Name: buoyant force from fluid pressure
Uploaded: 2023-04-30T11:04:43.000Z
Duration: 1 min 13 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

Archimedes' Principle

完全に流体に浸された静止している物体を考えてみましょう。それはそのスペースを占めるために等量の流体を変位させます。
上向きの浮力により、物体は静止したままになります。
オブジェクトが流体から除去されると、スペースは以前にそれによって置換されたのと同じ量の流体で満たされます。
この流体ボリュームは、以前にオブジェクトに作用したのと同じ浮力によって作用されます。流体の体積が静止しているため、浮力はその重量によってバランスが取れています。
したがって、流体中の物体に作用する浮力の大きさは、それによって変位する流体の重量に等しくなります。これはアルキメデスの原理として知られています。
数学的には、浮力は、変位した流体の質量と重力による加速度の積に等しくなります。
質量と密度の関係を利用して、浮力は流体の密度、変位した流体の体積、重力による加速度の積としても表すことができます。

アルキメデスの原理

アルキメデスの原理では、流体に部分的または全体的に浸された物体にかかる上向きの浮力は、物体によって押しのけられる流体の重量に等しいと述べています。 物体を浮かせるにはどのくらいの浮力が必要かを理解するために、水中に沈んだ物体を流体から取り出すときに何が起こるかを考えてみましょう。 物体が流体内にない場合、物体が占める空間は、 重さwflを持つ流体によって満たされます。 この重量は周囲の流体によって支えられるため、浮力は物体によって押しのけられる流体の重量である wfl に等しくなければなりません。

この原理は、力の概念が十分に確立されるずっと前にこの原理を述べたギリシャの数学者アルキメデス (紀元前 287 ～ 212 年頃) にちなんで名付けられました。 アルキメデスの原理は、物体が部分的または全体的に流体に浸されたときに生じる浮力を指します。 流体の圧力をもたらす力は、物体の表面に対して垂直に作用します。 言い換えれば、下部の圧力による力は上を向き、上部では圧力による力は下を向きます。 側面の圧力による力が物体に向けられます。 本体の底部は本体の上部よりも深いため、本体の下部の圧力は上部の圧力よりも高くなります。 したがって、本体には正味の上向きの力が働きます。 この上向きの力が浮力です。

このテキストは<a href="https://openstax.org/books/university-physics-volume-1/pages/14-4-archimedes-principle-and-buoyancy"> から転載されています。 Openstax, University Physics Volume 1, Section 14.4: Archimedes’ Principle and Buoyancy. </a>

アルキメデスの原理は、流体に部分的または完全に浸された物体に加えられる上向きの浮力は、...

Education

JoVE Core

物理学

Unit 1

ユニット1

Fluid Mechanics

流体力学

KEY TERMS AND CONCEPTS

主要用語と概念

buoyant force from fluid pressure

Archimedes' principle explains buoyant force in a fluid. An upward buoyant force acts on an object that is partially or fully immersed in a fluid. This force is equal to the weight of the fluid displaced by the object.

To see why an object floats, imagine removing a submerged object from the fluid. The space it occupied would be filled by fluid with weight w_fl. That weight is supported by the surrounding fluid, so the buoyant force must match the weight of the displaced fluid.

The principle is named for the Greek mathematician Archimedes, who lived around 287 to 212 BCE. He stated the idea long before the modern concept of force was fully developed. The same principle applies whether the body is only partly submerged or completely under the fluid.

Fluid pressure creates the buoyant force. The pressure force acts perpendicular to the surface of the body. The force at the bottom points upward, while the force at the top points downward. Forces at the sides point inward.

Because the bottom of the body is deeper than the top, the pressure is greater lower down than higher up. This difference creates a net upward force on the object. That upward force is buoyancy.

Buoyant Force from Fluid Pressure

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 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...

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

Hydraulic Force Transfer with Pascal's Law

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archimedes' principle

Archimedes' principle explains buoyant force in a fluid. An upward buoyant force acts on an object that is partially or fully immersed in a fluid. This force is equal to the weight of the fluid displaced by the object. To see why an object floats, imagine removing a submerged object from the fluid. The space it occupied would be filled by fluid with weight w_fl. That weight is supported by the surrounding fluid, so the buoyant force must match the weight of the displaced fluid. The principle...

density and archimedes' principle

How Buoyancy Depends on Density

Buoyancy and density explain why some objects float and others sink. A lump of clay sinks in water, but the same clay can float when it is shaped like a boat. The change in shape lets it displace more water, which increases the buoyant force even though its mass stays the same. The same idea applies to steel ships. An object with an average density lower than the surrounding fluid will float. The fluid has more mass in the same volume, so it has more weight. The buoyant force equals the weight...

accelerating fluids

Fluid Pressure in an Accelerating Elevator

Fluid pressure changes when a liquid accelerates inside a moving container. A beaker placed in an elevator that moves upward with constant acceleration is one example. In this setup, the pressure and buoyant force equations must be modified. To find the pressure difference, the liquid is modeled as a thin vertical cylinder with height h and an infinitesimal cross-sectional area, ΔS. Three vertical forces act on this small fluid element. The liquid below the bottom surface pushes upward, the...

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excess pressure inside a drop and a bubble

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

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rise of liquid in a capillary tube

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laminar and turbulent flow

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bernoulli's equation

Fluid Pressure and Speed in Motion

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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

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energy conservation and bernoulli's equation

energy-conservation-and-bernoulli-s-equation

Bernoulli’s Equation in Streamlined Flow

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viscosity

Viscosity and How Fluids Flow

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poiseuille's law and reynolds number

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stokes' law

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Viscous forces act like friction in a liquid. They resist the relative motion of molecules and pull back on a solid body moving through the fluid. The size of this force depends on the body’s shape and size, its speed, and the liquid’s viscosity, density, and temperature. For a spherical solid object, the force in a fluid is described by Stokes’ law. This law applies only at low Reynolds number, when the flow is laminar and not turbulent. Experiments show that the force is proportional to the...

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key terms and concepts

Learn how fluid pressure creates buoyancy and why the upward force on a submerged object equals the weight of the fluid it displaces.

Buoyant Force from Fluid Pressure | JoVE K12

physics

JoVE Core 物理学は、運動学、力学、流体力学、静力学、波動、音響などの基礎的な物理学の理論と応用を、簡潔でわかりやすいアニメーション ビデオで紹介します。さらに、科学者の実際のビデオでは、世界中の今日の研究所で行われている古典的および独自の研究実験における関連概念の適用を示しています。

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Buoyant Force from Fluid Pressure

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