About the National Junior High School Applied Physics Knowledge Competition

Second grade physics

☆ Kelvin: K Celsius: ℃ The relationship between thermodynamic temperature T and degrees Celsius t: T(K)=t(℃) 273℃

┌————Sublimation————————Emission of heat——————┐

│┌——Liquefaction——Emission of heat——┐┌—— Solidification - release of heat - ┐│

↓↓ │↓ ││

Gaseous liquid solid state

││ ↑│ ↑↑

│└——Gasification——Endotherm——┘└——Melting——Endotherm——┘│

└————Condensation———————— ——Endotherm————┘

☆ m→mass V→volume ρ→density ρ=m/V

Unit: 1kg/m3=1×10-3g /cm3 International unit: kg/m3

☆ The change of the position of one object relative to another object is called mechanical motion, usually referred to as motion

All objects are in motion, absolutely not There are no moving objects, which means that movement is absolute. The movement and stillness we usually talk about are relative to another object (reference object), so the description of movement is relative

The object selected as a standard when studying mechanical motion is called a reference object

Reference objects are not all objects that are stationary relative to the ground. We just choose which object is the reference object, and we assume that the object is not stationary. Moving

The reference object can be selected arbitrarily, but the description of the movement of the same object may be different if the selected reference object is different

Two objects moving at the same speed and in the same direction , or the position between them does not change, then the two objects are relatively stationary

The motion that does not change speed and takes a straight line is called uniform linear motion

Uniform linear motion It is the simplest mechanical motion

v→speed s→distance t→time v=s/t

Unit: 1m/s=36km/h International unit: m/s

☆ The propagation of sound requires a medium, and sound cannot be transmitted in a vacuum

During the propagation process of sound, when it encounters obstacles and is reflected back, the sound that people hear again is called echo

Conditions for distinguishing echo from original sound: The echo reaches people's ears more than 01 seconds later than the original sound

When it is less than 01 seconds, the reflected sound can only strengthen the original sound

☆ Light propagates along a straight line in a uniform medium. The speed of light propagation in different materials is generally different, and it is the fastest in vacuum. The propagation speed of light in vacuum: C = 3×108 m/s, in air The speed is close to this speed. The speed in water is 3/4C and in glass is 2/3C

When light is emitted from one medium to the interface of another medium, part of the light returns to the original medium. This changes the propagation direction of light. This phenomenon is called reflection of light

The angle between the incident light and the normal is called the incident angle, and the angle between the reflected light and the normal is called the reflection angle

Incidence angle = reflection angle Reflection: specular reflection and diffuse reflection

The real image is formed by the convergence of actual light rays. It can be received with a screen, and of course it can also be seen with the eyes

The virtual image is not formed by the convergence of actual light rays, but the intersection of the reverse extension lines of actual light rays. It can only be seen with the eyes and cannot be received with a screen

The image formed by a plane mirror is an upright virtual image. and the size of the object, the line connecting the image and the object is perpendicular to the mirror, and the distance from the image and the object to the mirror is equal

Light travels from a medium

When the light is incident obliquely into another medium, the propagation direction will generally change. This phenomenon is called refraction of light

☆ Lens: Made of transparent material, at least one surface is part of the sphere, and the lens is far thicker Much smaller than its spherical radius

Convex lens: thin at the edges, thick in the center, converging light Concave lens: thick at the edges, thin in the center, diverging light

Main optical axis: passing through two The linear optical center of the center of the sphere: there is a special point on the main optical axis through which the direction of light propagation remains unchanged. Focus: a convex lens can make light rays parallel to the main axis converge at a point on the main optical axis. This point is called the focus of the lens. , use "F" to represent the focal length: the distance from the focus to the optical center is called the focal length, use "f" to represent the object distance: the distance from the lens

Object distance (u) The distance between the virtual and real images of the large and small images (f) Application

u gt; 2f shrink the two sides of the real image lens f lt; v lt; 2f camera

u = 2f and equal the two sides of the real image lens v = 2f

f lt; u lt; 2f magnifies v on both sides of the real image lens gt; 2f slide projector

u = f does not image

u lt; f magnifies v on the same side of the virtual image lens gt; u Magnifying glass

Convex lens imaging rules

1: "The first focus divides the virtual and the real, the second focus divides the size, the virtual image is upright on the same side, the real image is inverted on the opposite side, and the object moving image becomes smaller."

Two: "Three object distances, three boundaries. The image changes with the object distance. The real image of far objects is small and close, and the real image of near objects is large and far. If the object is placed in the focus, the upright magnified virtual image appears, and the slideshow The image is so big, the object is between one focus and the other. If you shrink the camera, the object is twice as far away as the focal length.” When an object is placed in the focal point, the virtual image will be larger on the same side as the object. Keep a rule in mind: the near object and the far image will become larger."

☆ Force is an object Effect on objects

The effect of force between objects is the three elements of mutual force: size, direction and point of action

When a force occurs, there must be two forces present, that is It is said that without an object, there would be no powerful action

It can change the state of motion of an object. Changes in the state of motion include changes in the speed and direction of motion

It can change the shape of an object Changes in magnitude

Unit of force: N-N

Magnitude of gravity: G = mg Direction: always vertically downward Point of action: The point of action of gravity is at the center of gravity of the object The center of gravity of an object with regular shape and uniform mass distribution is at its geometric center

When the two forces have the same direction, the resultant force is equal to the sum of the two forces, and the direction is the same as the direction of the two forces. Mathematical expression: F Total = F1 F2

When the two forces have opposite directions, the magnitude of the resultant force is equal to the difference between the two forces, and the direction is the direction of the larger force. Mathematical expression: F Total = │F1 — F2 │

☆ The force acting vertically on the unit area of ??an object is called pressure. The magnitude of the pressure on an object per unit area is called pressure. When the pressures are equal, the smaller the force-bearing area, the more obvious the effect of pressure. When the force-bearing areas are equal, the greater the pressure. , the more obvious the effect of pressure. Standard atmospheric pressure = 1.01×105Pa

p→Pressure F→Pressure S→Stressed area p=F/S

Unit: Pa=1N/m2

The calculation formula of liquid pressure is p=ρgh ρ liquid, the unit is kg/m3 g=9.8N/kg h liquid depth, the unit is m p is the liquid pressure, the unit is Pa.

F float = ρ water gV matter F float = ρ matter gV matter F float = ρ liquid gV discharge

F float = p water gV matter F float = p matter gV matter F float = p liquid gV Platoon

Answer: xiupengyuan - General Soldier Level 11

2009-6-28 10:40

Report the review outline for the second grade physics review

1. Measurement of length

1. Measurement of length

The measurement of length is the most basic measurement, and the most commonly used tool is a scale.

2. Units and conversions of length

The international unit of length is meter (m). Commonly used units are kilometers (Km), decimeter (dm) and centimeters (cm). , millimeter (mm) micron (um) nanometer (nm)

1Km 103 m 10 m 10 dm 10 cm 10 mm 103um 103 nm

When converting length units, small units change Use multiplication for large units, and division for large units to small units

3. Use the scale correctly

(1) Before use, pay attention to the zero scale line, range, and graduation value

(2) Pay attention when using it

① The ruler should be placed along the length to be measured, and the edge of the ruler should be aligned with the object being measured. It must be placed upright and overlap, and cannot be skewed.

② Do not use the worn zero scale line. If the zero scale line is worn and another whole scale line is used as the zero scale line, do not forget to subtract the replaced zero scale line from the final reading. value.

③ Thick rulers should be placed vertically

④ When reading, the line of sight should be perpendicular to the ruler surface

4. Record the measurement values ??correctly

The measurement results are composed of numbers and units

(1) Recording only numbers without units is meaningless

(2) When reading, it is necessary to estimate the graduation value of the scale. Next digit

5. Error

The difference between the measured value and the true value

The error cannot be avoided, but it can be minimized. If the error can be avoided, it is not This should happen

The basic method to reduce the error: take multiple measurements and find the average. In addition, using precision instruments and improving the measurement method can also reduce the error

6. Special method measurement

(1) Cumulative method, such as measuring the diameter of thin metal wire or measuring the thickness of paper, etc.

(2) Caliper method

(3) Substitution method

2. Simple motion

1. Mechanical motion

The change in the position of an object is called mechanical motion

All objects are in motion, absolutely There is no immovable object, which means that motion is absolute. The motion and rest we usually talk about are relative to another object (reference object), so the description of motion is relative

2. Reference object

The object selected as a standard when studying mechanical motion is called a reference object

(1) Reference objects are not always stationary relative to the ground. Object, just choose which object is the reference object, we assume that the object does not move

(2) The reference object can be selected arbitrarily, but if the selected reference object is different, the description of the motion of the same object may be different

3. Relative stationary

If two objects move at the same speed and in the same direction, or if their positions remain unchanged, then the two objects are relatively stationary.

4. Uniform linear motion

Movement that does not change speed and travels in a straight line is called uniform linear motion

Uniform linear motion is the simplest machine sports.

5. Speed

(1) Speed ??is a physical quantity that indicates how fast an object moves.

(2) In uniform linear motion, the speed is equal to the distance traveled by the moving object in unit time

(3) Speed ??formula: v= S t

(4) Unit of speed

International unit: m/s Common unit: km/h 1m/s = 3.6 km/h

6. Average speed

The ratio of the time it takes an object moving at variable speed to pass a certain distance and the time it takes to pass this distance is called the average speed of the object during this distance

To find the average speed, you must specify which distance it is. or the average speed within time

7. Measuring average speed

Principle: v = s / t

Measuring tools: scale, stop watch (or Other timers)

3. Sound phenomenon

1. Occurrence of sound

All objects that are making sound are vibrating. When the vibration stops, the sound will stop. .

Sound is produced by the vibration of objects, but not all vibrations will emit sound

2. Propagation of sound

Propagation of sound A medium is required, and sound cannot be transmitted in a vacuum

(1) Sound must be transmitted through all gases, liquids, and solids as media. These substances as transmission media are called media. Even if the astronauts on the moon talk face to face, they still need to rely on radio. That is because there is no air on the moon and sound cannot be transmitted in a vacuum

(2) Sound propagates at different speeds in different media

3. Echo

In the process of sound propagation, the sound that is reflected back by obstacles and heard again is called echo

(1) Conditions for distinguishing echo from original sound : The echo reaches the human ear more than 0.1 second later than the original sound.

(2) When it is less than 0.1 seconds, the reflected sound can only strengthen the original sound.

(3) The echo can be used to measure the depth of the sea or the distance between the sound-emitting body and the obstacle.

4. Pitch

The pitch of the sound is called pitch. Determined by the vibration frequency of the sound-emitting body, the greater the frequency, the higher the pitch.

5. Loudness

The size of sound is called loudness. Loudness is related to the amplitude of the vibration of the sound source and the distance from the sound source to the human ear

6. Timbre

The quality of the sounds emitted by different sound emitters is called timbre

7. Noise and its sources

From a physical point of view, noise refers to The sound produced when the sound-emitting body vibrates in an irregular and chaotic manner. From an environmental perspective, any sound that interferes with people's normal rest, study and work is noise.

8. Classification of sound levels

People use decibels to divide sound levels. 30dB-40dB is an ideal quiet environment. If it exceeds 50dB, it will affect sleep. Above 70dB, it will affect sleep. It interferes with conversations and affects work efficiency. Living in a noise environment above 90dB for a long time will affect hearing.

9. Ways to reduce noise

It can be reduced at the sound source, during propagation and at the human ear

4. Thermal phenomena

1. Temperature

The degree of hotness and coldness of an object is called temperature

2. Celsius temperature

Set the temperature of the ice-water mixture as 0 degrees, and set 1 The temperature of boiling water under standard atmospheric pressure is 100 degrees.

3. Thermometer

(1) Principle: Made of the thermal expansion and contraction of liquid

(2) Structure: glass shell, capillary tube, Glass bubble, scale and liquid

(3) Usage: Before using the thermometer, pay attention to the measuring range and recognize the graduation value

Do the following three things when using the thermometer

① The thermometer is in full contact with the object to be measured

② Wait for the reading to stabilize before reading

③ When reading, the line of sight should be level with the surface of the liquid, and the thermometer should still be in contact with the surface of the liquid. The object to be measured is in close contact

4. The main differences between thermometers, experimental thermometers and thermometers

Construction and usage of measuring range graduation values

There is a shrinkage above the glass bulb of the thermometer Mouth 35-42℃ 0.1℃ ① Take readings away from the human body

② Need to throw them away before use

Experimental thermometer None - 20-100℃ 1℃ Do not take readings away from the object being measured, nor can they be thrown away

The cold and heat table has no -30 -50℃ 1℃ Same as above

5. Melting and solidification

The change of a substance from a solid state to a liquid state is called melting, and melting needs to absorb heat

The change of a substance from a liquid state to a solid state is called solidification, and solidification requires the release of heat

6. Melting point and freezing point

(1) Solids are divided into two categories: crystalline and amorphous.

(2) Melting point: Crystals have a certain melting temperature, called melting point

Freezing point: Crystals have a certain solidification temperature, called freezing point

Same kind The freezing point of a substance is the same as its melting point

7. The change of a substance from a liquid state to a gaseous state is called vaporization. There are two different ways of vaporization: evaporation and boiling, both of which absorb heat

8. Evaporation phenomenon

(1) Definition: Evaporation is a vaporization phenomenon that can occur in liquid at any temperature and only occurs on the surface of the liquid

(2 ) Factors that affect the speed of evaporation: the temperature of the liquid, the surface area of ??the liquid, the speed of air flow on the surface of the liquid

9. Boiling phenomenon

(1) Definition: Boiling is the internal and Violent vaporization phenomenon on the surface at the same time

(2) Conditions for liquid boiling: ①The temperature reaches the boiling point ②Continue to absorb heat

10. Lifting and condensation phenomena

(1) The direct change of a substance from a solid state to a gaseous state is called sublimation, and the direct change of a substance from a gaseous state to a solid state is called sublimation

(2) Sublimation and sublimation phenomena in daily life (frozen wet clothes become Dry, see frost in winter)

11. Sublimation absorbs heat, condensation releases heat

5. Reflection of light

1. Light source: capable of emitting light The object is called a light source

2. Light propagates in a straight line in a uniform medium

The atmosphere is uneven. When light is emitted from the atmosphere to the ground, the light is bent. Folding

3. Speed ??of light

The speed of light propagating in different materials is generally different, and it is the fastest in vacuum.

The speed of light propagating in vacuum: C = 3×108 m/s, the speed in air is close to this speed, the speed in water is 3/4C, and the speed in glass is 2/3C

4. Application of linear propagation of light

Can explain many optical phenomena: laser collimation, shadow formation, lunar and solar eclipse formation, pinhole imaging, etc.

5. Light

Light: represents light A straight line in the direction of propagation, that is, draw a straight line along the propagation route of light, and draw an arrow on the straight line to indicate the propagation direction of light (the light is imaginary and does not actually exist)

6. Reflection of light

When light is emitted from one medium to the interface of another medium, part of the light returns to the original medium, causing the propagation direction of the light to change. This phenomenon is called reflection of light

7. Law of reflection of light

The reflected light, the incident light and the normal are on the same plane; the reflected light and the incident light are separated on both sides of the normal; the reflection angle is equal to the incident angle

It can be summarized as: "Three lines and one side, two lines separate

", the two angles are equal"

Understanding:

(1) The reflected light is determined by the incident light, and the word "anti" should be at the forefront of the description

(2) The conditions for reflection to occur: the junction of two media; the place of occurrence: the point of incidence; the result: return to the original medium

(3) The reflection angle increases as the incident angle increases, and decreases as the incident angle increases. Small, when the incident angle is zero, the reflection angle also becomes zero

8. Two reflection phenomena

(1) Specular reflection: Parallel light reflects along a certain The direction is parallel and the reflected light can only be received in a certain direction

(2) Diffuse reflection: Parallel light is reflected in different directions after being reflected by the interface, that is, it can be received in all different directions. To the reflected light

Note: Whether it is specular reflection or diffuse reflection, it follows the law of reflection of light

9. In the reflection of light, the optical path is reversible

10 , The effect of plane mirror on light

(1) Imaging (2) Changing the propagation direction of light

11. Characteristics of plane mirror imaging

(1) Like an upright virtual image (2) The size of the image and the object (3) The line connecting the image and the object is perpendicular to the mirror surface, and the distance from the image and the object to the mirror is equal

Understanding: The image and object formed by a plane mirror It is a symmetrical figure with the mirror as the axis

12. The difference between real image and virtual image

The real image is formed by the convergence of actual light rays. It can be received with a screen, and of course it can also be seen with the eyes. . The virtual image is not formed by the convergence of actual light rays, but the intersection of the reverse extension lines of the actual light rays. It can only be seen with the eyes and cannot be received by the screen.

13. Application of plane mirrors

(1) Reflection in water (2) Plane mirror imaging (3) Periscope

6. Refraction of light

1. Refraction of light

Light When one medium is incident obliquely into another medium, the direction of propagation generally changes. This phenomenon is called refraction of light

Understanding: Refraction of light and reflection of light occur in two media. At the junction of , this is the refraction of light.

Note: At the intersection of two media, both refraction and reflection occur.

2. The law of refraction of light

When light is incident obliquely from air into water or other media, the refracted ray is on the same plane as the incident ray and the normal line, and the refracted ray and the incident ray are separated on both sides of the normal line; the refraction angle is smaller than the incident angle; when the incident angle increases, The refraction angle also increases; when the light hits the medium surface vertically, the propagation direction remains unchanged, and the light path is reversible during refraction.

Understanding: The law of refraction is divided into three points: (1) three lines and one side (2). ) Two lines are separated (3) There are three situations in the relationship between two angles: ① When the incident light is incident perpendicular to the interface, the refraction angle is equal to the incident angle equal to 0°; ② When the light is incident obliquely from the air into a medium such as water, the refraction angle is smaller than the incident angle; ③ When light enters the air obliquely from a medium such as water, the refraction angle is greater than the incident angle

3. The optical path is reversible in the refraction of light

4. Lenses and classification

Lens: Made of a transparent material (usually glass), at least one surface is part of the spherical surface, and the thickness of the lens is much smaller than the radius of the spherical surface.

Classification: Convex lens: thin at the edge, thick at the center

Concave lens: thick at the edge, thin at the center

5. Main optical axis, optical center, focus, focal length

Main optical axis: a straight line passing through the centers of two spheres

Optical center: there is a special point on the main optical axis through which the direction of light propagation remains unchanged. (The center of the lens can be considered as the optical center)

Focus: A convex lens can make light rays parallel to the main axis converge at a point on the main optical axis. This point is called the focus of the lens, represented by "F"

Virtual focus: The light rays parallel to the main optical axis become divergent after passing through the concave lens. The reverse extension lines of the divergent rays intersect at a point on the main optical axis. This point is not the actual convergence point of the light rays, so it is called virtual focus.

Focal length: The distance from the focus to the optical center is called the focal length, represented by "f".

Each lens has two focal points, focal lengths and an optical center. As shown in the picture

6. The effect of lens on light

Convex lens: Converging light (as shown)

Concave lens: Diverging light (such as Figure)

7. Imaging rules of convex lenses

Object distance

(u) Imaging

Small and large images

Virtual real image object position image distance

( v ) Application

u gt; 2f Reduce both sides of the real image lens f lt; v lt; 2f camera

u = 2f v on both sides of the same large real image lens = 2f

f lt; u lt; 2f v gt on both sides of the magnified real image lens; 2f slide projector

u = f does not image

u lt; f magnifies the virtual image on the same side as the lens v gt; u magnifying glass

Memory method of the imaging rules of convex lenses

Oral Judgment 1:

"The first focus divides the virtual and the real, and the second focus divides the big and small; the virtual image is upright on the same side; the real image is inverted on the opposite side, and the image of objects moving becomes smaller."

Oral Judgment 2:

Three object distances, three The boundary, the image changes with the distance of the object;

The real image of far objects is small and close, and the real image of near objects is large and far away.

If the object is placed in focus, a virtual image will appear when the upright magnification occurs;

The slide show image is so large that the object is between one focus and two focus;

The camera is zoomed out You are little, and objects are twice as far away as the focal length.

Oral Judgment 3:

Convex lenses are great for photography, slide shows and magnification;

The outside of the double focal point is small, and the double focal length is full. Large;

If the object is placed in focus, the virtual image on the same side as the object will be larger;

Keep a rule in mind, the near object will become larger as the far image.

8. In order to make the image on the screen "upright" (upright), the slides should be inserted upside down.

9. The lens of the camera is equivalent to a convex lens, and the film in the camera obscura is equivalent to a light screen. When we adjust the focusing ring, we do not adjust the focal length, but the distance from the lens to the film. The farther the object is from the lens. , the film should be close to the lens.

7. Mass and Density

1. Mass

(1) Definition: The amount of matter contained in an object is called mass. Represented by the letter "m".

(2) Mass is an attribute of an object:

For a given object, its mass is determined and does not vary with the shape and position of the object

Changes due to changes in location, status and temperature.

(3) Units and conversions of mass:

The main unit of mass is kilogram (kg). Commonly used units are tons (t), grams (g) and milligrams (mg)

1t 103 kg 103 g 103 mg

2. Measurement of mass

The tool for measuring mass in life is a scale. In the physics laboratory, balances are used to measure mass, including pallet balances and physical balances.

(1) How to use the balance:

① Place the balance on the horizontal platform, and place the scale at the zero mark on the left end of the ruler

② Adjust the balance nut on the right end of the beam so that the pointer points to the center line of the indexing plate. At this time, the beam is balanced

③ Estimate the mass of the object to be measured, place the object to be measured in the left plate, and use tweezers to Add or subtract weights in the right pan and adjust the position of the free weight on the scale until the beam returns to balance.

(2) Precautions for using the balance:

①After the balance is adjusted, the left and right pallets cannot be interchanged, otherwise the beam balance must be readjusted

② The mass of the object being measured cannot exceed the maximum weighing capacity

③The weights should be handled with care. Do not hold them by hand. Use tweezers to avoid corrosion of the weights due to sweat on your hands

④ Keep the balance plate dry and clean. Do not place wet or corrosive objects directly.

(3) Weighing and sense of balance:

The maximum mass that each balance can weigh is called the maximum weighing capacity of the balance, also called weighing.

The sense quantity represents the minimum mass that the balance can measure, which is the mass represented by the smallest scale on the scale.

3. Density

Density is a characteristic of matter.

(1) Definition: The mass of a certain substance per unit volume is called density. Represented by the letter "ρ".

(2) Density calculation formula:

(3) Unit: The international unit is kg/m3, the commonly used unit in experiments is g/cm3, 1g/cm3=103kg/m3

8. Force

1. Definition of force

(1) Definition: Force is the effect of an object on an object

(2 ) Explanation: The "action" in the definition is an abstract summary of specific actions such as pushing, pulling, lifting, lifting, and pressing.

2. Understanding the concept of force

(1) Occurrence When a force is exerted, there must be two (or more than two) objects present. In other words, there would be no force without an object

(2) When an object is acted upon by a force, there must be Another object exerts a force on it. The object receiving the force is called the force-receiving object, and the object exerting force is called the force-exerting object. Therefore, force does not exist without an object exerting force or without an object receiving force.

(3) There may not be a force between objects that are in contact with each other, and there may not be a force between objects that are not in contact. "Contact or not" cannot be the basis for judging whether a force occurs.

(4) The effects of forces between objects are mutual.

① The effects of the force-exerting object and the force-receiving object are mutual. This pair of forces always generates and disappears at the same time.

② The force-exerting object and the force-receiving object are relative. When the research object changes, the force-exerting object and the force-receiving object also change

3. The effect of force— —This can determine whether there is force

(1) It can change the motion state of the object. Changes in movement status include changes in movement speed and movement direction.

(2) It can change the shape and size of objects.

4. Unit of force

In the International System of Units, the unit of force is Newton, referred to as Newton, represented by the symbol N. 1N is equivalent to the force required to pick up 2 eggs.

5. Measurement of force

(1) Tool: dynamometer. The dynamometer commonly used in the laboratory is a spring balance

(2) Spring balance Principle: The greater the tension on the spring, the longer the spring stretches

6. Correct use of the spring scale

(1) Observe whether the range, graduation value and pointer of the spring scale are pointing. On the zero scale

(2) When reading, the sight, pointer and scale should be on the same horizontal plane

7. Three elements of force

Force The three elements of force, namely size, direction and point of action, can all affect the effect of force

8. Illustration of force: Use a line segment with an arrow to express the three elements of force

9. How to draw force diagrams

(1) Draw the force-bearing object: generally it can be represented by a square or rectangle, and the sphere can be represented by a circle.

(2) Determine the point of action: The point of action is drawn on the force-receiving object and at the midpoint of the contact surface between the force-receiving object and the force-exerting object. If the force-receiving object and the force-exerting object are not in contact or When two or more forces are exerted on the same object, the point of action is drawn at the geometric center of the object receiving the force.

(3) Determine the scale: For example, how many Newtons does a 1 cm line segment represent?

(4) Draw a line segment: starting from the action point of the force, draw a straight line along the direction of the force according to the specified scale to express the magnitude of the force

(5) Force Direction: Draw an arrow at the end of the line segment to indicate the direction of the force

(6) Mark the symbol and value of the force shown near the arrow

10. Schematic diagram

In some cases, you only need to qualitatively describe the force on the object and do not need to accurately express the magnitude of the force, so you can draw a schematic diagram of the force.

11. The concept of gravity

(1) Definition: The force exerted by objects near the ground due to the attraction of the earth is called gravity

(2) Understanding: ①Gravity The force-exerting object is the earth, and its force-receiving objects are all objects near the ground. ②The magnitude of gravity is related to the mass of the object.

12. Three elements of gravity

(1) Size: G = mg

(2) Direction: always vertically downward (vertical and horizontal (Bottom)

(3) Point of action: The point of action of gravity is at the center of gravity of the object. Among them, the shape is regular and the mass distribution is uniform. The center of gravity of the object is at its geometric center

13. The concept of resultant force

(1) Resultant force: If the effect of one force is the same as that of two forces* **The effect produced by the same action is the same, and this force is called the resultant force of those two forces

(2) Understanding: ①The concept of resultant force is based on "equivalence", that is, resultant force" It replaces the component force, so the resultant force is not another force acting on the object. It just replaces the two forces that originally acted. Don't mistakenly think that the object is also affected by the resultant force at the same time. ②The conditions for the synthesis of the two forces are this. Two forces must act on an object at the same time, otherwise it is meaningless to find the resultant force.

14. Synthesis of forces

Given the magnitude and direction of several forces, find the magnitude of the resultant force. The sum direction is called the resultant force

(1) When the two forces have the same direction, the magnitude of the resultant force is equal to the sum of the two forces; the direction is the same as the direction of the two forces

Mathematical expression: F combined = F1 F2

(2) When the two forces are in opposite directions, the resultant force is equal to the difference between the two forces, and the direction is the direction of the larger force

Mathematical expression: F combined = F1 — F2