Материал: Андрианова English Thesaurus of Student-Physicist 2013

2. Match the words in A with the words in B to form word combinations:

3. Fill in the gaps with the missing words from the list:

tangential acceleration, net torque, center of gravity, moment of inertia, rate

1.An object remains in a state of uniform rotational motion unless acted on by a _____.

2.The _____ of rotation of an object doesn't change, unless the object is acted on by a net torque.

3.The _____ of a homogeneous rod lies midway between the ends of the rod.

4.Because there is no force to oppose this tangential force, the object undergoes a _____ in accordance with Newton's second law.

5.The torque on the object is proportional to the angular acceleration of the object, where the constant of proportionality is called the

_____.

UNIT 9

SOLIDS AND FLUIDS

Read the following texts. Study the thesaurus on fig. 9, answer the questions after the texts

There are four known states of matter: solids, liquids, gases, and plasmas. In the universe at large, plasmas - systems of charged particles interacting electromagnetically - are the most common. In our environment on Earth, solids, liquids, and gases predominate.

An understanding of the fundamental properties of these different states of matter is important in all the sciences, in engineering, and in medicine.

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Forces put stresses on solids, and stresses can strain, deform, and break those solids, whether they are steel beams or bones. Fluids under pressure can perform work, or they can carry nutrients and essential solutes, like the blood flowing through our arteries and veins. Flowing gases cause pressure differences that can lift a massive cargo plane or the roof off a house in a hurricane. High-temperature plasmas created in fusion reactors may someday allow humankind to harness the energy source of the sun. The study of any one of these states of matter is itself a vast discipline.

States of matter

Everyday experience tells us that a solid has a definite volume and shape. A brick, for example, maintains its familiar shape and size day in and day out. A liquid has a definite volume but no definite shape. A gas differs from solids and liquids in that it has neither definite volume nor definite shape. Because gas can flow, however, it shares many properties with liquids.

All matter consists of some distribution of atoms or molecules. The atoms in a solid, held together by forces that are mainly electrical, are located at specific positions with respect to one another and vibrate about those positions. At low temperatures, the vibrating motion is slight and the atoms can be considered essentially fixed. As energy is added to the material, the amplitude of the vibrations increases. A vibrating atom can be viewed as being bound in its equilibrium position by springs attached to neighboring atoms. Solids can be classified as either crystalline or amorphous.

For any given substance, the liquid state exists at a higher temperature than the solid state. The intermolecular forces in a liquid aren't strong enough to keep the molecules in fixed positions, and they wander through the liquid in random fashion. Solids and liquids both have the property that when an attempt is made to compress them, strong repulsive atomic forces act internally to resist the compression.

In the gaseous state, molecules are in constant random motion and exert only weak forces on each other. The average distance between the molecules of a gas is quite large compared with the size of the molecules. Occasionally the molecules collide with each other, but most of the time they move as nearly free, noninteracting particles. As a result, unlike solids and liquids, gases can be easily compressed.

When a gas is heated to high temperature, many of the electrons surrounding each atom are freed from the nucleus. The resulting system is a collection of free, electrically charged particles - negatively charged elec-

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trons and positively charged ions. Such a highly ionized state of matter containing equal amounts of positive and negative charges is called a plasma. A Unlike a neutral gas, the long-range electric and magnetic forces allow the constituents of a plasma to interact with each other. Plasmas are found inside stars and in accretion disks around black holes, for example, and are far more common than the solid, liquid, and gaseous states because there are far more stars around than any other form of celestial matter, except possibly dark matter. Dark matter, inferred by observations of the motion of stars around the galaxy, makes up about 90% of the matter in the universe and is of unknown composition.

The deformation of solids

While a solid may be thought of as having a definite shape and volume, it's possible to change its shape and volume by applying external forces. A sufficiently large force will permanently deform or break an object, but otherwise, when the external forces are removed, the object tends to return to its original shape and size. This is called elastic behavior.

The elastic properties of solids are discussed in terms of stress and strain. Stress is the force per unit area causing a deformation; strain is a measure of the amount of the deformation. For sufficiently small stresses, stress is proportional to strain, with the constant of proportionality depending on the material being deformed and on the nature of the deformation. We call this proportionality constant the elastic modulus.

We define the tensile stress as the ratio of the magnitude of the external force to the cross-sectional area. The SI unit of stress is the newton per square meter (N/m2), called the pascal (Pa). The tensile strain is defined as the ratio of the change in length to the original length and is therefore a dimensionless quantity. We can write an equation relating tensile stress to tensile strain, where the constant of proportionality is Young's modulus.

It's possible to exceed the elastic limit of a substance by applying a sufficiently great stress. A material subjected to a stress beyond this limit ordinarily doesn't return to its original length when the external force is removed. As the stress is increased further, it surpasses the ultimate strength: the greatest stress the substance can withstand without breaking. The breaking point for brittle materials is just beyond the ultimate strength.

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Another type of deformation occurs when an object is subjected to a force parallel to one of its faces while the opposite face is held fixed by a second force. This kind of stress is called a shear stress. There is no change in volume with this kind of deformation. A material having a large shear modulus is difficult to bend.

The volume stress is defined as the ratio of the magnitude of the change in the applied force to the surface area. The bulk modulus characterizes the response of a substance to uniform squeezing. The reciprocal of the bulk modulus is the compressibility of the material.

Density and pressure

Equal masses of aluminum and gold have an important physical difference: The aluminum takes up over seven times as much space as the gold. While the reasons for the difference lie at the atomic and nuclear levels, a simple measure of this difference is the concept of density.

Fluids don't sustain shearing stresses, so the only stress that a fluid can exert on a submerged object is one that tends to compress it, which is bulk stress. The force exerted by the fluid on the object is always perpendicular to the surfaces of the object. The pressure at a specific point in a fluid can be measured. As the device is submerged in a fluid, the fluid presses down on the top of the piston and compresses the spring until the inward force exerted by the fluid is balanced by the outward force exerted by the spring. Notice that the force that compresses the spring is spread out over the entire area, motivating our formal definition of pressure.

Buoyant forces and Archimedes’s principle

A fundamental principle affecting objects submerged in fluids was discovered by the Greek mathematician and natural philosopher Archimedes. Archimedes's principle can be stated as follows: Any object completely or partially submerged in a fluid is buoyed up by a force with magnitude equal to the weight of the fluid displaced by the object.

Fluids in motion

When a fluid is in motion, its flow can be characterized in one of two ways. The flow is said to be streamline, or laminar, if every particle that passes a particular point moves along exactly the same smooth path followed by previous particles passing that point. This path is called a streamline. The flow of a fluid becomes irregular, or turbulent, above a certain velocity or under any conditions that can cause abrupt changes in velocity.

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