Response plan

1. Interaction of bodies.

2. Types of interaction.

4. Forces in mechanics.

Simple observations and experiments, for example with carts (Fig. 1), lead to the following qualitative

conclusions: a) a body on which other bodies do not act retains its speed unchanged; b) acceleration of a body occurs under the influence of other bodies, but also depends on the body itself;

c) the actions of bodies on each other always have the nature of interaction.

These conclusions are confirmed by observing phenomena in nature, technology, and outer space only in inertial reference systems.

The interactions differ from each other both quantitatively and qualitatively.

For example, it is clear that the more a spring is deformed, the greater the interaction of its coils. Or the closer two like charges are, the stronger they will attract.

In the simplest cases of interaction, the quantitative characteristic is force.

Force- the reason for the acceleration of bodies relative to the inertial frame of reference or their deformation.

Force is a vector physical quantity that is a measure of the acceleration acquired by bodies during interaction.

The force is characterized by: a) modulus; b) point of application; c) direction.

The unit of force is newton.

1 newton is the force that imparts an acceleration of 1 m/s to a body weighing 1 kg in the direction of action of this force, if other bodies do not act on it.

Resultant several forces is a force whose action is equivalent to the action of those forces that it replaces. The resultant is the vector sum of all forces applied to the body.

R g = F g 1 + F g 2 + ... + F g n.

Based on experimental data, Newton's laws were formulated.

Newton's second law. The acceleration with which a body moves is directly proportional to the resultant of all forces acting on the body, inversely proportional to its mass and directed in the same way as the resultant force:

a → = F → /t.

To solve problems, the law is often written in the form: F → =m a → .

Ticket No. 13 Body impulse. Law of conservation of momentum.

Response plan

1. Body impulse.

2.Law of conservation of momentum.

3. Jet propulsion.

Rest and motion are relative, the speed of a body depends on the choice of reference system; according to Newton's second law, regardless of whether the body was at rest or moving, a change in the speed of its movement can only occur under the action of force, i.e. as a result of interaction with other bodies. There are quantities that can be conserved when bodies interact. These quantities are energy And pulse .

Body impulse is called a vector physical quantity, which is a quantitative characteristic of the translational motion of bodies. The impulse is designated r → .

Pulse unit r →- kg m/s.

The momentum of a body is equal to the product of the mass of the body and its speed : p → = t υ → .

Pulse vector direction r → coincides with the direction of the body velocity vector υ → (Fig. 1).

The momentum of bodies obeys the conservation law, which is valid only for closed physical systems.

In mechanics closed called a system that is not affected by external forces or the action of these forces is compensated.

In this case р → 1 = р → 2, Where p → 1 is the initial impulse of the system, and p → 2- final.

In the case of two bodies included in the system, this expression has the form t 1 υ → 1 + t 2 υ → 2 = m 1 υ → 1 " + m 2 υ → 2 ", Where t 1 And t 2- masses of bodies, and υ → 1 and υ → 2 - speed before interaction, υ → 1 " And υ → 2 "- speed after interaction.

This formula for the law of conservation of momentum is: the momentum of a closed physical system is conserved under any interactions, occurring within this system.

. In the case of an open system, the momentum of the bodies of the system is not conserved.

However, if there is a direction in the system in which external forces do not act or their action is compensated, then the projection of the impulse in this direction is preserved.

If the interaction time is short (shot, explosion, impact), then during this time, even in the case of an open system, external forces slightly change the impulses of the interacting bodies.

Experimental studies of the interactions of various bodies - from planets and stars to atoms and elementary particles - have shown that in any system of interacting bodies, in the absence of action on the part of other bodies not included in the system, or the sum of the acting forces is equal to zero, the geometric sum of the momenta of the bodies really remains unchanged .

In mechanics, the law of conservation of momentum and Newton's laws are interconnected.

If the body weighs T for a time t a force acts and the speed of its movement changes from υ → 0 to υ → , then the acceleration of movement a → body is equal a → =(υ → - υ → 0)/ t.

Based on Newton's second law

for strength F → can be written down F → = ta → = m(υ → - υ → 0) / t, this implies

F → t = mυ → - mυ → 0.

F → t- a vector physical quantity characterizing the action of a force on a body over a certain period of time is called impulse of power. The SI unit of momentum is 1H s.

The law of conservation of momentum underlies jet propulsion.

Jet propulsion- This This is the movement of a body that occurs after separation of a part of it from the body.

Example: a body of mass T rested. Some part of the body has been separated t 1 with speed υ → 1 . Then the remaining part will move in the opposite direction with a speed υ → 2, the mass of the remaining part t 2. Indeed, the sum of the impulses of both parts of the body before separation was equal to zero and after separation will be equal to zero:

t 1 υ → 1 + m 2 υ → 2 =0, hence υ → 1 = -m 2 υ → 2 / m 1 .

K. E. Tsiolkovsky developed the theory of flight of a body of variable mass (a rocket) in a uniform gravitational field and calculated the fuel reserves necessary to overcome the force of gravity.

Tsiolkovsky's technical ideas are used in the creation of modern rocket and space technology. Movement using a jet stream according to the law of conservation of momentum is the basis of a hydrojet engine. The movement of many marine mollusks (octopus, jellyfish, squid, cuttlefish) is also based on the reactive principle.

Ticket number 17

The law of universal gravitation. Gravity. Body weight. Weightlessness.

Response plan

1. Gravity forces.

2. The law of universal gravitation.

3. Physical meaning of the gravitational constant.

4. Gravity.

5. Body weight, overload.

6. Weightlessness.

Isaac Newton suggested that there are forces of mutual attraction between any bodies in nature.

These forces are called forces of gravity, or forces of universal gravity. The force of universal gravity manifests itself in Space, the Solar System and on Earth. Newton derived the formula:

t 1 t 2

F=G----, Where G- proportionality coefficient, called gravitational

R 2

Constant.

The law of universal gravitation: between any material points there is a force of mutual attraction, directly proportional to the product of their masses and inversely proportional to the square of the distance between them, acting along the line connecting these points.

The physical meaning of the gravitational constant follows from the law of universal gravitation.

If t 1 = t 2 = 1 kg, R= 1 m, then G = F, i.e. the gravitational constant is equal to the force with which two bodies of 1 kg are attracted at a distance of 1 m. Numerical value: G= 6.67 10 -11 N m 2 / kg 2. The forces of universal gravity act between any bodies in nature, but they become noticeable at large masses. The law of universal gravitation is satisfied only for material points and balls (in this case, the distance between the centers of the balls is taken as the distance).

A particular type of universal gravitational force is the force of attraction of bodies towards the Earth (or to another planet). This force is called gravity.

Under the influence of this force, all bodies acquire gravitational acceleration. According to Newton's second law g = F T /m, hence, F T = tg.

The force of gravity is always directed towards the center of the Earth.

On the surface of the Earth, the acceleration of gravity is 9.831 m/s 2 .

Body weight called the force with which a body presses on a support or suspension as a result of gravitational attraction to the planet (Fig. 1).

Body weight is indicated p → . The unit of weight is 1 N. Since weight is equal to the force with which the body acts on the support, then, in accordance with Newton’s third law, the largest weight of the body is equal to the reaction force of the support. Therefore, to find the weight of a body, it is necessary to find what the support reaction force is equal to.

Rice. 1 Fig. 2

Let us consider the case when the body and the support do not move. In this case, the ground reaction force and body weight are equal to the force of gravity (Fig. 2):

P → = N → = tg → .

In the case of a body moving vertically upward together with a support with acceleration, according to Newton’s second law, we can write tg → + N → = ta →(Fig. 3, A).

In projection onto the axis OH:

-тg + N = ta, from here

N= t(g + a).

When moving vertically upward with acceleration, the weight of the body increases and is found according to the formula R= t(g + a).

An increase in body weight caused by accelerated movement of a support or suspension is called overload.

The effects of overload are experienced by astronauts and car drivers during sudden braking.

If a body moves down vertically,

tg → + N → = ta → ; tg - N = ta; N = m(g - a); P = m(g - a),

that is, the weight when moving vertically with acceleration will be less than the force of gravity (Fig. 3, b).

If the body is falling freely, in this case P = (g – g)m = 0

The state of a body in which its weight is zero is called weightlessness. The state of weightlessness is observed in an airplane or spacecraft when moving with free fall acceleration, regardless of the direction and value of the speed of their movement.

Ticket No. 24 Conversion of energy during mechanical vibrations. Free and forced vibrations. Resonance.

Response plan

1. Definition of oscillatory motion.

2. Free vibrations.

3. Energy transformations.

4. Forced vibrations. Mechanical vibrations

are movements of the body that are repeated exactly or approximately at equal intervals of time. The main characteristics of mechanical vibrations are: displacement, amplitude, frequency, period. Offset is a deviation from the equilibrium position. Amplitude- module of maximum deviation from the equilibrium position. Frequency- the number of complete oscillations performed per unit of time. Period- the time of one complete oscillation, i.e. the minimum period of time after which the process is repeated. Period and frequency are related by the relation: ν = 1 /T.

The simplest type of oscillatory motion is harmonic vibrations, in which the oscillating quantity changes over time according to the law of sine or cosine (Fig. 1 ).

Free are called oscillations that occur due to the initially imparted energy in the subsequent absence of external influences on the system performing the oscillations. For example, vibrations of a load on a thread (Fig. 2).

Rice. 1 Fig. 2

Let us consider the process of energy conversion using the example of oscillations of a load on a thread (see Fig. 2).

When the pendulum deviates from the equilibrium position, it rises to a height h relative to the zero level, therefore, at the point A a pendulum has potential energy tgh. When moving towards the equilibrium position, towards the point 0, the height decreases to zero, and the speed of the load increases, and at the point 0 all potential energy tgh turns into kinetic energy tυ 2 /2. At equilibrium, kinetic energy is at its maximum and potential energy is at its minimum. After passing the equilibrium position, the kinetic energy is converted into potential energy, the speed of the pendulum decreases and, at the maximum deviation from the equilibrium position, becomes equal to zero. With oscillatory motion, periodic transformations of its kinetic and potential energy always occur.

With free mechanical vibrations, energy loss inevitably occurs to overcome resistance forces. If oscillations occur under the influence of a periodic external force, then such oscillations are called forced. For example, parents swing a child on a swing, a piston moves in a car engine cylinder, an electric razor blade and a sewing machine needle vibrate. The nature of forced oscillations depends on the nature of the action of the external force, on its magnitude, direction, frequency of action and does not depend on the size and properties of the oscillating body. For example, the foundation of the motor on which it is attached performs forced oscillations with a frequency determined only by the number of revolutions of the motor - and does not depend on the size of the foundation.

When the frequency of the external force and the frequency of the body’s own vibrations coincide, the amplitude of the forced vibrations increases sharply. This phenomenon is called mechanical resonance. Graphically, the dependence of forced oscillations on the frequency of the external force is shown in Figure 3.

The phenomenon of resonance can cause the destruction of cars, buildings, bridges if their natural frequencies coincide with the frequency of a periodically acting force. Therefore, for example, engines in cars are installed on special shock absorbers, and military units are prohibited from keeping pace when moving across the bridge.

In the absence of friction, the amplitude of forced oscillations during resonance should increase with time without limit. In real systems, the amplitude in the steady state of resonance is determined by the condition of energy loss during the period and the work of the external force during the same time. The less friction, the greater the amplitude during resonance.

Ticket No. 16

Capacitors. Capacitance of the capacitor. Application of capacitors.

Response plan

1. Definition of a capacitor.

2. Designation.

3. Electrical capacity of the capacitor.

4. Electrical capacity of a flat capacitor.

5. Connection of capacitors.

6. Application of capacitors.

To accumulate significant quantities of opposite electrical charges, capacitors are used.

Capacitor is a system of two conductors (plates) separated by a dielectric layer, the thickness of which is small compared to the size of the conductors.

Example, two flat metal plates placed in parallel and separated by a dielectric form a flat capacitor.

If the plates of a flat capacitor are given charges of equal magnitude and opposite sign, then the voltage between the plates will be twice as great as the voltage of one plate. Outside the plates the tension is zero.

Capacitors are designated in the diagrams as follows:

The electrical capacity of a capacitor is a value equal to the ratio of the charge on one of the plates to the voltage between them. Electrical capacity is designated C.

A-priory WITH= q/U. The unit of electrical capacitance is the farad (F).

1 farad is the electrical capacity of such a capacitor, the voltage between the plates of which is equal to 1 volt when the plates are charged with opposite charges of 1 coulomb.

The electrical capacity of a flat capacitor is found by the formula:

C = ε ε 0 - ,

where ε 0 is the electrical constant, ε is the dielectric constant of the medium, S is the area of ​​the capacitor plate, d- distance between plates (or dielectric thickness).

If capacitors are connected to form a battery, then with parallel connection C O = C 1 + C 2(Fig. 1). For serial connection

- = - + - (Fig. 2).

C O C 1 C 2

Depending on the type of dielectric, capacitors can be air, paper, or mica.

Capacitors are used to store electricity and use it during rapid discharge (photo flash), to separate DC and AC circuits, in rectifiers, oscillating circuits and other electronic devices.

Ticket No. 15

Work and power in a DC circuit. Electromotive force. Ohm's law for a complete circuit.

Response plan

1. Current work.

2. Joule-Lenz law.

3. Electromotive force.

4. Ohm's law for a complete circuit.

In an electric field from the formula for determining voltage

U = A / q

then to calculate the work of electric charge transfer

A = U q since for current the charge q = I t

then the work of the current:

A = UIt or A = I 2 Rt = U 2 / R t

Power by definition N = A / t hence, N = UI = I 2 R = U 2 /R

Joule-Lenz law: When current passes through a conductor, the amount of heat released in the conductor is directly proportional to the square of the current strength, the resistance of the conductor and the time of passage of the current, Q = I 2 Rt.

A complete closed circuit is an electrical circuit that includes external resistances and a current source (Fig. 1).

As one of the sections of the circuit, the current source has resistance, which is called internal , r.

In order for the current to flow through a closed circuit, it is necessary that additional energy be imparted to the charges in the current source; this energy is taken from the work of moving the charges, which is produced by forces of non-electric origin (external forces) against the forces of the electric field.

The current source is characterized by EMF - electromotive force of the source.

EMF - characteristic of a non-electric energy source in an electrical circuit necessary to maintain electric current in it .

EMF is measured by the ratio of the work done by external forces to move a positive charge along a closed circuit to this charge

Ɛ = A ST / q.

Let it take time t an electric charge will pass through the cross section of the conductor q.

Then the work of external forces when moving a charge can be written as follows: A ST = Ɛ q.

According to the definition of current q=I t,

A ST = Ɛ I t

When performing this work on the internal and external sections of the circuit, the resistance of which R and r, some heat is released.

According to the Joule-Lenz law, it is equal to : Q = I 2 R t + I 2 r t

According to the law of conservation of energy A = Q. Hence, Ɛ = IR + Ir .

The product of current and the resistance of a section of a circuit is often called voltage drop in this area.

The EMF is equal to the sum of the voltage drops in the internal and external sections of the closed circuit. ABOUT

I = Ɛ / (R + r).

This relationship is called Ohm's law for a complete circuit

The current strength in a complete circuit is directly proportional to the emf of the current source and inversely proportional to the total resistance of the circuit .

When the circuit is open, the emf is equal to the voltage at the source terminals and, therefore, can be measured with a voltmeter.

Ticket No. 12

Interaction of charged bodies. Coulomb's law. Law of conservation of electric charge.

Response plan

1. Electric charge.

2. Interaction of charged bodies.

3. Law of conservation of electric charge.

4. Coulomb's law.

5. Dielectric constant.

6. Electric constant.

The laws of interaction of atoms and molecules are explained on the basis of the structure of the atom, using the planetary model of its structure.

At the center of the atom there is a positively charged nucleus, around which negatively charged particles rotate in certain orbits.

The interaction between charged particles is called electromagnetic.

The intensity of electromagnetic interaction is determined by the physical quantity - electric charge, which denoted by q.

Unit of electric charge - pendant (Cl).

1 pendant- this is an electric charge that, passing through the cross-section of a conductor in 1 s, creates a current of 1 A in it.

The ability of electric charges to both mutually attract and repulse is explained by the existence of two types of charges.

One type of charge is called positive, The carrier of the elementary positive charge is the proton.

Another type of charge was called negative, its carrier is an electron. The elementary charge is e = 1.6 × 10 -19 Cl.

Electric charge is neither created nor destroyed, but only transferred from one body to another.

This fact is called law of conservation of electric charge.

In nature, an electric charge of the same sign does not appear or disappear.

The appearance and disappearance of electric charges on bodies in most cases is explained by the transitions of elementary charged particles - electrons - from one body to another.

Electrification- this is a message to the body of an electric charge.

Electrification can occur through contact (friction) of dissimilar substances and during irradiation.

When electrification occurs in the body, an excess or deficiency of electrons occurs.

If there is an excess of electrons, the body acquires a negative charge, and if there is a deficiency, it acquires a positive charge.

The fundamental law of electrostatics was established experimentally by Charles Coulomb:

The modulus of the force of interaction between two point fixed electric charges in a vacuum is directly proportional to the product of the magnitudes of these charges and inversely proportional to the square of the distance between them.

F = k q 1 q 2 / r 2,

where q 1 and q 2 are the charge modules, r is the distance between them, k is the proportionality coefficient, depending on the choice of system of units, in SI

k = 9 10 9 N m 2 /Cl 2.

The quantity showing how many times the force of interaction between charges in a vacuum is greater than in a medium is called dielectric constant of the mediumε.

For a medium with dielectric constant ε, Coulomb’s law: F = k q 1 q 2 /(ε r 2).

Instead of the coefficient k, a coefficient called electrical is often used constant ε 0 .

The electrical constant is related to the coefficient k as follows:

k = 1/4πε 0 and is numerically equal to ε 0 = 8.85 10 -12 C/N m 2

Using the electric constant, Coulomb's law is:

1 q 1 q 2

F = --- ---

4 π ε 0 r 2

The interaction of stationary electric charges is called electrostatic, or Coulomb interaction. Coulomb forces can be depicted graphically (Fig. 1).

The Coulomb force is directed along the straight line connecting the charged bodies. It is the attractive force for different signs of charges and the repulsive force for the same signs.

In classical mechanics it is believed that:

a) The mass of a material point does not depend on the state of motion of the point, being its constant characteristic.

b) Mass is an additive quantity, i.e. the mass of a system (for example, a body) is equal to the sum of the masses of all material points that are part of this system.

c) The mass of a closed system remains unchanged during any processes occurring in this system (law of conservation of mass).

Density ρ body at a given point M called mass ratio dm small body element including a point M, to the value dV volume of this element:

The dimensions of the element under consideration must be so small that by changing the density within its limits many times greater intermolecular distances can be achieved.

The body is called homogeneous , if the density is the same at all its points. The mass of a homogeneous body is equal to the product of its density and volume:

Mass of a heterogeneous body:

dV,

where ρ is a function of coordinates, and integration is carried out over the entire volume of the body. Medium density (ρ) of an inhomogeneous body is called the ratio of its mass to volume: (ρ)=m/V.

Center of mass of the system material points is called point C, radius vector

which is equal to: and – mass and radius vector i th material point, n is the total number of material points in the system, and m= is the mass of the entire system.

Center of mass speed:

Vector quantity

, equal to the product of the mass of a material point and its speed, is called impulse, or amount of movement , this material point. Impulse of the system of material points is called a vector p, equal to the geometric sum of the momenta of all material points of the system:

The momentum of the system is equal to the product of the mass of the entire system and the speed of its center of mass:

Newton's second law

The basic law of the dynamics of a material point is Newton’s second law, which talks about how the mechanical motion of a material point changes under the influence of forces applied to it. Newton's second law reads: rate of change of momentum ρ material point is equal to the force acting on it F, i.e.

, or

where m and v are the mass and speed of the material point.

If several forces simultaneously act on a material point, then under the force F in Newton's second law, you need to understand the geometric sum of all acting forces - both active and reaction reactions, i.e. resultant force.

Vector quantity Fdt called elementary impulse strength F in a short time dt her actions. Impulse force F for a finite period of time from

to is equal to a definite integral:

Where F, in general, depends on time t.

According to Newton's second law, the change in the momentum of a material point is equal to the momentum of the force acting on it:

d p= F dt And

, is the value of the momentum of the material point at the end ( ) and at the beginning ( ) of the time period under consideration.

Since in Newtonian mechanics the mass m material point does not depend on the state of motion of the point, then

Therefore, the mathematical expression of Newton's second law can also be represented in the form

– acceleration of a material point, r is its radius vector. Accordingly, the wording Newton's second law states: the acceleration of a material point coincides in direction with the force acting on it and is equal to the ratio of this force to the mass of the material point.

The tangential and normal acceleration of a material are determined by the corresponding components of the force F

, is the magnitude of the velocity vector of the material point, and R– radius of curvature of its trajectory. The force imparting normal acceleration to a material point is directed towards the center of curvature of the point’s trajectory and is therefore called centripetal force.

If several forces simultaneously act on a material point

, then its acceleration. Consequently, each of the forces simultaneously acting on a material point imparts to it the same acceleration as if there were no other forces (the principle of independence of the action of forces).

Differential equation of motion of a material point called the equation

In projections onto the axes of a rectangular Cartesian coordinate system, this equation has the form

, ,

where x, y and z are the coordinates of the moving point.

Newton's third law. Movement of the center of mass

The mechanical action of bodies on each other is manifested in the form of their interaction. This is what he says Newton's third law: two material points act on each other with forces that are numerically equal and directed in opposite directions along the straight line connecting these points.

– force acting on i- yu material point from the side k- th, a is the force acting on the kth material point from the i-th side, then, according to Newton’s third law, they are applied to different material points and can be mutually balanced only in those cases when these points belong to the same absolutely solid body.

Newton's third law is an essential addition to the first and second laws. It allows you to move from the dynamics of a single material point to the dynamics of an arbitrary mechanical system (system of material points). From Newton’s third law it follows that in any mechanical system the geometric sum of all internal forces is equal to zero: where

– the resultant of external forces applied to i th material point.

From Newton's second and third laws it follows that the first derivative with respect to time t from impulse p mechanical system is equal to the main vector of all external forces applied to the system,

.

This equation expresses law of change in the momentum of the system.

The reason for a change in the speed of movement of a body is always its interaction with other bodies.

After turning off the engine, the car gradually slows down and stops. The main reason

changes in vehicle speed - the interaction of its wheels with the road surface.

A ball lying motionless on the ground never moves by itself. The speed of the ball changes only as a result of the action of other bodies on it, for example the legs of a football player.

Constancy of the ratio of acceleration modules.

When two bodies interact, the velocities of both the first and second bodies always change, i.e., both bodies acquire acceleration. The acceleration modules of two interacting bodies may be different, but their ratio turns out to be constant for any interaction:

Inertia of bodies.

The constancy of the ratio of the acceleration modules of two bodies during any of their interactions shows that the bodies have some property on which their acceleration during interactions with other bodies depends. The acceleration of a body is equal to the ratio of the change in its speed to the time during which this change occurred:

Since the time of action of bodies on each other is the same, the change in speed is greater for the body that acquires greater acceleration.

The less the speed of a body changes when interacting with other bodies, the closer its motion is to uniform rectilinear motion by inertia. Such a body is called more inert.

All bodies have the property of inertia. It consists in the fact that it takes some time to change the speed of a body when it interacts with any other bodies.

The manifestation of the property of inertia of bodies can be observed in the following experiment. We hang a metal cylinder on a thin thread (Fig. 20, a), and tie exactly the same thread from below. Experience shows that with gradual tension of the lower thread, the upper thread breaks (Fig. 20, b). If you sharply pull the lower thread, the upper thread remains intact, but the lower thread breaks (Fig. 20, c). In this case, the inertia of the cylinder is affected, which does not have time in a short time to sufficiently change its speed and make a noticeable movement sufficient to break the upper thread.

Body mass.

The property of a body on which its acceleration when interacting with other bodies depends is called inertia. A quantitative measure of body inertia is body mass. The more mass a body has, the less acceleration it receives during interaction.

Therefore, in physics it is accepted that the ratio of the masses of interacting bodies is equal to the inverse ratio of the acceleration modules:

The unit of mass in the International System is the mass of a special standard made from an alloy of platinum and iridium. The mass of this standard is called a kilogram (kg).

The mass of any body can be found by interacting this body with a standard mass.

By definition of the concept of mass, the ratio of the masses of interacting bodies is equal to the inverse ratio of the modules of their accelerations (5.2). By measuring the acceleration modules of the body and the standard, one can find the ratio of the mass of the body to the mass of the standard

The ratio of the body mass to the mass of the standard is equal to the ratio of the acceleration modulus of the standard. To the acceleration module of a body during their interaction.

Body mass can be expressed through the mass of the standard:

Body mass is a physical quantity that characterizes its inertia.

Mass measurement.

To measure the masses of bodies in science, technology and everyday practice, the method of comparing the mass of a body with the mass of a standard by determining the accelerations of bodies during their interaction is rarely used. A commonly used method is to compare the masses of bodies using scales.

When weighing, the ability of all bodies to interact with the Earth is used to determine masses. Experiments have shown that bodies with the same mass are equally attracted to the Earth. The equality of the attraction of bodies to the Earth can, for example, be established by the equal stretching of the spring when bodies with equal masses are alternately suspended from it.

Question 4

Inertial reference systems

Inertial frames of reference. Newton's first law

Question 3

Newton's first law– (the law of inertia) there are such reference systems relative to which a translationally moving body, while maintaining its speed unchanged, is at rest or moves rectilinearly and uniformly, if it is not acted upon by external bodies or their action is equal to zero, that is, it is compensated.

A reference system in which the law of inertia is valid: a material point, when no forces act on it (or mutually balanced forces act on it), is in a state of rest or uniform linear motion. Any reference system moving with respect to an axis. O. progressively, uniformly and rectilinearly, there is also I. s. O. Consequently, theoretically there can be any number of equal i.s. o., possessing the important property that in all such systems the laws of physics are the same (the so-called principle of relativity).

Interaction of bodies. The reason for a change in the speed of movement of a body is always its interaction with other bodies.

After turning off the engine, the car gradually slows down and stops. The main reason for changes in vehicle speed is the interaction of its wheels with the road surface.

A ball lying motionless on the ground never moves by itself. The speed of the ball changes only as a result of the action of other bodies on it, for example the legs of a football player.

Constancy of the ratio of acceleration modules. When two bodies interact, the velocities of both the first and second bodies always change, i.e., both bodies acquire acceleration. The acceleration modules of two interacting bodies may be different, but their ratio turns out to be constant for any interaction:

The interactions differ from each other both quantitatively and qualitatively. For example, it is clear that the more a spring is deformed, the greater the interaction of its turns. Or the closer two charges of the same name are, the stronger they will attract. In the simplest cases of interaction, the quantitative characteristic is force.

Body mass. The property of a body on which its acceleration when interacting with other bodies depends is called inertia.

A quantitative measure of body inertia is body mass. The more mass a body has, the less acceleration it receives during interaction.

Therefore, in physics it is accepted that the ratio of the masses of interacting bodies is equal to the inverse ratio of the acceleration modules:

The unit of mass in the International System is the mass of a special standard made from an alloy of platinum and iridium. The mass of this standard is called kilogram(kg).

The mass of any body can be found by interacting this body with a standard mass.

By definition of the concept of mass, the ratio of the masses of interacting bodies is equal to the inverse ratio of the modules of their accelerations (5.2). By measuring the acceleration modules of the body and the standard, one can find the ratio of the body mass to the mass of the standard:

The ratio of the mass of the body to the mass of the standard is equal to the ratio of the acceleration module of the standard to the acceleration module of the body during their interaction.

Body mass can be expressed through the mass of the standard:

Body mass is a physical quantity that characterizes its inertia.

Force is the reason for the acceleration of bodies relative to an inertial reference frame or their deformation. Force is a vector physical quantity, which is a measure of the acceleration acquired by bodies during interaction. The force is characterized by: a) modulus; b) point of application; c) direction.

Newton's second law - the force acting on a body is equal to the product of the mass of the body and the acceleration imparted by this force.

Physics

Body mass

Interaction of bodies. The reason for a change in the speed of movement of a body is always its interaction with other bodies.

Constancy of the ratio of acceleration modules. When two bodies interact, the velocities of both the first and second bodies always change, i.e. both bodies acquire acceleration. The acceleration modules of two interacting bodies may be different, but their ratio turns out to be constant for any interaction:

Inertia of bodies. The constancy of the ratio of the acceleration modules of two bodies during any of their interactions shows that the bodies have some property on which their acceleration during interactions with other bodies depends.

The less the speed of a body changes when interacting with other bodies, the closer its motion is to uniform rectilinear motion by inertia. Such a body is called more inert.

All bodies have the property of inertia. It consists in the fact that it takes some time to change the speed of a body when it interacts with other bodies.

Body mass. The property of a body on which its acceleration when interacting with other bodies depends is called inertia. A quantitative measure of inertia is body weight. The more mass a body has, the less acceleration it receives during interaction.

Therefore, in physics it is accepted that the ratio of the masses of interacting bodies is equal to the inverse ratio of the acceleration modules:

m 1 /m 2 =a 2 /a 1 (5.2)

Body mass is a physical quantity that characterizes its inertia.

Density of matter. Mass ratio m body to its volume V is called the density of the substance:

Density is expressed in kilograms per cubic meter, the unit of density is 1 kg/m3.

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