force

1. The force generated when an object is elastically deformed is called elasticity. Tension and pressure are elastic.

2. The greater the external force acting on the object, the greater the deformation of the object. According to this characteristic, a spring dynamometer is made.

3. in the international system of units, the unit of force is Newton and the symbol is n.

4. How to use the spring balance: (1): Know the range of the spring dynamometer, and don't measure the force beyond the range when using it. (2): Observe the calibration value of the spring dynamometer; (3): zero correction: put the spring dynamometer at the position to be measured and check whether the pointer is at zero scale; If not, it should be set to zero. (4) When measuring, make the stress direction of the spring dynamometer along the axial direction of the spring; When observing, the line of sight must be perpendicular to the dial.

An object with elastic deformation has energy, which is called elastic potential energy.

6. Usage of the spring scale: (1) Check whether the pointer is at zero scale, and if not, set it to zero; (2) Identify the measuring range and dividing value; (3) Gently pull the scale hook several times to see if the pointer returns to zero scale after each release. (4) After completing the above three steps, you can use the spring scale to measure the force, and the force cannot exceed the range of the spring scale.

7. The force that the earth's gravity acts on an object is called gravity, and the magnitude of gravity is simply called the weight of the object. Symbol "g", the center of gravity is called the center of gravity, and the center of gravity is not necessarily on the object;

8. The gravity on an object is directly proportional to its mass, and the relationship between them is G=mg, where the constant is expressed by "g=9.8N/Kg": an object with a mass of 1kg weighs 9.8N. ..

9. The gravity direction is always vertical downward, which can be used as a double vertical line or a level.

10. The force that the contact surface hinders the movement of an object is collectively called friction.

1 1. When an object is about to move, the force on the contact surface that hinders the movement of the object is called static friction; In the process of sliding, the force on the contact surface that hinders the movement of the object is called sliding friction.

12. Sliding friction is related to pressure and roughness of contact surface, but has nothing to do with the size of contact surface. Its direction is opposite to that of the object. The rougher the contact surface between objects, the greater the sliding friction, the greater the pressure of objects on the contact surface, and the greater the sliding friction. When sliding becomes rolling, the friction between objects becomes smaller.

13. There are many frictions in life, and the pencil sharpener is sliding friction; The sole is patterned to increase friction by increasing the roughness of the contact surface; Tighten the belt when threshing, and hold the brake handle when braking to increase friction by increasing pressure; The purpose of adding lubricating oil to the relatively slippery part of the machine is to reduce friction by separating the contact surface; The purpose of installing ball bearings is to change sliding into rolling and reduce friction. The friction direction of bicycle is that the front wheel is backward and the rear wheel is forward.

14. hovercraft and maglev train use the method of separating contact surfaces to reduce friction.

15. The action of objects on objects is called force. There must be at least two objects to produce force, and the actions of 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. The action of force is mutual.

16. When an object is deformed or its motion state changes, it can be judged that the object is acted by a force. The function of force is related to the size, direction and action point of force, which is called the three elements of force. Also known as the role of influence, the factor of effect.

17. The schematic diagram of the force is a line segment with an arrow. The starting point of the line segment indicates the point of action of the force, and the direction of the arrow indicates the direction of the force. If there are several forces in the same diagram, the greater the force, the longer the line segment should be.

Pressure and buoyancy

1. The force acting vertically on the surface of an object is called pressure. The point of pressure action is on the surface of the object, and the direction points to the pressed object. The effect of pressure action is related to the size of pressure and the size of stress area. In physics, the function of pressure is usually expressed by pressure.

2. The pressure per unit area of an object is called pressure. The calculation formula of pressure is P=F/S, and the unit is Pascal (Pa). Pressure is affected by pressure and stress area. If the pressure is high, the pressure may not be great. The pressure is great, but it is not necessarily great.

3. The methods to increase the pressure are: (1) increasing the pressure when the stress area is the same, (2) reducing the stress area when the pressure is the same, and (3) increasing the pressure while reducing the stress area. The methods to reduce the pressure are: (1) reducing the large pressure when the stress area is the same, (2) increasing the stress area when the pressure is the same; (3) Reduce the pressure and increase the stress area.

4. Three situations of increasing pressure: ① skates of speed skaters are equipped with skates; (2) throwing darts at the target; (3) sudden braking.

Three situations of decompression: ① the truck is equipped with many wheels; ② The house is built on a large foundation; ③ The schoolbag belt is made wider.

5. The kitchen knife needs to be polished for a long time to reduce the stress area and increase the pressure; The backpack belt should be wider, and the rails of the railway should be laid on sleepers to increase the stress area and reduce the pressure; The reason why the jaws of pliers are threaded is to increase roughness and friction.

6. Because the liquid is affected by gravity and has fluidity, there is pressure inside the liquid, which is at the bottom and side wall of the container.

7. There are pressures in all directions in the liquid, and the pressures in all directions are equal at the same depth. The internal pressure of liquid increases with the increase of depth, and the size of liquid is also related to the density of liquid. Different liquids at the same depth, the greater the density, the greater the pressure.

8. The liquid pressure is only related to the density and depth of the liquid, and has nothing to do with the gravity and volume of the liquid. The pressure formula of liquid is P=ρ liquid gh.

9. The tool for measuring liquid pressure is called pressure gauge, which judges the internal pressure of liquid according to the height difference of liquid level on both sides of U-shaped pipe.

10. It has been proved that there is pressure in the gas by the hemispheric experiment and the cup cover experiment in Madeborg. The standard atmospheric pressure is 1.0× 105Pa or 76cm high mercury column. Atmospheric pressure will decrease with the increase of altitude, which is also related to the weather.

1 1. Italian scientist Torricelli first measured the atmospheric pressure accurately, and the boiling point of liquid increased with the increase of atmospheric pressure; The principle of pressure cooker application is that the boiling point increases with the increase of atmospheric pressure. The reasons why alpine eggs can't be cooked well are as follows: ① With the increase of height, the atmospheric pressure decreases; ② With the decrease of air pressure, the boiling point of liquid decreases.

12. The hemispheric experiment in Madeborg was conducted by Otto, the mayor of Madeborg, Germany. Gehrig succeeded.

13. Liquid and gas are called fluids; The greater the velocity in a liquid, the smaller the pressure.

Force and motion

1. Buoyancy: All objects immersed in liquid or gas are pushed upward by liquid or gas. This force is called buoyancy. The buoyancy direction is always vertical. The buoyancy applying object is liquid or gas; (Objects are also affected by buoyancy in air.)

2. Conditions for the ups and downs of objects: (Immersed in liquid at first), that is, the relationship between buoyancy and gravity is considered in the submerged state.

Method 1: (Specific gravity and buoyancy of objects)

( 1)F float < g sinks； (2)F floating >; G floating; (3)F float =G float or float

Method 2: (Compare the densities of objects and liquids)

(1) > sinking; (2)& lt； Floating (3) = Pause.

The floating condition of the object: f floating = g.

3. Causes of buoyancy: An object immersed in a liquid is subjected to the upward and downward pressure difference of the liquid. Buoyancy is resultant force;

4. Archimedes principle: the buoyancy of an object immersed in a liquid is equal to the gravity that displaces the liquid; Formula: F floating =G leakage =m leakage = ρ gV leakage.

5. The methods for calculating buoyancy are:

(1) weighing method: f float =G-F(G is the gravity of the object, and f is the reading of the spring scale when the object is immersed in liquid).

(2) differential pressure method: F float = F down-F up；; (3) Unconditional immersion or sinking:

F flotation =G drainage =m drainage = ρ gV drainage.

(4) Floating and floating (note the similarities and differences between them): F floating = G.

7. Buoyancy utilization: (1) Ship: Make the material with density greater than water hollow, so that more water can be discharged.

(2) Submarine: change its own gravity to achieve ups and downs.

(3) Balloons and airships: filled with gas with density less than air. They rise and fall by changing their own gravity.

(4) Principle of densimeter: People who work by floating on the surface of liquid are subjected to the same buoyancy in different liquids, and the liquid displaced in the liquid with higher density is smaller and more exposed.

8. Based on the research of British physicist Newton and other scientists, Newton's first law is summarized: the content is that all objects always keep moving in a straight line at a uniform speed or at rest without being forced. Newton's first law is further inferred and generalized on the basis of experiments, so it cannot be proved by experiments.

9. The property that an object keeps its state of motion unchanged is called inertia. Newton's first law is also called the law of inertia. Inertia is not a force. The magnitude of inertia is only related to the mass of the object, and has nothing to do with the speed and direction of the object. All objects have inertia.

10. Balance of two forces: When an object is acted by several forces, we say that these forces are in a state of balance if it remains in a state of static or uniform linear motion.

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

12. An object will remain stationary or move in a straight line at a constant speed without external force or balancing force.

13. Force is the reason for changing (maintaining/changing) the motion state of an object.