Force in physics - types, formulas and definitions with examples

Force:

When studying natural phenomena, different physical quantities are used. In order to describe qualitatively and quantitatively the interaction of bodies, a physical quantity is introduced, which is called force.

Newton as a unit of measurement:

Newton is a unit of force in the International System of Units (SI), named after the English physicist Isaac Newton. Newton is a derived unit.

Newton as a unit of measurement has a Russian designation - N and an international designation - N.

Based on Newton's second law, a force of 1 newton (N) is defined as a force that changes the speed of a body weighing 1 kg by 1 m/s in 1 second in the direction of the force.

H = kg m/s2.

1 N = 1 kg m/s2.

There are also other definitions. A force of 1 newton (N) is a force that, when applied to a body weighing 1 kg, acquires an acceleration of 1 m/s2, or it is a force that does 1 Joule of work when moving any body 1 meter.

N = J/m.

1 N = 1 J/m.

Based on the definitions presented, one can discover the connection between Newton and gravity. It turns out that a gravity force of just 1 newton acts on a body weighing 102 grams, and therefore the force of gravity acting on a body weighing 1 kilogram is equal to 9.8 N.

The newton was introduced into the International System of Units by the decision of the XI General Conference on Weights and Measures in 1960, simultaneously with the adoption of the SI system as a whole. In accordance with the SI rules regarding derived units named after scientists, the name of the unit newton is written with a lowercase letter, and its designation is written with a capital letter (N). This spelling of the designation is also preserved in the designations of other derived units formed using Newton.

Definition of strength

Force is a physical quantity that serves as a measure of the interaction of bodies and causes changes in the speeds of bodies or their parts.

Observation. If we consider, for example, the interaction of a hand with a volleyball, then we say: “The ball exerts a force on the hand or the hand exerts a force on the ball.”

Experience. Let's hang an apple on a spring (Fig. 66).

The spring will lengthen. If you hang two apples on it, it will lengthen more. So, two apples exert a greater force on the spring than one.

The result of the action of one body on another depends on the value of the applied force.

The tighter the door is closed, the more force we have to push or pull it to open it.

To make it easier to open the door, its handle is attached as far as possible from the hinges. Try opening the door by pushing it at a point located near the hinges. You will find that this is much more difficult to do than with a pen. The result of the action of one body on another depends on the point of application of the force.

To achieve a certain result of an action, for example, stretching or compressing a spring, closing or opening a door, you need to apply forces in different directions.

The action of one body on another depends on the direction of the force.

Graphically, force is depicted as a straight line segment with an arrow at the end (Fig. 67).

The beginning of the segment is combined with the point of application of force. The length of a segment on a certain scale is equal to the value of the force. The arrow shows the direction of the force. Quantities that, in addition to their numerical value, are also characterized by a direction in space are called vectors, or vectors (from the Latin word vector - leading, carrying).

Why do bodies change their state in space?

Any changes in nature occur as a result of interaction between bodies. To change the position of the car on the rails, railway workers send a locomotive towards it, which displaces the car from its place and puts it in motion (Fig. 32).

A sailboat can stand near the shore for a long time until a fair wind blows and affects its sails (Fig. 33). The wheels of a toy car can rotate at any speed, but the toy will not change its position unless a board or ruler is placed under the toy (Fig. 34). The shape or size of a spring can be changed only by hanging a weight from it or by pulling one of its ends with your hand.

All bodies in nature are connected in one way or another and act on each other either directly or through physical fields. This action is always reciprocal. If a diesel locomotive acts on a car and changes its speed, then the speed of the diesel locomotive also changes due to the reverse action of the car. The Sun acts on all bodies on Earth and on the Earth itself, keeping it in orbit. But the Earth also attracts the Sun and, in turn, changes its trajectory. Thus, in all cases we can only talk about the mutual action of bodies - interaction.

During interaction, the speeds of bodies or their parts can change.

However, interacting with different bodies, a given body will change its speed in different ways. Thus, a sailboat can acquire speed due to the action of wind on it. But the same result can be obtained by turning on the engine located on the sailboat. A sailboat can also move a boat by acting on it through a cable. In order not to name all the interacting bodies each time, all these actions are united by one concept of force.

Multiples and submultiples of Newton:

Multiples and submultiples are formed using standard SI prefixes.

Change in body weight

The action of a body on support due to interaction with the Earth is often called body weight. Interestingly, the amount of body weight depends on the acceleration of movement in the vertical direction. In the case where the direction of acceleration is opposite to the acceleration of gravity, an increase in weight is observed. If the acceleration of the body coincides with the direction of free fall, then the weight of the body decreases. For example, being in an ascending elevator, at the beginning of the ascent a person feels an increase in weight for some time. There is no need to say that its mass changes. At the same time, we separate the concepts of “body weight” and its “mass”.

Basic statement of mechanics

To describe the motion of a body, you can take any reference system. Typically, a reference frame associated with the Earth is used for this. If a body changes its speed, next to it you can always find another body that acts on it. So, if you lift a stone and let it go, it will not remain hanging in the air, but will fall down. Therefore, something had an effect on him. In this case, the Earth itself pulled the stone towards itself. This implies the main statement of mechanics:

Basic statement of mechanics

A change in speed (acceleration) of a body is always caused by the influence of other bodies on it.

According to the statement, if no forces act on a body, its acceleration will be zero, and it will either be at rest or move uniformly and in a straight line (at a constant speed).

But in our world we don’t always see this. And there is an explanation for this. If a body is at rest, it does not really change its speed. So, the ball lies on the grass until it is kicked. After he is kicked, he starts to roll, but then stops. While the ball is rolling, it is no longer touched. It would seem that, according to the basic statement of mechanics, the ball should roll forever. But this does not happen, because the ball is affected by the frictional force that arises between its surface and the grass.

The main statement of mechanics can be illustrated in outer space in a place where the force of attraction of cosmic bodies is negligible. If you give a body speed in space and release it, it will move at that speed in a straight line until other forces act on it. A striking example is intergalactic stars, or rogue stars. They are not gravitationally connected to any of the galaxies, and therefore move at a constant speed. Thus, the star HE 0437-5439 is moving away from our galaxy at a constant speed of 723 km/s.

A free body is a body that is not acted upon by other bodies. A free body is either at rest or moving rectilinearly and uniformly.

Types of devices

Depending on the design and principle of operation, all dynamometers are divided into mechanical, hydraulic, and electrical. A special category of force meters are disposable sensors.

Mechanical (lever or spring) dynamometers

Current unit

Mechanical dynamometers measure force and torque through physical processes such as elastic tension and compression.

The main types of such devices are:

• Lever - in such devices, the elastic body is a lever, the deformation of which is transmitted to a sensor or measuring device connected to it;
• Mechanical - these are the simplest and most common dynamometers, they consist of an elastic spring connected to a pointer moving along a circular or vertical scale, with marked divisions, or a sensor that transmits an electrical signal to an electronic unit with an electronic scoreboard (monochrome liquid crystal display).

On a note. Before measuring force using a mechanical dynamometer, which is an ordinary steelyard in its essence and design, be sure to make sure that the arrow on the round or vertical scale is located at the value “0”. If the arrow is lost and shows a value greater than zero in the absence of load, it means that the elastic element has undergone irreparable deformation caused by the application of a load to it that significantly exceeds the maximum permissible. Such a device will no longer be accurate and will eventually fail.

Steelyard - a device for measuring the weight (heaviness) of various objects

Hydraulic dynamometer

The hydraulic meter consists of:

• Several cylinders, inside of which there are movable rods with pistons;
• A lever attached to the top of the rods;
• Measuring device (pressure gauge).

Oil is used as a working fluid in such meters.

This device works as follows:

1. The force applied to the lever through the rods and pistons acts on the liquid in the cylinders;
2. The displaced liquid flows through the tubes to the pressure gauge;
3. The pressure gauge measures the pressure of the fluid coming from the cylinders and displays it on a round analog dial scale or liquid crystal monochrome digital display in the form of a certain value of the force acting on the lever.

Hydraulic force meter

Such devices make it possible to determine the value of force with greater accuracy than mechanical analogues. However, compared to the latter, such dynamometers are characterized by a higher price, expensive repairs and maintenance, inaccuracy in case of cylinder depressurization and the occurrence of working fluid leaks.

Electric dynamometer

Electric dynamometers consist of:

• An elastic element connected to an inductive, capacitive, piezoelectric, vibration-frequency or strain-resistor type sensor that reacts to its deformation;
• Amplifier of the electrical signal coming from the sensor;
• Electronic unit equipped with a display.

The operating principle of such a device is quite simple:

1. The force applied to the elastic body is recorded by a sensor;
2. The sensor sends an electrical signal to the amplifier, which, in turn, transmits it to the electronic unit;
3. An electronic unit with a built-in microcircuit translates the signal received from the amplifier into a graphical representation of the force value on the display.

Electric dynamometer

On a note. Since such electrical devices, unlike most mechanical and hydraulic ones, are equipped with an electronic unit and a display, they must be turned on with a special button before use. These devices are powered by built-in rechargeable batteries. Some models can be connected to a 220 V network to provide power. Devices that have a low power supply or are not connected to the network will not turn on and work.

Disposable sensors

Such sensors, unlike the analogues described above, are used to measure destructive loads that have enormous power: a very strong blow, a powerful explosion. However, before they lose integrity and completely fail, they quite accurately measure and transmit data about the force that destroyed them to an electronic unit located at a safe distance.

Disposable sensor

Interaction of magnetic field with current

The effect of a magnetic field on a direct current is described. In this case, the force with which the magnetic field acts on a conductor with current placed in it is called the Ampere force.

The interaction of the magnetic field with causes force manifestation. Ampere's force, the formula of which is F = IBlsinα, depends on (B), the length of the active part of the conductor (l), (I) in the conductor and the angle between the direction of the current and the magnetic induction.

Thanks to the last dependence, it can be argued that the vector of action of the magnetic field can change when the conductor is rotated or the direction of the current changes. The left hand rule allows you to establish the direction of action. If the left hand is positioned in such a way that the magnetic induction vector enters the palm, the four fingers are directed along the current in the conductor, then the thumb bent 90 ° will show the direction of the magnetic field.

Mankind has found applications for this effect, for example, in electric motors. Rotation of the rotor is caused by a magnetic field created by a powerful electromagnet. The force formula allows you to judge the possibility of changing engine power. As the current or field strength increases, the torque increases, which leads to an increase in motor power.

Application of dynamometers

Dynamometers have a very wide range of applications. For example, in medicine, special medical dynamometers . They are designed to measure the strength of various human muscle groups.

One such device is a hand-held dynamometer called a strength meter (Figure 3). It measures the muscle strength of the arm when clenching the hand into a fist.

Figure 3. Strength meter - electronic hand dynamometer.

In order to measure the traction forces of locomotives, tractors, sea tugs and other equipment, special traction dynamometers (Figure 4).

Figure 4. Application of a traction dynamometer.

Such dynamometers are capable of measuring forces up to several tens of thousands of newtons. Modern models have a remote control with a display (Figure 5).

Figure 5. Traction dynamometer.

When installing wires and cables, dynamometers are used to determine the tension force of the wire (Figure 6). There are special installation tables with the required values.

Figure 6. Dynamometer for installation work.

Dynamometers are used not only in special equipment, but also in places common to us: in the subway, on buses and even in the elevator. Here these devices are used to measure the compression force of the leaves of various automatic doors.

Plasma problems

The interaction of a magnetic field and matter is used in cyclotrons. Problems associated with the laboratory study of plasma do not allow it to be kept in closed vessels. High can only exist at high temperatures. Plasma can be kept in one place in space using magnetic fields, twisting the gas in the form of a ring. Controlled ones can also be studied by twisting high-temperature plasma into a cord using magnetic fields.

An example of the effect of a magnetic field under natural conditions on ionized gas is the Aurora Borealis. This majestic spectacle is observed above the Arctic Circle at an altitude of 100 km above the earth's surface. The mysterious colorful glow of the gas could only be explained in the 20th century. The earth's magnetic field near the poles cannot prevent the solar wind from entering the atmosphere. The most active radiation, directed along magnetic induction lines, causes ionization of the atmosphere.

Particle trajectories

The interaction of a magnetic field with a charge is widely used in mass spectrographs in the study of elementary particles.

The action of the field in this case causes the appearance of a force called the Lorentz force. When a charged particle moving at a certain speed enters a magnetic field, the formula of which is F = vBqsinα, causes the particle to move in a circle.

In this mathematical model, v is the modulus of the velocity of a particle whose electric charge is q, B is the magnetic induction of the field, α is the angle between the directions of velocity and magnetic induction.

The particle moves in a circle (or arc of a circle), since the force and speed are directed at an angle of 90 ° to each other. Changing the direction of linear velocity causes acceleration to appear.

The rule of the left hand, discussed above, also occurs when studying the Lorentz force: if the left hand is positioned in such a way that the magnetic induction vector enters the palm, four fingers extended in a line are directed along the speed of a positively charged particle, then bent by 90 ° the thumb will indicate the direction of the force.

Content

Force is a vector physical quantity that has a direction and a numerical value. How to determine its numerical value?

As you already know, for this we need to define a unit of measurement - a certain standard accepted as a unit. Any force can be taken as such a unit. For example, the force of gravity that acts on a specific body.

You can also take the elastic force of the selected spring, stretched to a certain length. In this lesson you will learn what force is taken as a unit, get a formula for determining the force of gravity and learn how to use it to solve problems.

One more unit

In addition to the newton, the SI also has a larger physical unit of force - the kilonewton (kN). It is equal to 1000 N. For example, the thrust from a class Y in a steam locomotive and the thrust from an F100 fighter jet are both about 130 kN.

One kilonewton is 102.0 kgf or about 100 kg of load.

1 kN=102 kg × 9.81 m/s2

So, for example, a platform that shows it rated at 321 kN (72,000 lb F) will reliably support a 32,100 kg (70,800 lb) load. This unit is used in the following safety specifications:

• holding values ​​of fasteners, ground anchors and other items used in the construction industry;
• working loads in tension and shear;
• climbing equipment;
• thrust in rocket engines and launch vehicles;
• clamping force of various molds in injection molding machines used to make plastic parts.