Buoyancy

Japanese: 浮力 - ふりょく(英語表記)buoyancy
Buoyancy

The force that acts on a material when the material is placed in a fluid in a gravitational field.

Consider a box placed in a stationary fluid on the Earth's surface. If the height of the box is h , the fluid pressure p on the top surface of the box is related to the pressure p 0 on the bottom surface as p = p 0 - ρ gh . Here, ρ is the density of the fluid, and g is the gravitational acceleration of the Earth. This relationship is obtained from Bernoulli's theorem with the flow velocity set to zero. If the area of ​​the top and bottom surfaces of the box is S , then the box is subjected to an overall upward force of F = Sp 0 (pressure is the force per unit area) and a downward force of F = Sp . The net result is that the upward force is larger by Δ F = S ( p 0 - p ) = ρg hS , which tries to float the box. This is buoyancy. Since the volume of the box is V = hS , the magnitude of this buoyancy is Δ F = ρg V , which is equal to the weight of the box when it is filled to the brim with fluid. Buoyancy is a force that acts in the opposite direction to gravity and acts on the center of gravity of an object. This is called the center of buoyancy. Therefore, if an object of mass M is placed in a fluid in a vacuum, gravity of f = Mg and buoyancy of ΔF = mg ( m is the mass of the fluid displaced by the object) act on the center of gravity, and the object is only subjected to a force of f - ΔF = ( M - m ) g. Archimedes' principle, which states that an object submerged in water becomes lighter by the amount of water it displaces, can be understood in this way. In the past, people were amazed that iron ships could float, but all that was needed was to design the ship to be hollow and the mass of the water displaced by the ship, m , to be greater than or equal to the total mass of the iron, M. However, when the ship tilts and the amount of water displaced decreases, the buoyancy decreases but the ship's weight remains the same, so it will sink, and water may enter and sink.

Buoyancy is also the reason why heated air becomes an updraft, and hot water rises from the bottom of a kettle due to convection. When this happens, the heated parts of the air or hot water expand and become less dense. Therefore, compared to the surrounding unheated part of the same volume, the heated parts are lighter because there are fewer air or water molecules in them. This is the same reason why helium balloons float because the weight of the helium gas (and the balloon) inside them is less than the weight of the air they displace. Recently, hot air balloons, which float by heating the air with a nearby burner, are more commonly used for leisure activities than expensive helium balloons.

[Ryo Ikeuchi]

Buoyancy regulation in animals

Many aquatic animals adjust their body's specific gravity to maintain a constant living depth and to move vertically. Plankton are generally small, with complex projections on the surface of their bodies that slow down their sinking speed, and some are adapted to float by storing gas and oil droplets inside their bodies. Noctiluca has many ammonium salts in its vacuoles, and radiolarians also have many vacuoles in their exosporium, which are said to help regulate buoyancy. Marine dinoflagellates have vacuoles (water sacs) that communicate with the outside world, and changes in their volume cause changes in their specific gravity. Many fish eggs obtain buoyancy from oil droplets and float near the water surface. Colonies of siphonophores (Portuguese man's warbler, Portuguese man-of-war) have gas sacs called bubble bodies that float on the water surface. Many fish have well-developed swim bladders. The volume of the swim bladder increases as gases, mainly oxygen, are secreted from the red gland, and decreases as the gas is absorbed by the oval gland. The volume of the swim bladder also changes depending on the depth at which the fish is, so if a fish rises too quickly and cannot keep up with the absorption of gases from the oval gland, the swim bladder will expand too much and lose its ability to regulate buoyancy. Fish such as sturgeon, salmon, and carp have swim bladders that are open to the outside world even when they become adults, so they regulate their buoyancy by letting air in and out. Cyclostomes, sharks, and rays do not have swim bladders. The carapace of the cuttlefish and the shell of the nautilus are buoyancy-regulating organs that lower the osmotic concentration of the body fluids inside to remove water and regulate the volume occupied by gas.

[Akira Murakami]

[References] | Archimedes' principle | Swim bladder | Vacuole | Weight | Balloon | Convection | Plankton | Bernoulli's principle
Buoyancy (box in stationary fluid)
©Shogakukan ">

Buoyancy (box in stationary fluid)


Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

重力場に置かれた流体内に物質を置いたときに、流体から物質にかかる力のこと。

 地球表面の静止流体中に置かれた箱を考えてみよう。箱の高さをhとすると、箱の上面での流体の圧力pは、下面の圧力p0pp0-ρghの関係にある。ここにρは流体の密度、gは地球の重力加速度である。この関係は、流速をゼロとしたベルヌーイの定理から得られる。箱の上下面の面積をSとすると、箱には全体として、上向きにFSp0(圧力は単位面積当りの力である)、下向きにFSpの力が働くことになり、差し引きΔFS(p0p)=ρghSだけ上向きの力が大きく、箱を浮かせようとする。これが浮力である。この浮力の大きさは、箱の体積がVhSであることから、ΔF=ρgVとなり、ちょうど箱の中に流体をいっぱいに満たしたときの重さに等しいことがわかる。浮力は重力と反対方向に働く力であり、物体の重心に作用する。これを浮力の中心という。したがって、真空中で質量Mの物体を流体中に置くと、重心にfMgの重力とΔFmgの浮力(mは、物体が押しのけた流体の質量)が働き、結局、fΔF=(Mm)gの力しか受けないことになる。「水の中の物体は、それが押しのけた水の量だけ軽くなる」というアルキメデスの原理は、このようにして理解できる。昔、鉄の船が浮かぶことに人々は驚いたが、中空にして船が押しのける水の質量mを、鉄の全質量M以上になるよう設計すればよいわけである。しかし、船が傾いて、押しのける水の量が減ったとき、浮力は減るが船の重量は変わらないため沈み、浸水がおこって沈没することがある。

 温められた空気が上昇気流になったり、やかんの底から湯が対流で上昇するのも、浮力のためである。このとき、空気や湯の温められた部分では膨張し密度が小さくなる。したがって、周りの温められていない部分と同じ体積で比べると、温められた部分の空気や水の分子の数は少ないため軽くなっているのである。これは、ヘリウムガスの風船が浮くのは、風船中のヘリウムガス(と風船)の重さが押しのけた空気の重さより小さいのと同じ理由である。最近では、高価なヘリウムガスの風船より、手近なバーナーで空気を熱して浮かせる熱気球がレジャーによく使われている。

[池内 了]

動物の浮力調節

水生動物には、体の比重を調節して自分が生活する深度を一定に保ったり、また垂直に移動したりするものが多い。浮遊生物(プランクトン)は一般に小形のものが多く、体表に複雑な突起があって沈降速度が遅くなっているものや、体内に気体、油滴などを蓄える浮遊への適応がみられる。ヤコウチュウは液胞にアンモニウム塩が多く、また放散虫でも外肉中に多数の液胞があり、それらが浮力調節に役だっているといわれている。海産渦鞭毛(うずべんもう)虫類には、外界と通じた液胞(水嚢(すいのう))があり、その体積変化により比重が変化する。魚卵には、油滴によって浮力を得て水面近くに浮遊するものが多い。クダクラゲ類(カツオノエボシ、カツオノカンムリ)の群体には、気泡体といわれるガスの袋があって水面に浮遊する。多くの魚類には、うきぶくろが発達している。うきぶくろの体積は赤腺(せきせん)から酸素を主とするガスが分泌されて大きくなり、そのガスが卵円腺から吸収されて小さくなる。また、うきぶくろの体積は魚のいる深さによって変化するので、魚が卵円腺からのガスの吸収にまにあわないほど急激に浮上すると、うきぶくろが膨張しすぎて浮力の調節能力を失う。チョウザメ、サケ、コイなどの魚は、成体になってもうきぶくろは外界に通じているので、浮力の調節は空気の出入によって行われる。また円口類、サメ・エイの類にはうきぶくろはない。コウイカの甲やオウムガイの殻は、中の体液の浸透圧濃度を下げることによって水を抜き、気体で占められる部分の体積を調節する浮力調節器官である。

[村上 彰]

[参照項目] | アルキメデスの原理 | うきぶくろ | 液胞 | 重さ | 気球 | 対流 | プランクトン | ベルヌーイの定理
浮力(静止流体中に置かれた箱の場合)
©Shogakukan">

浮力(静止流体中に置かれた箱の場合)


出典 小学館 日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について 情報 | 凡例

<<:  Free rider

>>:  Prisoner of War - Furyoki

Recommend

Takeuchi Style

It is one of the styles that played a role as the...

Consonant interval

A musical term. The degree of consonance or disson...

Elton's Pyramid - Elton's Pyramid

…If these relationships are illustrated and stack...

Meal portion - divide

〘Other ka next 1〙 Kuhiwa-ku 〘Other ka next 2〙 To e...

Cante Intermedio - Cante Intermedio

...However, in the 20th century, choreography was...

Unkei

A Chinese literary figure from the Qing dynasty. ...

Kanaya Shonin

⇒ Yokoi Kanaya Source: Kodansha Digital Japanese N...

Song scriptures marking style

A book on waka poetry from the Nara period. Writt...

Motsugo (Japanese parrotfish) - Motsugo (English spelling) Pseudorasbora parva

A freshwater fish of the Cyprinidae family (illust...

Awaken - to remember

(A variation of "omohoyu (thought)") [1]...

Crime of forgery of securities

It is a crime to forge, alter, or make false entr...

Unwin, R.

… His proposal was seen by many as a utopian drea...

Clothing - Ifuku

A general term for clothing worn by humans, exclu...

Ohara Yugaku

A thinker of rural reform in the late Edo period....

Kalmar Union - Kalmar Union

The union of Denmark, Sweden, and Norway was forme...