What's the formula of pressure and buoyancy?

What's the formula of pressure and buoyancy?

10、 Pressure
1. Pressure
Pressure: the force acting vertically on the surface of an object is called pressure. The direction of pressure is perpendicular to the surface of the object being pressed
Note: pressure and gravity: (1) gravity can produce pressure, but pressure is not always produced by gravity. (2) the direction of pressure is always perpendicular to the surface of the object being pressed, while the direction of gravity is always vertical downward. (3) the object applying pressure can be various objects, while the object applying gravity must be the earth
2. Pressure
(1) A physical quantity used to describe the effect of pressure
(2) Definition: the pressure on an object per unit area
(3) Formula: P = f / s, which is applicable to the pressure of solid, gas and liquid
S refers to the force area of an object. For example, when a person walks, the force area is the area of one foot, while standing is the total area of two feet
② For a cylindrical object placed on a horizontal plane, when it is not subjected to external force, the pressure produced by different objects on the supporting surface can be compared according to the density and height. P = ρ GH
(4) Unit: Pascal (PA)
(5) Methods of increasing and decreasing pressure
Method and principle of pressure change
Using the formula: P = f / s, this formula is applicable to the pressure of solid, gas and liquid
Methods of increasing and decreasing pressure
Methods to increase the pressure:
If the stress area s remains unchanged, the pressure f increases and the pressure P increases
If the pressure f is constant, the stress area s becomes smaller and the pressure P becomes larger
Three examples of increasing pressure: 1. Skates of speed skaters are equipped with skates; 2. Darts thrown at the target plate; 3
Methods to reduce the pressure:
If the stress area s remains unchanged, the pressure f becomes smaller and the pressure P becomes smaller
If the pressure f is constant, the stress area s becomes larger and the pressure P becomes smaller
Three examples of pressure reduction: 1. Heavy trucks are equipped with many wheels; 2. Houses are built on larger foundations; 3. Schoolbags are made wider
3. Liquid pressure
(1) The characteristics of the internal pressure of liquid: ① there is pressure in all directions of the liquid; ② the pressure increases with the increase of depth; ③ the pressure in all directions of the same liquid at the same depth is equal
(2) The cause of liquid pressure: the liquid is subject to gravity
(3) Calculation formula: P = ρ GH
This formula is only applicable to the calculation of the pressure inside the liquid, where ρ is the density of the liquid and H is the vertical height from the research point to the free surface
(4) Measuring tool: manometer
(5) Application: connector (ship lock, livestock automatic water feeder, etc.)
Principle of connecter: for the same kind of liquid still in the connecter, the liquid level in direct contact with the atmosphere is always flat
4. Gas pressure
(1) The cause of atmospheric pressure: the atmosphere is subject to gravity
(2) Experiments to verify the existence of atmospheric pressure -- madeburg hemisphere experiment, cup covering experiment, egg swallowing experiment, etc
(3) Determination of atmospheric pressure -- Torricelli experiment
1atm＝1.013×105Pa＝76cmHg＝10.34mH2O
The method to judge whether there is air in the tube: tilt the glass tube to see if mercury can fill the whole tube
② The height of the mercury column in the glass tube is related to the atmospheric pressure of the outside world, and has nothing to do with the thickness of the tube, the depth of insertion into the mercury, and whether it is inclined or not
(4) The influence factors of atmospheric pressure are related to height and climate
Atmospheric pressure measuring tools: barometers (mercury barometers and aneroid barometers)
(5) The relationship between gas pressure and volume: under the condition of constant temperature, the volume of a certain mass of gas decreases and the pressure increases
(6) The relationship between liquid pressure and flow velocity: the pressure of fluid is lower in the place with high flow velocity, and higher in the place with low flow velocity
11、 Buoyancy
1. The cause of buoyancy: the pressure difference between upward and downward caused by liquid or gas
2. Archimedes principle
① Content: the object immersed in liquid or gas is subject to the vertical buoyancy of liquid or gas. The buoyancy is equal to the weight of liquid or gas
② Formula: F floating = g row = m row g = ρ liquid GV row
Principle (1) the buoyancy is only related to the volume and density of the liquid, but not to the depth of the object. (2) if only a part of the object is immersed in the liquid, the buoyancy is equal to the weight of the liquid. (3) Archimedes' law is applicable not only to liquids, but also to gases, Equal to the weight of the gas displaced by the object
When the liquid density is constant, the larger the volume of liquid, the greater the buoyancy
When the volume of liquid is constant, the higher the density of liquid, the greater the buoyancy
When the product of liquid density and liquid volume is larger, the buoyancy is larger
On the contrary, the smaller
The buoyancy is only related to the volume of liquid and the density of liquid,
It has nothing to do with the density or volume of the object (when the object floats, it is half on the water surface and half under the water. Only when it is submerged, the volume of the liquid discharged by the object is equal to the volume of the object). It has nothing to do with the depth of the object
3. Floating and sinking conditions of objects
Floating up: F floating > G floating: F floating = g sinking: F floating