Force F: Force is the action of an object on an object. The forces between objects always interact. Unit of force: Newton (n). Instruments for measuring force: dynamometer; Spring scales are used in laboratories. The function of force: to deform an object or change its motion state. The change of the motion state of an object refers to the change of the speed or direction of the object. The three elements of force: the size, direction and action point of force are called the three elements of force. The diagram of force should be proportional; Schematic diagram of forces, not to scale. 3. Gravity G: the force exerted on an object due to the attraction of the earth. Direction: vertical downward. The relationship between gravity and mass: G=mg m=G/g g=9.8 N/kg. Reading: 9.8 N/kg, that is to say, the weight of an object with a mass of 1 kg is 9.8 N. Center of gravity: the point of gravity is called the center of gravity of the object. The center of gravity of a regular object is in the geometric center of the object. 2. Two-force balance condition: acting on the same object; These two forces are equal in magnitude and opposite in direction; Linear motion. Under the balance of two forces, an object can be at rest or move in a straight line at a uniform speed. The equilibrium state of an object means that the object is in a state of static or uniform linear motion. The resultant force of external forces on an object in equilibrium is zero. 5. Synthesis of two forces on the same line: same direction: resultant force f = f1+F2; The direction of resultant force is the same as that of F 1 and F2; Opposite directions: the resultant force F=F 1-F2, and the resultant force direction is the same as the strong force direction. Under the same conditions, rolling friction is much smaller than sliding friction. Sliding friction is related to positive pressure, material characteristics and contact surface roughness. Sliding friction, rolling friction and static friction. Newton's first law, also known as the law of inertia, means that all objects are always in a state of static or uniform linear motion when they are not affected by external forces. Inertia: The property that an object maintains its original static or uniform linear motion is called inertia. 4. density ⒈ density ρ: the mass per unit volume of a substance, and density is a characteristic of the substance. Formula: m=ρV international unit: kg/m3, common unit: g/cm3, relationship:1g/cm3 =1×103kg/m3; ρ water =/kloc-0 /×103kg/m3; Reading: per cubic meter 103kg, which means that the mass of 1 cubic meter of water is 103kg. 1. Density measurement: measure the mass with a pallet balance, and measure the volume of solid or liquid with a measuring cylinder. Area unit conversion:1cm2 =1x10-4mm2,1mm2 =1x10-6mm2. V. Pressure 1. Pressure P: The pressure per unit area of an object is called pressure. Pressure F: the force acting vertically on the surface of an object, in N ... The function of pressure is expressed by pressure, which is related to the pressure and the stressed area. Pressure unit: n/m2; Scientific name: Pascal (Pa) formula: F=PS S: stress area, the common part of contact between two objects; Unit: square meters. Methods of changing pressure: ① reducing pressure or increasing stress area can reduce pressure; ② Increasing the pressure or decreasing the stress area can increase the pressure. 1. liquid internal pressure: measure the liquid internal pressure: use a liquid manometer (u-tube manometer). Reason: Because the liquid has gravity, it exerts pressure on the bottom of the container; Due to the fluidity of the liquid, pressure is generated on the wall of the device. Law: ① At the same depth, the pressure in all directions is equal; ② The greater the depth, the greater the pressure; ③ Different liquids at the same depth, the higher the liquid density, the greater the pressure. [Depth h, the vertical height from the liquid surface to a certain point of the liquid. ] formula: p = p = P=ρgh h: unit: meter; ρ: kg/m3; G=9.8 N/kg. 13. Atmospheric pressure: gravity produces atmospheric pressure, which proves the existence of atmospheric pressure and is very large. It was the Madeborg Hemisphere Experiment and Torricelli (an Italian scientist) who measured the atmospheric pressure. After the Torricelli tube is tilted, the height of the mercury column remains unchanged and the length becomes longer. 1 standard atmospheric pressure = 76 cm, mercury column height =1.01×105pa =10.336m, and atmospheric pressure is measured by barometer (mercury barometer, box barometer). The law of atmospheric pressure changing with altitude: the higher the altitude, the smaller the pressure, that is, the lower the boiling point with the elevation. The intransitive verb buoyancy 1. Buoyancy and its causes: the force that an object immersed in liquid (or gas) pushes it upward is called buoyancy. Direction: vertically upward; Cause: the pressure difference between liquid and object. 2. Archimedes principle: An object immersed in a liquid is subjected to upward buoyancy, and the buoyancy is equal to the gravity when the object displaces the liquid. That is, f float = G liquid discharge = ρ liquid gV discharge. (Line V represents the volume of liquid displaced by the object) 3. Buoyancy calculation formula: F float = G-T = ρ gV row of liquid = F pressure difference 4. When the object floats: F float = G object and ρ object; When the object sinks, G and ρ < ρ liquid: F floats: ρ liquid VII. Simple machinery 1. Lever balance condition: f 1L 1 = f2l2. Force arm: the vertical distance from the fulcrum to the line of force action. By adjusting the nuts at both ends of the lever, the lever is in water position. Usage: It is convenient to directly measure the length of power arm and resistance arm. Crown block: equivalent to an arm. It can't save energy, but it can change the direction of force. Moving pulley: it is equivalent to a lever with twice the power arm as the resistance arm, which can save half of the force, but cannot change the direction of the force. Work: two necessary factors: ① the force acting on the object; ② The passing distance of the object in the direction of force. W = unit of fs work: Joule 3. Power: the work done by an object in unit time. A physical quantity indicating the speed at which an object does work, that is, an object with high power does work quickly. W = unit W = Pt P:watt；; Unit of w: joule; Unit of t: seconds.