Name: density, floating, and specific gravity
Uploaded: 2023-04-30T11:04:05.000Z
Duration: 56 s
Channel: JoVE

K12 Standards

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

9-12

Physics

Forces, Momentum And Motion

Air resistance and drag

Density

Se un tronco di legno e una pietra della stessa massa vengono gettati in un fiume, la pietra affonda, mentre il tronco galleggia. Ciò è dovuto a una proprietà fisica del materiale nota come densità.
La densità è definita come la massa di un oggetto per unità di volume e può aiutare a determinare se un oggetto galleggerà o affonderà in un fluido.
Nell'esempio precedente, la densità della pietra è maggiore di quella dell'acqua e quindi affonda. Considerando che la densità del legno è inferiore e quindi galleggia.
Oggetti dello stesso materiale con masse diverse hanno le stesse densità.
Se un oggetto ha una densità variabile nel suo volume o per una miscela eterogenea di due o più materiali diversi, si definisce una densità media.
La densità media di questi oggetti è uguale alla massa totale divisa per il suo volume totale.

densità

La densità è un'importante caratteristica delle sostanze, è cruciale nel determinare se un oggetto in un fluido affonda o galleggia. La sua unità nell'SI è il kg/m3, e nel cgs la sua unità è il g/cm3. La densità di un oggetto aiuta ad identificarne la composizione, e rivela anche informazioni riguardanti la fase della materia e la sua struttura interna. Le densità di liquidi e solidi sono approssimativamente comparabili, coerenti con il fatto che i loro atomi sono a stretto contatto. Ciò nonostante, i gas hanno densità molto più basse dei liquidi e dei solidi perché i loro atomi sono separati da grandi quantità di spazio vuoto.

La densità di una sostanza non è necessariamente costante in tutto il suo volume. Se la densità è costante in tutta la sostanza, essa è detta essere una sostanza omogenea. Un esempio di una sostanza omogenea è una barra di ferro solida, la cui densità è costante in ogni sua parte, e quindi la densità di ogni barra di ferro è uguale alla sua densità media. Se la densità di una sostanza non è costante, è detta essere una sostanza eterogenea. Un pezzo di formaggio svizzero è un esempio di materiale eterogeneo, contenente sia formaggio solido che vuoti pieni di gas. La densità in una posizione specifica all'interno di un materiale eterogeneo è chiamata densità locale ed è data in funzione della posizione. Visto che i gas sono liberi di espandersi e contrarsi, le loro densità variano considerevolmente con la temperatura, mentre le densità dei liquidi variano poco con la temperatura. Pertanto, le densità dei liquidi sono spesso trattate come costanti, con la densità uguale alla densità media.

La densità è una proprietà dimensionale; pertanto, quando confrontiamo le densità di due sostanze, devono essere prese in considerazione le unità. Per questa ragione, spesso si usa una quantità più conveniente e adimensionale chiamata gravità specifica, per confrontare le densità. La gravità specifica è definita come il rapporto della densità di un materiale e la densità dell'acqua a 4.0°C e un'atmosfera di pressione, che è di 1000 kg/m3. Il confronto usa l'acqua perché la densità di quest'ultima è di 1 g/cm3, il quale era originariamente usato per definire il chilogrammo. La gravità specifica, essendo adimensionale, fornisce un confronto immediato tra i materiali senza doversi preoccupare dell'unità della densità.

Questo testo è adattato da <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>

Densità

La densità è una caratteristica importante delle sostanze, cruciale per determinare se un oggetto affonda o galleggia in un fluido. La sua unità SI è kg/m3 e la sua unità ...

Education

Didattica

JoVE Core

Fisica

Unit 1

Unità 1

Fluid Mechanics

Meccanica dei Fluidi

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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 stay the same throughout a material. A substance with the same density everywhere is called homogeneous. A solid iron bar is a homogeneous example, because its density is constant through the whole bar and any sample has the same density as the average density.

If density changes from place to place, the material is heterogeneous. A chunk of Swiss cheese is one example, because it contains solid cheese and gas-filled voids. The density at one specific spot in a heterogeneous material is called local density, and it is given as a function of location.

Gas density can change a lot with temperature because gases can expand and contract freely. Liquid density changes very little with temperature, so liquids are often treated as having constant density. In that case, the density is taken to be the average density.

Density is a dimensional property, so the units matter when comparing two materials. A more convenient dimensionless quantity called specific gravity is often used for comparisons. Specific gravity is the ratio of a material’s density to the density of water at 4.0 °C and one atmosphere of pressure, which is 1000 kg/m3. Water is used for the comparison because its density is 1 g/cm3, a value that was originally used to define the kilogram. Since specific gravity has no units, it makes density comparisons easier without worrying about unit conversion.

Density, Floating, and Specific Gravity

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 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

Pascal’s principle explains how pressure changes move through a fluid that is not flowing. A fluid at rest is in static equilibrium. If the fluid is water, it is in hydrostatic equilibrium. In this state, the net force on any part of the fluid must be zero, or the fluid will begin to flow. Blaise Pascal described this idea in 1653 in Treatise on the Equilibrium of Liquids. He showed that when pressure changes in an enclosed fluid, that change is transmitted without loss through the fluid and...

application of pascal's law

Hydraulic Force Transfer with Pascal's Law

Hydraulic force transfer uses Pascal's law to move pressure through an enclosed fluid. Pascal showed experimentally that a change in pressure applied to a confined fluid is transmitted undiminished throughout the fluid and to the walls of its container. That idea is the basis of hydraulics, a major part of modern mechanical technology. Hydraulic systems appear in automotive brakes, hydraulic jacks, and many other machines. A simple hydraulic system uses two pistons at the same height. Because...

pressure gauges

Gauge vs Absolute Pressure

Pressure gauges often show pressure relative to atmospheric pressure. Many gauges, including those on scuba tanks, are set to read zero at atmospheric pressure. That reading is called gauge pressure. When the pressure inside the tank is higher than atmospheric pressure, the gauge shows a positive value. Some gauges can also measure negative pressure. This is useful in a vacuum chamber, which is a rigid chamber where some air has been pumped out. Because the pressure inside the chamber is below...

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archimedes' principle

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...

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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surface tension and surface energy

Why Liquids Form Drops and Float

Surface tension and surface energy explain why liquids behave the way they do at their surface. A paint brush moves freely in water, but its bristles stick together after it is taken out. That change is caused by surface tension. Inside a liquid, bulk molecules feel attractive forces from nearby molecules in all directions. At the surface, molecules feel a net downward pull from the molecules below them. Because of this imbalance, the liquid surface acts like a stretched membrane and tries to...

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excess pressure inside a drop and a bubble

excess-pressure-inside-a-drop-and-a-bubble

Surface Tension in Drops and Bubbles

Surface tension sets the pressure difference in small drops and bubbles. For a tiny liquid drop, gravity can be neglected, so the drop is treated as spherical. It can also be viewed as two equal hemispherical drops joined together by surface tension. The forces on the drop come from three sources. The liquid inside pushes outward, the air outside pushes inward, and surface tension acts along the diameter between the two hemispheres. Since surface tension is force per unit length, it creates a...

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buoyancy

Buoyant Force in Sinking and Floating

Buoyant force explains why objects float, sink, or stay suspended in a fluid. A fluid can be a liquid or a gas, and every object placed in it experiences this upward force. A metal ball sinks, while a rubber ball floats. A submarine also changes its buoyancy to move up or down. The force comes from pressure changes in the fluid. Fluid pressure increases with depth because the weight of the fluid above also increases. That means the bottom of an object feels more pressure than the top. This...

contact angle

Wetting and Surface Curvature

Wetting and surface curvature describe how a liquid behaves where it meets a solid. When a solid is dipped into a liquid, the liquid surface can bend near the contact line. In some cases, the liquid rises along the solid. In others, the surface becomes convex or depressed near the solid. This behavior comes from cohesive and adhesive forces. Adhesive force is the attraction between molecules of different materials, such as a solid and a liquid. Cohesive force is the attraction between...

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rise of liquid in a capillary tube

Capillary Action in Thin Tubes

Capillary action explains why liquid can rise or fall in a very thin tube compared with the liquid around it. A capillary is a narrow cylindrical tube, and the effect becomes noticeable when it is dipped into a liquid. The movement happens because two forces act in opposite directions. Cohesive forces make liquid molecules stick to each other, so the liquid tends to hold together and form a rounded shape. Adhesive forces pull the liquid toward the walls of the container. In a narrow glass...

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laminar and turbulent flow

Fluid Motion Patterns in Air and Water

Fluid dynamics studies fluids in motion. Scientists often use velocity vectors to show fluid motion in places like meteorology. For example, wind in the atmosphere can be mapped with vectors that show the speed and direction of the wind at each point. Another way to show fluid motion is with a streamline. A streamline shows the path of a small volume of fluid as it flows. When the flow changes with time, the streamlines do not match the flow lines. The fluid velocity is always tangential to...

equation of continuity

Fluid Flow in Narrowing Pipes

Fluid flow in a pipe is described by flow rate and average speed. Flow rate, or volume flow rate, is the amount of fluid that passes a point through an area in a given time. It depends on both how fast the fluid moves and the size and shape of the conduit. For a river or pipe, a greater speed usually means a greater flow rate. A larger cross-sectional area also increases flow rate. That is why the same fluid can move differently through conduits of different sizes. The relationship becomes...

bernoulli's equation

Fluid Pressure and Speed in Motion

Bernoulli's equation connects fluid pressure with fluid speed. It helps explain why moving fluids can create pressure changes in everyday situations. One example is when two trains pass each other quickly and seem to pull inward. The same effect can happen when two cars pass each other at high speed. As the air between the trains or cars speeds up, its pressure drops. The air on the outside stays at atmospheric pressure. That pressure difference creates a net inward force on the vehicles.

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

Bernoulli's principle helps explain how many fluid devices work when the fluid moves at a constant height. It is useful for devices that use fast flow to create low pressure and for devices that measure fluid speed. Some devices use this pressure change to pull one fluid into another. These are called entrainment devices. A Bunsen burner uses an adjustable gas nozzle to entrain air for proper combustion. An atomizer uses a squeeze bulb to make a jet of air that pulls perfume drops into the...

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energy conservation and bernoulli's equation

energy-conservation-and-bernoulli-s-equation

Bernoulli’s Equation in Streamlined Flow

Bernoulli’s equation describes energy in a moving fluid. It comes from the conservation of energy principle, or the work-energy theorem, applied to an incompressible, inviscid fluid in laminar, steady, irrotational flow. The equation says that pressure energy, potential energy, and kinetic energy per unit volume stay constant along a streamline. Each term has the units of energy per unit volume. The kinetic energy per unit volume is called kinetic energy density, and the potential energy per...

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viscosity

Viscosity and How Fluids Flow

Viscosity describes how easily a fluid flows. Water moves quickly, while honey and maple syrup flow slowly and stick to a surface. That difference comes from fluid friction between liquid layers and between a fluid and the material around it. In this example, water has a lower viscosity than honey and maple syrup. Viscosity is measured in newton-second per meter squared (N·s/m2) or pascal-second (Pa·s). It also changes with temperature and can differ from one fluid to another by several orders...

poiseuille's law and reynolds number

Fluid Resistance, Viscosity, and Flow Regime

Fluid resistance, viscosity, and flow regime shape how liquid moves through a horizontal tube. Fluid flows from high pressure to low pressure. The flow rate, Q, is the ratio of the pressure difference to the resistance in the tube. A larger pressure difference gives a higher flow rate. Poiseuille's law describes laminar flow of an incompressible fluid in a tube. In this relation, the factors that affect flow rate except pressure are grouped into resistance. Resistance depends on the tube size...

stokes' law

Terminal Velocity in a Viscous Fluid

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

Explore how density affects floating and sinking, how it varies in homogeneous and heterogeneous materials, and how specific gravity compares substances.

Density, Floating, and Specific Gravity | JoVE K12

physics

JoVE Core Physics introduces the theory and application of fundamental physics topics, including kinematics, dynamics, fluid mechanics, statics, waves, and acoustics, with concise and easy-to-understand animated videos. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Physical Geology

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Igneous, Metamorphic And Sedimentary Rocks

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Alabama Course of Study Science

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Matter and Its Interactions

9-12.PS.3.5.a

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Motion and Stability: Forces and Interactions

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Newton's First Law: Application

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Internal and External Forces

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Newton's Second Law

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Mass and Weight

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Newton's Third Law: Introduction

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Newton's Third Law: Examples

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Second Law: Motion under Same Force

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Second Law: Motion under Same Acceleration

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Frictional Force

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Newton's laws applied to complex problems

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newton-s-laws-applied-to-complex-problems

Newton's laws applied to complex problems

Gravitation

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Newton's Law of Gravitation

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Gravitation Between Spherically Symmetric Masses

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Gravity between Spherical Bodies

Newton&#39;s law of gravitation describes the gravitational force between any two point masses. However, for extended spherical objects like the Earth, ...

Apparent Weight

True weight is the measure of the gravitational force acting on an object. However, if the object accelerates, its measured weight is different from its ...

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Apparent Weight and the Earth's Rotation

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The Principle of Superposition and the Gravitational Field

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Gravitational force and fields

Content elaboration: Gravitational interactions are very weak compared to other interactions and are difficult to observe unless one of the objects is extremely massive (e.g., the sun, planets, moons). The force law for gravitational interaction states that the strength of the gravitational force is proportional to the product of the two masses and inversely proportional to the square of the distance between the centers of the masses, $F_{g}=(G\cdot m_{1}\cdot m_{2})/r^{2}$. The proportionality constant, G, is called the universal gravitational constant and has a value of $6.674\cdot10^{-11}m^{3}/(kg\cdot s^{2})$. Problem solving may involve calculating the net force for an object between two massive objects (e.g., Earth-moon system, planet-sun system) or calculating the position of such an object given the net force. The strength of an object's (i.e., the source's) gravitational field at a certain location, g, is given by the gravitational force per unit of mass experienced by another object placed at that location, $g=F_{g}/m$. Comparing this equation to Newton's second law can be used to explain why all objects on Earth's surface accelerate at the same rate in the absence of air resistance. While the gravitational force from another object can be used to determine the field strength at a particular location, the field of the object is always there, even if the object is not interacting with anything else. The field direction is toward the center of the source. Given the gravitational field strength at a certain location, the gravitational force between the source of that field and any object at that location can be calculated. Greater gravitational field strengths result in larger gravitational forces on masses placed in the field. Gravitational fields can be represented by field diagrams obtained by plotting field arrows at a series of locations. Field line diagrams are excluded from this course. Distinctions between gravitational and inertial masses are excluded. A scale indicates weight by measuring the normal force between the object and the surface supporting it. The reading on the scale accurately measures the weight if the system is not accelerating. However, if the scale is used in an accelerating system, as in an elevator, the reading on the scale does not equal the actual weight. The scale reading can be referred to as the "apparent weight." This apparent weight in accelerating elevators can be explained and calculated using force diagrams and Newton's laws.

Gravitational force and fields

Simple Harmonic Motion

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Characteristics of Simple Harmonic Motion

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Oscillations about an Equilibrium Position

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Hooke's Law

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Elasticity

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Elastic forces

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Elastic forces

Density, Floating, and Specific Gravity

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