Summary of nine-year physics knowledge

Chapter 5 XI's colorful material world

I. Universe and microcosm

Universe → Galaxy → Solar System → Earth

Matter consists of molecules; Molecules are particles that maintain the original properties of matter; The general size is only tens of billions of meters (0.3-0.4nm).

Properties of three states of matter:

Solid: molecules are arranged closely, and there is a strong force between particles. Solids have a certain shape and volume.

Liquid: molecules have no fixed position and move freely, and the force between particles is smaller than that of solids; Liquid has no fixed shape and fluidity.

Gas: Molecules are extremely dispersed, with large spacing, and move in all directions at high speed. The interaction between particles is weak and easy to be compressed. Gas has fluidity.

Molecules are composed of atoms, atoms are composed of nuclei and (extra-nuclear) electrons (similar to the solar system), and nuclei are composed of protons and neutrons.

Nanotechnology: (1 nm = 10 nm), nanotechnology: (0. 1- 100 nm). The object of study is a small pile of molecules or a single atom or molecule.

Second, quality.

Mass: How much matter does an object contain? Mass is an attribute of the object itself, and its size has nothing to do with shape, state, position, temperature, etc. Symbol of physical quantity: m.

Units: kilograms, tons, grams and milligrams.

1t= 103kg， 1kg= 103g， 1g= 103mg。

Balance: 1, principle: lever principle.

2. Precautions: The measured object cannot exceed the weighing of the balance; Use tweezers to add and subtract weights on the plate, and the weights should not be dirty or wet; Wet objects and chemicals cannot be placed directly on the plate of the balance.

3. Use: (1) Put the balance on a horizontal platform; (2) Put the traveling code on the ruler on the left zero line, and adjust the balance nut on the beam to balance the balance (the pointer points to the center line of the reticle or the swing amplitude is equal from left to right). (3) Put the object on the left board, put the weight on the right board, increase or decrease the weight and adjust the stroke code to balance the balance. (4) Reading: the total weight plus the scale value corresponding to the travel code.

Note: In weightlessness (such as a spaceship), you can't weigh the mass with a balance.

Third, density.

Density is a special property of matter; The mass of the same substance is directly proportional to its volume, and the ratio of mass to volume is constant.

Density: The mass per unit volume of a substance is called the density of this substance.

Density is related to the kind and state of matter, affected by temperature, and has nothing to do with mass and volume.

Formula:

Unit: kg/m3 g/cm3/kloc-0 /×103kg/m3 =1g/cm3.

1L = 1dm 3 = 10-3 m3； 1 ml = 1 cm3 = 10-3L = 10-6 cubic meters.

Fourth, measure the density of matter.

Experimental principle:

Experimental equipment: balance, measuring cylinder, beaker, thin thread.

Measuring cylinder: measures the volume of liquid (which can indirectly measure the volume of solid), and the reading is based on the lowest point of concave liquid level.

Measuring the density of solids (the density is greater than water):

1. Weigh the mass m of solids with a balance; 2. Pour a proper amount of water into the measuring cylinder (which can immerse the object without exceeding the maximum scale) and read the volume of water (v1); 3. Tie the object with thin thread, put it into the measuring cylinder, and read out the total volume V2.

Note: If the density of solid is lower than that of water, needle pressing method and drop hammer method can be used.

Measure the density of liquid: Step: 1. Weigh the total mass of beaker and liquid with a balance M 1; 2. Pour the liquid in the beaker into a part of the measuring cylinder and read out the volume v2 of the liquid; 3. Weigh the mass m2 of the remaining liquid and beaker with a balance.

V. Density and social life

Density is the basic attribute (characteristic) of matter, and each matter has its own density.

Density and temperature: temperature can change the density of matter; The thermal expansion of gas is the most significant, and its density is most affected by temperature. Solids and liquids are less affected by temperature.

Abnormal expansion of water: the highest density is at 4℃; Water gets bigger when it freezes.

Density application: 1, identification of substances (density measurement) 2, calculation of mass 3, calculation of volume.

Chapter XII Movement and Strength

Description of movement

Motion is a universal phenomenon in the universe.

Mechanical motion: the change of object position is called mechanical motion.

Reference: An object selected as a standard (or an object assumed to be stationary) is called a reference.

Relativity between motion and stillness: whether the same object is moving or still depends on the selected reference object.

Second, the speed of movement.

Speed: describes the speed at which an object moves. Speed is equal to the distance traveled by a moving object in unit time.

Formula:

The unit of speed is: m/s; Kilometers per hour.

Uniform linear motion: to move at a uniform speed along a straight line. This is the simplest mechanical movement.

Variable speed motion: the speed of motion of an object is variable.

Average speed: In variable-speed movement, the speed of an object within this distance can be obtained by dividing the total distance by the time spent, which is the average speed.

Third, the measurement of time and length.

Time measuring tool: clock. Stopwatch (for laboratory use)

Unit: s minimum h

Measuring tool of length: ruler.

Unit of length: meter kilometer mark centimeter millimeter micron nautical mile

Conversion relationship between primary unit and common unit:

1km = 103m 1m = 10dm 10cm 1cm = 10mm 1mm = 103 μm 1m = 106 μm 1m = 100

Length estimation: (approximate) blackboard length is 2.5m, desk height is 0.7m, basketball diameter is 24cm, nail width is 1cm, pencil lead diameter is 1mm, new pencil length is 1.75dm, palm width is 8cm, and ink bottle height is 6cm.

Correct use of scale;

(1). Pay attention to its zero score, range and dividing value before use; (2) When measuring with a scale, the scale should be along the measured length, and the worn zero line shall not be used; (3) The scribed line of the coarse scale should be close to the measured object. (4) When reading, the line of sight should be perpendicular to the ruler surface. When measuring accurately, it is necessary to estimate the next position of the dividing value. (5). The measurement result consists of numbers and units.

Error: The difference between the measured value and the real value is called error.

Errors are inevitable and can only be reduced as much as possible, but not eliminated. One of the common methods to reduce the error is to measure the average value many times.

Special measurement method of length:

1. The common accumulation method is used to measure the diameter of thin copper wire and the thickness of a piece of paper (when the measured length is small and the accuracy of the measuring tool is not enough, the smaller objects can be accumulated, and then a single length can be obtained after measuring with a scale).

☆ How to measure the thickness of a piece of paper in physics textbooks?

Answer: Count the pages of physics textbooks, write down the total number of pages n, and measure the thickness L of n pieces of paper with millimeter scale, then the thickness of a piece of paper is L/N. ..

☆ How to measure the diameter of thin copper wire?

Answer: The thin copper wire is tightly wound around the pencil shaft for N turns to form a solenoid, and the length L of the solenoid is measured with a scale, so the diameter of the thin copper wire is L/N. ..

2. Measure the distance between two points on the map, the circumference of the cylinder, etc. It is often converted into straight line method (by superimposing flexible wires that are not easy to stretch, marking the starting point and ending point on the curve to be measured, and then straightening measurement).

☆ Given a soft copper wire and a scale, can you estimate the railway length from Beijing to Guangzhou with the atlas?

Answer: Concentrate the railway line from Beijing to Guangzhou with a thin copper wire lap diagram, then straighten the thin copper wire, measure the length L with a scale, find out the scale and calculate the length of the railway line.

3. Measure the length of the playground runway and other common wheel rolling methods (roll the wheel with known circumference along the curve to be measured, write down the number of turns of the wheel, and calculate the length of the curve).

4. Commonly used auxiliary methods for measuring coins, balls and cylindrical cones (for the length of objects that cannot be directly measured by the scale, it can be measured by combining the scale triangle).

Fourth, force.

Force: Force is the action of objects on objects. The forces between objects are mutual. When an object exerts a force on another object, it is also subjected to the force exerted on it by the latter.

Conditions for force generation: ① There must be two or more objects. ② There must be interaction (no contact) between objects.

Function of force: Force can change the motion state of an object (change of speed or direction of motion) [the case of constant motion state: stationary or doing uniform linear motion], and can also change the shape of an object.

The unit of force is Newton (n), 1N is about the force you use to pick up two eggs.

The three elements of force are: the size, direction and action point of force. They can all affect the effect.

Schematic diagram of force: the three elements of force are represented by line segments with arrows, which is called the schematic diagram of force.

General drawing method: 1. First draw a sketch of the object (represented by a rectangle or a square, usually drawn in the title) 2. Determine the point of action and draw a real point (if multiple forces are drawn at the same time, put the point of action of multiple forces in the center [center of gravity].

3. Draw a straight line from the action point along the direction of the force, and use the length of the straight line to indicate the magnitude of the force. Finally, draw an arrow at the end of the line segment to indicate the direction of the force.

Fifth, Newton's first law.

Aristotle's point of view: the motion of an object needs force to maintain.

Galileo inclined plane experiment;

The purpose of the three experiments is to ensure that the car starts to move along the plane at the same speed.

⑵ Experimental conclusion: Under the same conditions, the more stable the plane is, the farther the car will go.

(3) Galileo's inference is that in an ideal situation, if the surface is absolutely smooth, the object will always move at a constant speed.

⑷ The excellence of Gaglio's inclined plane experiment lies not in the experiment itself, but in the unique method used in the experiment-idealized reasoning based on the experiment. (also called idealized experiment).

Galileo's point of view: the motion of an object needs no force to maintain. The reason why the movement stops is because of the influence of resistance. If there is no resistance, the object will move at a constant speed forever.

3 Reply: People's Education Edition ninth grade physics basic knowledge induction

Newton's first law: when no force acts on all objects, they always remain at rest or move in a straight line at a constant speed. Newton's first law is further deduced on the basis of empirical facts, so it can't be proved by experiments.

Understanding: A, Newton's first law is further inferred and summarized on the basis of a large number of empirical facts, and has stood the test of practice, so it has become one of the basic laws of mechanics recognized by everyone. But there can't be no pressure around us, so it's impossible to prove Newton's first law directly through experiments.

B, the connotation of Newton's first law: an object is at rest without force.

The object will remain stationary, while the original moving object will move in a straight line at a uniform speed no matter what it does.

C. Newton's first law tells us that an object can move in a straight line at a uniform speed without force, that is, force has nothing to do with the state of motion, so force is not the reason for producing or maintaining motion.

Inertia: The property that an object keeps its state of motion unchanged is called inertia.

All objects have inertia under any circumstances; The size of inertia is only related to mass.

Newton's first law is also called the law of inertia.

The difference between inertia and inertia law;

A. Inertia is an attribute of the object itself, and the law of inertia is the law of motion that the object follows when it is not stressed.

B, any object has inertia under any circumstances (i.e., no matter whether the object is stressed, balanced or unbalanced). When the object is unbalanced, inertia shows the change of motion state (the greater the inertia, the less likely the change is); The law of inertia is conditional.

Six, two force balance

Balance force: an object is at rest or moving in a straight line at a constant speed under the action of force because of the action of balance force.

Balance of two forces: When an object is acted by two forces, we say that the two forces are in a state of balance if it remains stationary or moving in a straight line at a constant speed.

Conditions for the balance of two forces: If two forces acting on the same object are equal in magnitude and opposite in direction, the two forces will balance each other on the same straight line.

○ (When two forces are balanced, the resultant force is zero).

An object will remain stationary or move in a straight line at a constant speed when it is not subjected to force or balanced force.

Comparison of balance force and interaction force;

Similarities: ① equal in size; ② The direction is opposite; ③ The points acting on a straight line are different: the balance force acting on an object can be different in nature; The interaction forces acting on different objects are forces of the same nature.

For example, the gravity and supporting force of an object placed on a horizontal plane are balanced forces, and the gravity of an electric lamp hoisted by a wire and the pulling force of the wire on the object are balanced forces.

Chapter XIII Force and Machinery

First, the elastic spring dynamometer

Elasticity: when an object is deformed by force, it will return to its original state when it is not stressed. This property of an object is called elasticity.

Plasticity: an object cannot automatically recover its original shape after being stressed. This property of an object is called plasticity.

Elastic force: the force produced by elastic deformation of an object.

Spring dynamometer: principle: within the elastic limit, the greater the tension the spring receives, the greater its elongation. (Within the elastic limit, the elongation of the spring is directly proportional to the tensile force. )

Use of spring dynamometer: (1) Identify the dividing value and measuring range; (2) check whether the pointer points to the zero scale, and if not, set it to zero; (3) Gently pull the scale hook several times to see whether the pointer returns to zero scale after each release; (4) When measuring the force, it should be along the axial direction of the spring (the elongation direction of the spring) and should not exceed the scale of the spring.

Common sense: In physics experiments, the size of some physical quantities is not suitable for direct observation, but when it changes, it is easy to observe the changes of other physical quantities. It is an idea to display unobservable quantities with easily observable quantities. This scientific method is called transformation. The instruments made by this method include thermometer, spring dynamometer, pressure gauge and so on.

Second, gravity.

Gravity: Any two objects in the universe, from celestial bodies to dust, have gravity. (Newton)

Gravity: the force exerted by the gravity of the earth on an object. (The object of gravity is the earth)

1, the magnitude of gravity is called weight, and the gravity of an object is directly proportional to its mass. G = mg. (g=9.8N/kg) means that the gravity of an object with a mass of 1kg is 9.8N.)

2. Gravity direction: vertically downward (pointing to the center of the earth). Application: Check whether the wall is vertical and the surface is horizontal with vertical line and level respectively.