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This force can be measured by a dynamometer (spring scale). In the international system of units, the unit of force is Newton, abbreviated as cow, and the symbol is n.

Force has not only magnitude, but also direction. The gravity of the object is vertical downward, and the buoyancy of the object in the liquid is vertical upward. The direction of the force is different, and the effect is different. If the direction of the force acting on a moving object is the same as the direction of motion, it will accelerate the movement of the object; If the direction is opposite to the direction of motion, it will hinder the movement of the object. It can be seen that to fully express a force, in addition to the magnitude of the force, it is also necessary to express the direction of the force, which can be represented by a schematic diagram of the force. A graph that accurately expresses the three elements of force with directed line segments is called a force graph.

Force is a vector and the international unit is Newton.

The action of force is mutual.

Influence of force:

1, changing the motion state of the object;

2. Deform the object.

The three elements of force:

1, size;

2. direction;

3. Action point.

The essence of force:

Force is the process of throwing elementary particles between objects, or the function of elementary particles to transfer force between objects.

For example, electromagnetic force is to throw photons between objects, release and absorb them.

Different forces are transmitted by different elementary particles, which is simple and clear.

Basic characteristics of force:

① Importance

② Reciprocity

③ Vector property

(4) independence (that is, the effect of one object acting on another object has nothing to do with whether the object is acted on by other forces at the same time. )

Classification of forces

Forces can be divided into two categories, one is named according to the nature of forces, such as gravity, elasticity, friction, molecular force, electricity, magnetism and so on; The other is named according to the function of force, such as tension, pressure, supporting force, power, resistance and so on.

What is sound?

Sound is produced by the vibration of objects, and it is also the source of the objects that emit sound.

Sound is just the movement of pressure waves in the air. Pressure waves vibrate small bones in the inner ear, and these vibrations are converted into tiny electronic brain waves, which are the sounds we perceive. The inner ear uses the same principle as the sound waves captured by a microphone or the pronunciation of a speaker. Is the relationship between moving mechanical parts and air pressure waves. Naturally, when the acoustic Boeing is turned down and moved slowly and loudly enough, we can actually "feel" the air pressure wave vibrating the body. So we use mixed body parts to perceive sound.

Return to the sound source?

Start with the sound source. Beat little snare drum with a drumstick. The drumstick hits the dome-shaped drum surface, and the drum surface vibrates. The vibrating drum head pushes the air and produces pressure waves, which are emitted from the drum head and drum body and scattered. Therefore, the "pressure wave" is emitted from the sound source and dispersed. To prove this point, throw a stone into a pond in a park or a pool at home, and see how the water waves generated by falling objects disperse from the disturbed wave source. Also notice how the ripple/vibration hits the edge and then bounces back from the wall if it is thrown into a sink or a closed container like a bowl. Observing the ripples/water waves in a closed container will give you an idea of how sound moves in a closed room and how it bounces off the wall. Also note that the bigger the stone/stone, the greater the distance between ripples than the distance between small objects.

The weight of sound

Sound has mass, but no weight. Sound is not an object, but a name. Sound is mechanical longitudinal wave, and wave is a form of energy transfer. It has energy, so it can produce effects, but it is different from light (electromagnetic waves). Light has mass, energy and momentum, while sound has only pressure and no mass in physics.

Sound characteristics

(1) Loudness: People subjectively think that sound is determined by "amplitude", and the greater the amplitude, the greater the loudness. (unit: decibel dB)

(2) Tone: The sound level is determined by "frequency". The higher the frequency, the higher the tone (frequency unit Hz, Hz [/url, human ear hearing range 20-20~20000Hz). Such as low-end sounds or higher sounds, such as thin string sounds.

(3) timbre: the characteristics of sound are determined by the material and structure of the object itself.

Frequency is the number of sound waves passing through a given point every second, and its measurement unit is hertz, which is named after an acoustic legend, Heric Hertz. The man made a table to demonstrate the relationship between frequency and cycles per second.

1 kHz or 1000 Hz means that sound waves pass through a given point with 1000 cycles per second, 1 MHz means 1 000,000 cycles per second, and so on.

Light can stimulate the retina to produce radiant energy of visual ability; The visible spectrum range of electromagnetic wave is 380 ~ 770 nm (10-9m).

Light is divided into artificial light and natural light. We can see the ever-changing multi-tone scene in the objective world because our eyes receive the light emitted, reflected or scattered by objects. Light is closely related to human life and social practice.

Light source: An object that can emit light by itself is called a light source. Light sources are divided into cold light sources and hot light sources;

Cold light source: it means to emit light without heating (or to generate heat at a very low temperature). Such as fireflies;

Thermal light source: refers to light emitting and heating (must be high temperature heat). Such as the sun;

Strictly speaking, light is a kind of radiation that can be observed by human eyes. Experiments show that light is electromagnetic radiation, and the wavelength range of this part of electromagnetic wave is about 0.77 micron of red light to 0.39 micron of purple light. Electromagnetic waves with a wavelength of more than 0.77 micron to about 1000 micron are called "infrared rays". Below 0.39 micron to about 0.04 micron is called "ultraviolet". Infrared and ultraviolet rays can't cause vision, but the existence of this luminous object can be measured and detected by optical instruments or photography. Therefore, the concept of light in optics can also be extended to infrared and ultraviolet fields. Even X-rays are considered as light, and the spectrum of visible light is only a part of the electromagnetic spectrum.

Light has wave-particle duality, that is, it can be regarded as a kind of high-frequency electromagnetic wave (10/2 ~1015hz), and it can also be regarded as a particle, that is, photon for short.

The speed of light replaced the platinum watch kept in the International Bureau of Metrology in Paris as the standard for defining "meter", and it was agreed that the speed of light was strictly equal to 299,792,458 m/s, which was consistent with the definition of meter and second at that time. Later, with the continuous improvement of the experimental accuracy, the numerical value of the speed of light changed, and the meter was defined as the distance traveled by light in 1/299792458 seconds.

Light is one of the sources of life on earth.

Light is the foundation of human life. Light is a tool for human beings to know the outside world. Light is an ideal carrier or media of information.

According to statistics, at least 90% of all information received by human senses from the outside world is transmitted through the eyes. ...

Light is essentially an electromagnetic wave, covering a wide electromagnetic spectrum (from X-ray to far infrared), but its wavelength is shorter than ordinary radio waves. Visible light visible to the naked eye is only a part of the whole electromagnetic spectrum.

When a beam of light is projected on an object, it will be reflected, refracted, interfered and diffracted.

Light travels in a straight line in a uniform medium.

Light waves, including infrared rays, have shorter wavelengths and higher frequencies than microwaves. Therefore, it is a natural and inevitable trend to develop from microwave communication in electrical communication to optical communication.

Ordinary light: Generally speaking, light is composed of many photons. Under fluorescence (ordinary sunlight, light, candlelight, etc.). ), there is no connection between photons, that is, different wavelengths, different phases, different polarization directions and different propagation directions, just like an unorganized and undisciplined photon army, all photons are stragglers and cannot act in unison.

When light is reflected, the reflection angle is equal to the incident angle. On the same plane, on both sides of the normal, the light path can be reversed.

Light incident obliquely from one medium to another will produce refraction. If the density of the injected medium is greater than that of the original light, the incident angle is less than the refraction angle. On the other hand, if it is less than, the incident angle is greater than the refraction angle. However, if the incident angle is zero and the refraction angle is zero, no refraction will occur. However, light refraction still occurs in the same inhomogeneous medium. Theoretically, it can be injected from one direction without refraction. However, due to the unclear boundary, it is generally divided into several levels and is not flat, so refraction will occur anyway. For example, looking at the calm lake bottom from the shore belongs to the first refraction, but seeing a mirage belongs to the second refraction. The function of convex lens and concave lens is due to the first refraction.