Name: fluid pressure in everyday life
Uploaded: 2023-04-30T11:04:12.000Z
Duration: 1 min 14 s
Channel: JoVE

K12 Standards

Alabama Course of Study Science

9-12

Physics

motion-and-stability-forces-and-interactions

Motion and Stability: Forces and Interactions

9-12.P.1.4

Pressure of Fluids

Vous êtes-vous déjà demandé comment un fluide s’écoule d’un point à un autre ?
Pour comprendre cela, considérons un fluide de densité constante au repos. Si une force est appliquée parallèlement à la surface du fluide, alors pour s’adapter à cette force, le fluide est mis en mouvement.
En revanche, si une force normale est appliquée à la surface du fluide, une force égale et opposée se développe dans le fluide en réaction. Cette force normale exercée par le fluide par unité de surface est appelée pression. L’unité SI de pression est pascal.
La pression est toujours normale à toute surface en contact avec le fluide, tout comme une quantité scalaire. En comparaison, la force a une direction spécifique et est une grandeur vectorielle.
La pression à une profondeur h de la surface du fluide est la somme de la pression atmosphérique et de la pression due au poids du fluide.
Lorsque nous substituons les expressions équivalentes du poids, de la masse et du volume, nous obtenons la pression due au poids du fluide comme le produit de la densité, de l'accélération due à la gravité et de h.

pression des fluides

Il existe de nombreux exemples de pression dans les fluides dans la vie quotidienne, comme dans le cas du sang (pression artérielle élevée ou basse) et en relation avec la météo (systèmes météorologiques à haute et basse pression). Une force donnée peut avoir un effet considérablement différent en fonction de la surface sur laquelle la force est exercée. Par exemple, une force appliquée sur une surface de 1 mm² a une pression 100 fois plus grande que la même force appliquée sur une surface de 1 cm2. C'est pourquoi une aiguille pointue peut percer la peau lorsqu'une petite force est appliquée, mais l'application de la même force avec le doigt ne perce pas la peau.

La pression est très importante lorsqu'on parle de fluides. Une caractéristique importante des fluides est qu'il n'y a aucune résistance significative à la composante de la force appliquée parallèlement à la surface du fluide. Les molécules du fluide s'écoulent simplement pour accommoder la force horizontale. Une force appliquée perpendiculairement à la surface compresse ou dilate le fluide. Par exemple, si on essaie de comprimer un fluide, une force de réaction se développe à chaque point à l'intérieur du fluide dans la direction extérieure, équilibrant la force appliquée sur les molécules à la limite. La pression en tout point d'un fluide statique ne dépend que de la profondeur à ce point.

La pression dans un fluide proche de la Terre varie en fonction de la profondeur en raison du poids du fluide au-dessus d'un niveau particulier. Ainsi, dans une piscine, la densité est approximativement constante et l'eau au fond est très peu compressée par le poids de l'eau au-dessus. En revanche, en montant dans l'atmosphère, la situation est assez différente, car la densité de l'air commence à changer de manière significative à une courte distance au-dessus de la surface de la Terre. De plus, la pression du fluide, en tant que quantité scalaire, n'a pas de direction, tandis que les forces dues à la pression ont des directions bien définies - elles sont toujours exercées perpendiculairement à toute surface. La raison en est que les fluides ne peuvent pas résister ou exercer des forces de cisaillement. Ainsi, dans un fluide statique enfermé dans un réservoir, la force exercée sur les parois du réservoir est exercée perpendiculairement à la surface interne. De même, la pression est exercée perpendiculairement aux surfaces de tout objet à l'intérieur du fluide.

Ce texte est adapté de<a href="https://openstax.org/books/university-physics-volume-1/pages/14-1-fluids-density-and-pressure"> Openstax, University Physics Volume 1, Section 14.1: Fluids, Density, and Pressure.</a>

Pression des fluides

Il existe de nombreux exemples de pression dans les fluides de la vie quotidienne, par exemple en relation avec le sang (pression artérielle élevée ou basse) et en relation avec les conditions météorologiques...

Education

Enseignement

JoVE Core

Physique

Unit 1

Unité 1

Fluid Mechanics

Mécanique des fluides

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

fluid pressure in everyday life

Fluid pressure appears in many everyday situations, including blood pressure and weather systems. The same force can have a very different effect depending on the area it acts on. A force spread over 1 mm² creates 100 times more pressure than the same force spread over 1 cm². That is why a sharp needle can pierce skin with a small force, while the same force from a finger usually does not.

Pressure is especially important when studying fluids. A fluid does not offer much resistance to a force applied parallel to its surface. Instead, the fluid molecules flow to fit that horizontal force. A force applied perpendicular to the surface behaves differently. It can compress or expand the fluid.

When a fluid is compressed, a reaction force forms inside the fluid at each point. This force acts outward and balances the force applied at the boundary. In a static fluid, the pressure at a point depends only on depth. Near Earth, fluid pressure changes with depth because of the weight of the fluid above that point.

This effect is easy to see in a swimming pool, where the water density is nearly constant. The water at the bottom is compressed very little by the water above it. The atmosphere is different because air density changes noticeably just a short distance above Earth’s surface. Pressure is a scalar quantity, so it has no direction. The forces caused by pressure do have direction, and they act perpendicular to any surface. Fluids cannot withstand or exert shearing forces, so pressure on tank walls or on objects in a fluid is always perpendicular to the surface.

Fluid Pressure in Everyday Life

characteristics of fluids

Fluid Behavior Under Force

Fluid behavior under force differs from the behavior of solids. When a force acts parallel to the top surface of a solid, the solid resists it because of internal friction between its layers. This is called shearing resistance. If the force is removed, the shearing forces help the solid return to its original shape. Other deformation forces can also change the shape of a solid for a short time, as long as the force stays below a threshold magnitude. Solids usually keep their shape, which makes...

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density

Density, Floating, and Specific Gravity

Density helps explain why objects sink or float in a fluid. It is also useful for identifying a substance and for learning about its phase and substructure. The SI unit of density is kg/m3, and the cgs unit is g/cm3. Liquids and solids usually have densities that are roughly comparable. That matches the fact that their atoms are in close contact. Gases have much lower densities than liquids and solids because their atoms are separated by large amounts of empty space. Density does not always...

pressure of fluids

Fluid pressure appears in many everyday situations, including blood pressure and weather systems. The same force can have a very different effect depending on the area it acts on. A force spread over 1 mm² creates 100 times more pressure than the same force spread over 1 cm². That is why a sharp needle can pierce skin with a small force, while the same force from a finger usually does not. Pressure is especially important when studying fluids. A fluid does not offer much resistance to a force...

variation of atmospheric pressure

Atmospheric Pressure and Altitude

Atmospheric pressure and altitude are closely linked. As height increases, atmospheric pressure decreases. This happens because the gravitational force per unit area becomes smaller as we move away from Earth’s surface. To describe this change, the atmosphere is treated with a constant air temperature at a given altitude. The ideal gas law of thermodynamics is also used as a good approximation for the air. Let p(y) be the atmospheric pressure at height y. The density ρ at y, the temperature T...

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pascal's law

Pascal’s Principle in Static Fluids

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application of pascal's law

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archimedes' principle

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excess pressure inside a drop and a bubble

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buoyancy

Buoyant Force in Sinking and Floating

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contact angle

Wetting and Surface Curvature

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rise of liquid in a capillary tube

Capillary Action in Thin Tubes

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bernoulli's equation

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bernoulli's principle

Fluid Pressure and Speed Changes

Bernoulli's equation links fluid pressure and fluid speed in a static, incompressible fluid. For horizontal flow, where gravitational energy density stays constant, the equation is especially useful. In that setting, the result is known as Bernoulli's principle. Bernoulli's principle says that fluid pressure drops when speed increases, and pressure rises when speed decreases. This pressure-speed relationship helps explain how moving fluids behave in simple flow situations. It also provides a...

bernoulli's principle: applications

Bernoulli's Principle in Fluid Devices

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energy conservation and bernoulli's equation

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Bernoulli’s Equation in Streamlined Flow

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viscosity

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stokes' law

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Fluid Pressure in Everyday Life | JoVE K12

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Fluid Pressure in Everyday Life

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