopedic dictionary

Exist., m., use. often Morphology: (no) what? weight and weight, what? weight, (see) what? weight what? weight, what? about weight; pl. what? weight, (no) what? weights, why? scales, (see) what? weight than? weights about what? about scales 1. The weight of any physical ... ... Dictionary of Dmitriev

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A fun encyclopedia for girls and boys. 300 questions Tamara Viktorovna Skiba. Every parent dreams of finding a book that, in a simple and entertaining way, would give answers to the endless questions of little whys. Children are interested in literally everything: -Where does the Sun disappear ...
Fun anatomy. Formation of ideas about oneself and one's body, Nishchev Valery Mikhailovich. A visual and didactic manual with colorful drawings and funny poems will help preschoolers to form ideas about the structure of their body, about elementary rules hygiene and...

Quite a lot of mistakes and non-random reservations of students are connected with the strength of the weight. The phrase “power of weight” itself is not very familiar, because. we (teachers, authors of textbooks and problem books, teaching aids and reference literature) are more accustomed to speaking and writing "body weight". Thus, the phrase itself leads us away from the concept that weight is force, and leads to the fact that body weight is confused with body weight (we often hear in the store when they are asked to weigh a few kilograms of a product). The second common mistake students make is that they confuse the force of weight with the force of gravity. Let's try to deal with the force of weight at the level of a school textbook.

To begin with, let's look at reference literature and try to understand the point of view of the authors on this issue. Yavorsky B.M., Detlaf A.A. (1) in a handbook for engineers and students, the weight of a body is the force with which this body acts due to gravity towards the Earth on a support (or suspension) that keeps the body from free fall. If the body and the support are stationary relative to the Earth, then the weight of the body is equal to its gravity. Let's ask some naive questions to the definition:

1. What reporting system are we talking about?

2. Is there one support (or suspension) or several (supports and suspensions)?

3. If the body gravitates not to the Earth, but, for example, to the Sun, will it have weight?

4. If a body in a spaceship moving with acceleration "almost" does not gravitate to anything in observable space, will it have weight?

5. How is the support located relative to the horizon, is the suspension vertical for the case of equality of body weight and gravity?

6. If the body moves uniformly and rectilinearly together with the support relative to the Earth, then the weight of the body is equal to its gravity?

In the reference guide to physics for applicants to universities and self-education, Yavorsky B.M. and Selezneva Yu.A. (2) provide an explanation on the last naive question, leaving the former unaddressed.

Koshkin N.I. and Shirkevich M.G. (3) it is proposed to consider the weight of the body as a vector physical quantity, which can be found by the formula:

The examples below will show that this formula works in cases where no other forces act on the body.

Kuchling H. (4) does not introduce the concept of weight as such at all, identifying it practically with the force of gravity; in the drawings, the force of weight is applied to the body, and not to the support.

In the popular "Physics Tutor" Kasatkina I.L. (5) body weight is defined as the force with which a body acts on a support or suspension due to attraction to the planet. In the following explanations and examples given by the author, answers are given only to the 3rd and 6th of the naive questions.

In most textbooks on physics, definitions of weight are given to some extent similar to the definitions of the authors (1), (2), (5). When studying physics in the 7th and 9th educational grades, perhaps this is justified. In the 10th profile classes with such a definition, when solving a whole class of problems, various kinds of naive questions cannot be avoided (in general, one should not at all strive to avoid any questions).

Authors Kamenetsky S.E., Orekhov V.P. in (6), delimiting and explaining the concepts of gravity and body weight, they write that body weight is a force that acts on a support or suspension. And that's it. You don't have to read between the lines. True, I still want to ask, how many supports and suspensions, and can the body have both support and suspension at once?

And, finally, let's look at the definition of body weight, which is given by Kasyanov V.A. (7) in a 10th grade physics textbook: “body weight is the total body elasticity force acting in the presence of gravity on all connections (supports, suspensions)”. If at the same time we remember that the force of gravity is equal to the resultant of two forces: the force of gravitational attraction to the planet and the centrifugal force of inertia, provided that this planet rotates around its axis, or some other force of inertia associated with the accelerated movement of this planet, then one could agree with this definition. Since, in this case, no one bothers us to imagine a situation where one of the components of gravity is negligible, for example, the case of a spaceship in deep space. And even with these reservations, it is tempting to remove the mandatory presence of gravity from the definition, because situations are possible when there are other forces of inertia that are not related to the movement of the planet or Coulomb forces of interaction with other bodies, for example. Or agree with the introduction of some "equivalent" gravity in non-inertial reference systems and define the force of weight for the case when there is no interaction of the body with other bodies, except for the body that creates gravitational attraction, supports and suspensions.

And yet, let's decide when the body weight equal to strength gravity in inertial reporting systems?

Suppose we have one support or one suspension. Is the condition sufficient that the support or suspension is stationary relative to the Earth (we consider the Earth to be an inertial frame of reference), or that it moves uniformly and rectilinearly? Take a fixed support, located at an angle to the horizon. If the support is smooth, then the body slides along the inclined plane, i.e. is not resting on a support and is not in free fall. And if the support is so rough that the body is at rest, then either the inclined plane is not a support, or the weight of the body is not equal to the force of gravity (you can, of course, go further and question that the weight of the body is not equal in absolute value and not opposite in direction support reaction force, and then there will be nothing to talk about at all). If, however, we consider the inclined plane as a support, and the sentence in brackets as irony, then, solving the equation for Newton's second law, which for this case will also be the equilibrium condition for the body on the inclined plane, written in projections onto the Y axis, we will obtain an expression for weight other than gravity:

So, in this case, it is not enough to say that the weight of the body is equal to the force of gravity, when the body and the support are motionless relative to the Earth.

Let us give an example with a suspension fixed relative to the Earth and a body on it. A positively charged metal ball on a thread is placed in a uniform electric field so that the thread makes some angle with the vertical. Let's find the weight of the ball from the condition that the vector sum of all forces is equal to zero for a body at rest.

As you can see, in the above cases, the weight of the body is not equal to the force of gravity when the condition of immobility of the support, suspension and body relative to the Earth is met. The features of the above cases are the existence of the friction force and the Coulomb force, respectively, the presence of which actually leads to the fact that the bodies are kept from moving. For vertical suspension and horizontal support, additional forces are not needed to keep the body from moving. Thus, to the condition of immobility of the support, suspension and body relative to the Earth, we could add that the support is horizontal and the suspension is vertical.

But would this addition solve our question? Indeed, in systems with a vertical suspension and a horizontal support, forces can act that reduce or increase the weight of the body. These can be the force of Archimedes, for example, or the force of Coulomb, directed vertically. To summarize for one support or one suspension: the weight of the body is equal to the force of gravity, when the body and the support (or suspension) are at rest (or move uniformly and rectilinearly) relative to the Earth, and only the reaction force of the support (or the elastic force of the suspension) and the force act on the body gravity. The absence of other forces, in turn, implies that the support is horizontal, the suspension is vertical.

Let us consider cases when a body with several supports and/or suspensions is at rest (or moves uniformly and rectilinearly with them relative to the Earth) and no other forces act on it, except for the reaction forces of the support, the elastic forces of the suspensions, and attraction to the Earth. Using the definition of the weight force Kasyanov V.A. (7), we find the total force of elasticity of the body bonds in the first and second cases presented in the figures. The geometric sum of the forces of elastic bonds F, equal in modulus to the weight of the body, based on the equilibrium condition, is really equal to gravity and opposite to it in direction, and the angles of inclination of the planes to the horizon and the angles of deviation of the suspensions from the vertical do not affect the final result.

Let us consider an example (figure below), when in a system that is motionless relative to the Earth, a body has a support and a suspension, and no other forces act in the system, except for the forces of elastic bonds. The result is similar to the above. The weight of the body is equal to the force of gravity.

So, if the body is on several supports and (or) suspensions, and rests together with them (or moves uniformly and rectilinearly) relative to the Earth, in the absence of other forces, except for the force of gravity and the forces of elastic bonds, its weight is equal to the force of gravity. At the same time, the location of supports and suspensions in space and their number do not affect the final result.

Consider examples of finding body weight in non-inertial frames of reference.

Example 1 Find the weight of a body of mass m moving in a spaceship with acceleration a in "empty" space (so far from other massive bodies that their gravity can be neglected).

In this case, two forces act on the body: the force of inertia and the reaction force of the support. If the modulus of acceleration is equal to the acceleration of free fall on the Earth, then the weight of the body will be equal to the force of gravity on the Earth, and the astronauts will perceive the nose of the ship as the ceiling, and the tail as the floor.

The artificial gravity created in this way for the astronauts inside the ship will not differ in any way from the “real” earth.

In this example, due to its smallness, we neglect the gravitational component of gravity. Then the force of inertia on the spacecraft will be equal to the force of gravity. In view of this, we can agree that the cause of the body weight in this case is gravity.

Let's go back to Earth.

Example 2

With respect to the ground with acceleration a a trolley is moving, on which a body is fixed on a thread of mass m, deviated by an angle from the vertical. Find the weight of the body, neglect the air resistance.

A task with one suspension, therefore, the weight is equal in modulus to the elastic force of the thread.

Thus, you can use any formula to calculate the elastic force, and, therefore, the weight of the body (if the air resistance force is large enough, then it will need to be taken into account as a term for the inertial force).

Let's work with the formula

Therefore, by introducing the "equivalent" force of gravity, we can assert that in this case the weight of the body is equal to the "equivalent" force of gravity. And finally, we can give three formulas for its calculation:

Example 3

Find the weight of a race car driver with mass m in a moving with acceleration a car.

At high accelerations, the reaction force of the seat back support becomes significant, and we will take it into account in this example. The total elastic force of the bonds will be equal to the geometric sum of both reaction forces of the support, which in turn is equal in absolute value and opposite in direction to the vector sum of the forces of inertia and gravity. For this problem, we find the module of the weight force by the formulas:

The effective free fall acceleration is found as in the previous problem.

Example 4

A ball on a thread of mass m is fixed on a platform rotating at a constant angular velocity ω at a distance r from its center. Find the weight of the ball.

Finding the body weight in non-inertial frames of reference in the given examples shows how well the formula for the body weight proposed by the authors in (3) works. Let's complicate the situation a bit in example 4. Let's assume that the ball is electrically charged, and the platform, together with its contents, is in a uniform vertical electric field. What is the weight of the ball? Depending on the direction of the Coulomb force, the weight of the body will decrease or increase:

It so happened that the question of weight naturally reduced to the question of gravity. If we define gravity as the resultant of the forces of gravitational attraction to a planet (or to any other massive object) and inertia, keeping in mind the principle of equivalence, leaving in the fog the origin of the force of inertia itself, then both components of gravity, or one of them, at least cause body weight. If there are other interactions in the system along with the force of gravitational attraction, the force of inertia and the forces of elasticity of the bonds, then they can increase or decrease the weight of the body, lead to a state when the weight of the body becomes equal to zero. And these other interactions can cause weight gain in some cases. Let's charge a ball on a thin non-conducting thread in a spaceship moving uniformly and rectilinearly in a distant "empty" space (we will neglect the forces of gravity because of their smallness). Let's put the ball in the electric field, the thread will be stretched, the weight will appear.

Summarizing the above, we conclude that the weight of the body is equal to the force of gravity (or the equivalent force of gravity) in any system where no other forces act on the body, except for the forces of gravity, inertia and elasticity of the bonds. Gravity, or "equivalent" gravity, is most often the cause of weight force. The force of weight and the force of gravity are of different nature and are applied to different bodies.

Bibliography.

1. Yavorsky B.M., Detlaf A.A. Handbook of physics for engineers and university students, M., Nauka, 1974, 944p.

2. Yavorsky B.M., Selezneva Yu.A. Physics Reference Guide for

entering universities and self-education., M., Nauka, 1984, 383p.

3. Koshkin N.I., Shirkevich M.G. Handbook of elementary physics., M., Nauka, 1980, 208s.

4. Kuhling H. Handbook of Physics., M., Mir, 1983, 520p.

5. Kasatkina I.L. Physics tutor. Theory. Mechanics. Molecular physics. Thermodynamics. Electromagnetism. Rostov-on-Don, Phoenix, 2003, 608s.

6. Kamenetsky S.E., Orekhov V.P. Methodology for solving problems in physics in high school., M., Enlightenment, 1987, 336s.

7. Kasyanov V.A. Physics. Grade 10., M., Bustard, 2002, 416s.

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Attention! The slide preview is for informational purposes only and may not represent the full extent of the presentation. If you are interested in this work, please download the full version.

This presentation is intended to help students in grades 9-10 in preparing the topic "Body Weight".

Presentation goals:

Repeat and deepen the concepts: "gravity"; "body weight"; "weightlessness".
Focus students' attention on the fact that gravity and body weight are different forces.
To teach students to determine the weight of a body moving vertically.

In everyday life, body weight is determined by weighing. From the 7th grade physics course, it is known that the force of gravity is directly proportional to the mass of the body. Therefore, the weight of a body is often identified with its mass or gravity. From the point of view of physics, this is a gross mistake. The weight of a body is a force, but gravity and the weight of a body are different forces.

The force of gravity is a special case of the manifestation of the forces of universal gravitation. Therefore, it is appropriate to recall the law of universal gravitation, as well as the fact that the forces of gravitational attraction appear when the bodies or one of the bodies have huge masses (slide 2).

When applying the law of universal gravitation for terrestrial conditions (slide 3), the planet can be considered as a homogeneous ball, and small bodies near its surface as point masses. The radius of the earth is 6400 km. The mass of the Earth is 6∙10 24 kg.

= ,
where g is the free fall acceleration.

Near the Earth's surface g = 9.8 m/s 2 ≈ 10 m/s 2.

Body weight - the force with which this body acts on a horizontal support or stretches the suspension.

Fig.1

On fig. 1 shows a body on a support. The reaction force of the support N (F control) is applied not to the support, but to the body located on it. The modulus of the reaction force of the support is equal to the modulus of the weight according to Newton's third law. The weight of the body is a special case of the manifestation of the force of elasticity. The most important feature weight is that its value depends on the acceleration with which the support or suspension is moving. Weight is equal to gravity only for a body at rest (or a body moving at a constant speed). If the body moves with acceleration, then the weight can be greater or less than the force of gravity, and even equal to zero.

In the presentation, using the example of solving problem 1, various cases of determining the weight of a load with a mass of 500 g suspended from a dynamometer spring are considered, depending on the nature of the movement:

a) the load is lifted up with an acceleration of 2 m / s 2;
b) the load is lowered down with an acceleration of 2 m / s 2;
c) the load is evenly lifted up;
d) the load falls freely.

Tasks for calculating body weight are included in the "Dynamics" section. The solution of problems on dynamics is based on the use of Newton's laws, followed by projection onto the selected coordinate axes. This determines the sequence of actions.

A drawing is made showing the forces acting on the body(s) and the direction of acceleration. If the direction of acceleration is unknown, it is chosen arbitrarily, and the solution of the problem gives an answer about the correctness of the choice.
Write down Newton's second law in vector form.
Select axes. It is usually convenient to direct one of the axes along the direction of the body's acceleration, the other one - perpendicular to the acceleration. The choice of axes is determined by considerations of convenience: so that the expressions for the projections of Newton's laws would have the simplest form.
The vector equations obtained in projections on the axis are supplemented with relations arising from the text of the problem conditions. For example, the equations of kinematic connection, definitions of physical quantities, Newton's third law.
Using the resulting system of equations, they try to answer the question of the problem.

Setting up animation in a presentation allows you to focus on the sequence of actions when solving problems. This is important, because the skills acquired while solving problems for calculating body weight will be useful to students when studying other topics and sections of physics.

Solution of problem 1.

1a. The body moves with an acceleration of 2 m / s 2 up (slide 7).

Fig.2

1b. The body moves with acceleration downwards (slide 8). We direct the OY axis down, then the projections of gravity and elasticity in equation (2) change signs, and it looks like:

(2) mg – F control = ma.

Therefore, P \u003d m (g-a) \u003d 0.5 kg ∙ (10 m / s 2 - 2 m / s 2) \u003d 4 N.

1c. At uniform motion(slide 9) equation (2) has the form:

(2) mg - F control = 0, since there is no acceleration.

Therefore, P \u003d mg \u003d 5 N.

1g In free fall = (slide 10). We use the result of solving Problem 1b:

P \u003d m (g - a) \u003d 0.5 kg (10 m / s 2 - 10 m / s 2) \u003d 0 H.

The state in which the weight of the body is zero is called the state of weightlessness.

Only the force of gravity acts on the body.

Speaking of weightlessness, it should be noted that astronauts experience a prolonged state of weightlessness during flight with the spacecraft engines turned off.

ship, and to experience a short-term state of weightlessness, just jump up. A running person at the moment when his feet do not touch the ground is also in a state of weightlessness.

The presentation can be used in the lesson when explaining the topic "Body Weight". Depending on the level of preparation of the class, students may not be offered all the slides with solutions to problem 1. For example, in classes with increased motivation to study physics, it is enough to explain how to calculate the weight of an accelerating body (task 1a), and the rest , c, d) provide for independent solution followed by verification. The conclusions obtained as a result of solving problem 1, students should try to draw on their own.

Conclusions (slide 11).

Body weight and gravity are different forces. They have a different nature. These forces are applied to different bodies: gravity - to the body; body weight - to the support (suspension).
The weight of the body coincides with the force of gravity only when the body is motionless or moves uniformly and rectilinearly, and other forces, except for the force of gravity and the reaction of the support (suspension tension), do not act on it.
The weight of the body is greater than the force of gravity (P> mg), if the acceleration of the body is directed in the direction opposite to the direction of gravity.
Body weight is less than gravity (P< mg), если ускорение тела совпадает по направлению с силой тяжести.
The state in which the weight of the body is zero is called the state of weightlessness. A body is in a state of weightlessness when it moves with free fall acceleration, that is, when only gravity acts on it.

Tasks 2 and 3 (slide 12) can be offered to students as homework.

The body weight presentation can be used to distance learning. In this case, it is recommended:

when viewing the presentation, write down the solution to problem 1 in a notebook;
independently solve problems 2, 3, using the sequence of actions proposed in the presentation.

The presentation on the topic “Body weight” allows you to show the theory of solving problems on dynamics in an interesting, accessible interpretation. Presentation activates cognitive activity students and allows you to form the right approach to solving physical problems.

Literature:

Grinchenko B.I. Physics 10-11. Theory of problem solving. For high school students and college students. - Velikiye Luki: Velikie Luki City Printing House, 2005.
Gendenstein L.E. Physics. Grade 10. At 2 p.m. H 1./L.E. Gendenstein, Yu.I. Dick. – M.: Mnemosyne, 2009.
Gendenstein L.E. Physics. Grade 10. At 2 o'clock. H 2. Task book./L.E. Gendenstein, L.A. Kirik, I.M. Gelgafgat, I.Yu. Nenashev.- M.: Mnemosyne, 2009.

Internet resources:

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