Physics
Units Physics
Units in physics refer to the standardized measures used to quantify physical quantities such as length, mass, time, and energy. These units provide a common language for scientists to communicate and compare measurements. The International System of Units (SI) is the globally accepted standard for units in physics and other scientific disciplines.
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10 Key excerpts on "Units Physics"
eBook - PDF- Robert Resnick, David Halliday, Kenneth S. Krane(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
1 MEASUREMENT D espite the mathematical beauty of some of its most complex and abstract theories, physics is above all an experimental science. It is therefore critical that those who make precise measurements be able to agree on standards in which to express the results of those measurements, so that they can be communicated from one laboratory to another and verified. In this chapter we begin our study of physics by introducing some of the basic units of physical quanti- ties and the standards that have been accepted for their measurement. We consider the proper way to ex- press the results of calculations and measurements, including the appropriate dimensions and number of significant figures. We discuss and illustrate the importance of paying attention to the dimensions of the quantities that appear in our equations. Later in the text, other basic units and many derived units are in- troduced as they are needed. 1-1 PHYSICAL QUANTITIES, STANDARDS, AND UNITS The laws of physics are expressed in terms of many differ- ent quantities: mass, length, time, force, speed, density, re- sistance, temperature, luminous intensity, magnetic field strength, and many more. Each of these terms has a precise meaning, and they form part of the common language that physicists and other scientists use to communicate with one another — when a physicist uses a term such as “kinetic en- ergy,” all other physicists will immediately understand what is meant. Each of these terms also represents a quantity that can be measured in the laboratory, and just as there must be agreement on the meaning of these terms, there must also be agreement about the units used to express their values. Without such agreement, it would not be possible for scien- tists to communicate their results to one another or to com- pare the results of experiments from different laboratories. Such comparisons require the development and accep- tance of a set of standards for units of measurement.
eBook - PDF- David Halliday, Robert Resnick, Jearl Walker(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
What Is Physics? Science and engineering are based on measurements and comparisons. Thus, we need rules about how things are measured and compared, and we need experiments to establish the units for those measurements and comparisons. One purpose of physics (and engineering) is to design and conduct those experiments. For example, physicists strive to develop clocks of extreme accuracy so that any time or time interval can be precisely determined and compared. You may wonder whether such accuracy is actually needed or worth the effort. Here is one example of the worth: Without clocks of extreme accuracy, the Global Positioning System (GPS) that is now vital to worldwide navigation would be useless. Measuring Things We discover physics by learning how to measure the quantities involved in physics. Among these quantities are length, time, mass, temperature, pressure, and electric current. C H A P T E R 1 Measurement 2 CHAPTER 1 MEASUREMENT We measure each physical quantity in its own units, by comparison with a standard. The unit is a unique name we assign to measures of that quantity— for example, meter (m) for the quantity length. The standard corresponds to exactly 1.0 unit of the quantity. As you will see, the standard for length, which corresponds to exactly 1.0 m, is the distance traveled by light in a vacuum during a certain fraction of a second. We can define a unit and its standard in any way we care to. However, the important thing is to do so in such a way that scientists around the world will agree that our definitions are both sensible and practical. Once we have set up a standard—say, for length—we must work out proce- dures by which any length whatever, be it the radius of a hydrogen atom, the wheelbase of a skateboard, or the distance to a star, can be expressed in terms of the standard. Rulers, which approximate our length standard, give us one such procedure for measuring length.
eBook - PDF- David Halliday, Robert Resnick, Jearl Walker(Authors)
- 2020(Publication Date)
- Wiley(Publisher)
1 C H A P T E R 1 Measurement 1-1 MEASURING THINGS, INCLUDING LENGTHS Learning Objectives After reading this module, you should be able to . . . 1.01 Identify the base quantities in the SI system. 1.02 Name the most frequently used prefixes for SI units. 1.03 Change units (here for length, area, and volume) by using chain-link conversions. 1.04 Explain that the meter is defined in terms of the speed of light in vacuum. What Is Physics? Science and engineering are based on measurements and comparisons. Thus, we need rules about how things are measured and compared, and we need experiments to establish the units for those measurements and comparisons. One purpose of physics (and engineering) is to design and conduct those experiments. For example, physicists strive to develop clocks of extreme accuracy so that any time or time interval can be precisely determined and compared. You may wonder whether such accuracy is actually needed or worth the effort. Here is one example of the worth: Without clocks of extreme accuracy, the Global Positioning System (GPS) that is now vital to worldwide navigation would be useless. Measuring Things We discover physics by learning how to measure the quantities involved in physics. Among these quantities are length, time, mass, temperature, pressure, and electric current. We measure each physical quantity in its own units, by comparison with a standard. The unit is a unique name we assign to measures of that quantity—for example, meter (m) for the quantity length. The standard corresponds to exactly 1.0 unit of the quantity. As you will see, the standard for length, which corresponds Key Ideas ● Physics is based on measurement of physical quanti- ties. Certain physical quantities have been chosen as base quantities (such as length, time, and mass); each has been defined in terms of a standard and given a unit of measure (such as meter, second, and kilogram).
eBook - PDF- Paul Regtien, F. van der Heijden, M. J. Korsten, W Otthius(Authors)
- 2004(Publication Date)
- Butterworth-Heinemann(Publisher)
Chapter 2 Basics of Measurement In this chapter we discuss various basic aspects of measurement science. First, the system of units is presented, and how this system has been developed from its introduction in 1795 to the present time with basically seven standard units. The materialization of a unit quantity is the next issue to be discussed. It will be shown that at present all standards (except for the kilogram) are related to funda- mental physical constants. Obviously, there are many more quantities than those seven basic quantities. In the third section we give an overview of the most import- ant quantities and properties, used in various physical domains: the geometric, electrical, thermal, mechanical, and optical domain. Relations between quantities from different domains that are fundamental to the measurement of non-electrical quantities and parameters are also given. Finally, some general aspects of sensors, the devices that convert information from one domain to another, are discussed. 2.1. System of units A unit is a particular physical quantity, defined and adopted by convention, with which other particular quantities of the same kind are compared to express their value. The value of a physical quantity is the quantitative expression of a particular physical quantity as the product of a number and a unit, the number being its numerical value. For example, the circumference of the earth around the equator is given by: Ce = 40,074.103 m (2.1) where Ce is the physical quantity and the number 40,074. 103 is the numerical value of this quantity expressed in the unit meter. Obviously, the numerical value of a particular physical quantity depends on the unit in which it is expressed. Therefore, to any measurement result the unit in which it is expressed should be added explicitly. To avoid misinterpretations the use of the international system of units (Systrme International d'unitrs, SI) is highly recommended when presenting measurement results.
eBook - PDF- A. Boros(Author)
- 2012(Publication Date)
- North Holland(Publisher)
PART I BASIC CONCEPTS This page intentionally left blank 1. PHYSICAL QUANTITIES, UNITS, THE SI SYSTEM The measurable properties and characteristics of physical phenomena, physical processes and/or states are called physical quantities. Physical quantities can be defined as the product of their measures and units. Be-fore standardization, the units to be specified in the standard may be arbitrarily chosen (preferably in a way that is easiest to reproduce) while their number provides information about the magnitude of the examined (measured, read) quantity, i.e. what fraction it is of the chosen unit, or how many times greater. In building up systems of units for the most important or the most frequently used physical quantities, the units are arbitrarily chosen. These are called basic units, from which the so-called derived units are then formed. In all this, coherence is an important criterion. The reader will meet the concept of coherence later in this chapter. The multiples and fractions of the basic and derived units are formed by prefixes.* Considerable efforts are being devoted worldwide to the introduction of the inter-nationally standardised system of units into general practice. This system has become known as the SI from the French term Systeme International des Unites. Table 1.1 shows the basic units of the SI system, Table 1.2 its supplementary units. TABLE 1.1. Basic quantities and basic units of the SI system Basic quantity Basic unit, symbol Length metre, m Mass kilogram, kg Time second, s Electric current ampere, A Thermodynamic temperature kelvin, Κ Luminous intensity candela, cd Amount of substance mole, mol * The prefixes are multiplying factors, the power of 10 concerned. For the prefixes of SI see Table 1 .3. 5
- David Halliday, Robert Resnick, Jearl Walker(Authors)
- 2023(Publication Date)
- Wiley(Publisher)
4. The meter is defined as the distance traveled by light during a precisely spec- ified time interval. LEARNING OBJECTIVES What Is Physics? Science and engineering are based on measurements and comparisons. Thus, we need rules about how things are measured and compared, and we need experiments to establish the units for those measurements and comparisons. One purpose of physics (and engineering) is to design and conduct those experiments. For example, physicists strive to develop clocks of extreme accuracy so that any time or time interval can be precisely determined and compared. You may wonder whether such accuracy is actually needed or worth the effort. Here is one example of the worth: Without clocks of extreme accuracy, the Global Positioning System (GPS) that is now vital to worldwide navigation would be useless. Measuring Things We discover physics by learning how to measure the quantities involved in physics. Among these quantities are length, time, mass, temperature, pressure, and electric current. Measurement 1 2 CHAPTER 1 Measurement We measure each physical quantity in its own units, by comparison with a standard. The unit is a unique name we assign to measures of that quantity— for example, meter (m) for the quantity length. The standard corresponds to exactly 1.0 unit of the quantity. As you will see, the standard for length, which corresponds to exactly 1.0 m, is the distance traveled by light in a vacuum during a certain fraction of a second. We can define a unit and its standard in any way we care to. However, the important thing is to do so in such a way that scientists around the world will agree that our definitions are both sensible and practical. Once we have set up a standard—say, for length—we must work out proce- dures by which any length whatever, be it the radius of a hydrogen atom, the wheelbase of a skateboard, or the distance to a star, can be expressed in terms of the standard.
eBook - PDFEngineering Technology NQF2 SB
TVET FIRST
- Jowaheer Consulting and Technologies Business Programme Developments(Author)
- 2013(Publication Date)
- Macmillan(Publisher)
Physical quantity is a physical property of a phenomenon, body or substance that can be quantified by measurement. A more formal definition is given in the International Vocabulary of Metrology , 3rd edition (VIM3), which defines quantity as: the property of a phenomenon, body, or substance, where the property has a magnitude that can be expressed as a number and a reference. SI base units As you have learnt, there are different types of SI units of measurement. There are seven SI base units and many derived units. The base units are combined to form the derived units. Table 5.1 shows the seven SI base units. Physical quantity Physical quantity symbol SI base unit SI base unit symbol length l metre m mass m kilogram kg time t second s electric current I ampere A temperature T kelvin K luminous intensity I v candela cd amount of a substance n mole mol Table 5.1 The seven SI base units Figure 5.2 SI units are used worldwide Did you know? The SI units were adopted by 36 countries at the 11 th General Conference on Weights and Measures held in France in 1960. unit of measurement: a defined, physical quantity in terms of which other quantities of the same kind may be expressed as simple multiples of the unit of measurement physical quantity: the property of a phenomenon, body, or substance, where the property has a magnitude that can be expressed as a number and a reference Words & Terms 177 Module 5: SI units of measurement For a measurement to be of value, a unit of a physical quantity must have the same magnitude regardless of where you take the actual measurement. By convention, the units of measurement of the seven base quantities have been defined as explained below. Metre (m) The metre (m) is the base unit of length ( l ). The metre is defined as follows: A metre is the length of the path travelled by light in a vacuum during a time interval of 1 ⁄ 299 792 458 of a second.
eBook - PDF- Patrick N. McDermott, Colin G. Orton(Authors)
- 2018(Publication Date)
- Medical Physics Publishing(Publisher)
2-1 Review of Basic Physics 2.1 Units for Physical Quantities 2.2 Mechanics 2.3 Electricity and Magnetism 2.4 Electromagnetic Spectrum 2.5 The Special Theory of Relativity 2.6 Review of Atomic Structure 2.7 What is Radiation? Problems Bibliography This review of physics is cast in such a way as to make direct contact with the subject of radiation therapy. 2.1 Units for Physical Quantities The standard metric system in use worldwide for scientific purposes is the International System of Units (Système International d’Unités), or SI. Scien- tific journals are becoming increasingly strict that papers submitted for publi- cation contain only SI units. Table 2.1 lists the fundamental quantities of the SI. We will consider quantities related to radiation measurement separately in chapter 7. Any true SI unit is a combination of the fundamental units listed in Table 2.1. A combination of the fundamental units is called a derived unit. An example is the SI unit of force, the newton (as in Isaac, not fig): 1 newton = 1 N = 1 kg m/s 2 . Unit abbreviations are capitalized when they refer to a proper name, thus the abbreviation for newton is “N,” whereas the abbreviation for 2 THE PHYSICS AND TECHNOLOGY OF RADIATION THERAPY 2-2 meter is “m.” Examples of non-SI units are electron volts, ergs, rads, roent- gens, etc. Even though the electron volt is a non-SI unit, its use with the SI is considered acceptable. Do not mix different systems of units when doing cal- culations or you will get a nonsensical result. Always make sure that your units are consistent before combining them in a calculation. In this book the units of a quantity will be represented by square bracket notation, [ ]. As an example, consider the SI unit of force, F: [F] = N. Some standard SI prefix multipliers are listed in Table 2.2. Do not confuse milli (m) and mega (M). 2.2 Mechanics Mechanics is a very broad subject.
- Cohen, Douglas L.(Authors)
- 2001(Publication Date)
Fortunately, there do exist procedures using elementary mechanical units that are analogous to what goes on when we change the kinds of units used in electromagnetic theory; we can use this analogy to in-troduce the appropriate perspective for understanding electromagnetic units. This chapter begins by presenting material with which the reader is probably already familiar—what a unit is, what a dimension is, the rules for manipulating units in-side equations—and then moves on to describe the procedures applied to standard mechanical units in quantum mechanics and relativistic physics to simplify the forms of complicated equations. It should be emphasized that all these equations are presented as “given,” with no expectation that the reader will gain or have any particular knowledge of how the equation is derived; we just show how to simplify the equation by changing the units in which it is expressed. By the end of the chap-ter the reader will have acquired the rules and terminology needed to describe how and why the equations of classical electromagnetism change form when moving from one system of units to another. 1 2 C HAPTER 1 1.1 T HE BASIC IDEA OF A UNIT Measurements create numbers, and units give meaning to numbers by connect-ing them to measurements. For example, to call a length “8.5” is completely am-biguous; but to call it “8.5 centimeters,” or “8.5 feet,” or “8.5 miles,” does have meaning—and the meaning changes when the attached unit changes. In equations, a physical quantity, such as a length L , is treated as if it were the product of a numeric or pure number, like the 8.5 in “8.5 centimeters,” and a unit, like the cen-timeters in “8.5 centimeters.” Length L is specified in centimeters (cm) using the equation L = 8 .
eBook - PDF- Paul Peter Urone, Roger Hinrichs(Authors)
- 2012(Publication Date)
- Openstax(Publisher)
English units: fundamental units: kilogram: law: meter: method of adding percents: metric system: model: modern physics: order of magnitude: percent uncertainty: physical quantity : physics: precision: quantum mechanics: relativity: scientific method: second: SI units : significant figures: theory: uncertainty: units : system of measurement used in the United States; includes units of measurement such as feet, gallons, and pounds units that can only be expressed relative to the procedure used to measure them the SI unit for mass, abbreviated (kg) a description, using concise language or a mathematical formula, a generalized pattern in nature that is supported by scientific evidence and repeated experiments the SI unit for length, abbreviated (m) the percent uncertainty in a quantity calculated by multiplication or division is the sum of the percent uncertainties in the items used to make the calculation a system in which values can be calculated in factors of 10 representation of something that is often too difficult (or impossible) to display directly the study of relativity, quantum mechanics, or both refers to the size of a quantity as it relates to a power of 10 the ratio of the uncertainty of a measurement to the measured value, expressed as a percentage a characteristic or property of an object that can be measured or calculated from other measurements the science concerned with describing the interactions of energy, matter, space, and time; it is especially interested in what fundamental mechanisms underlie every phenomenon the degree to which repeated measurements agree with each other the study of objects smaller than can be seen with a microscope the study of objects moving at speeds greater than about 1% of the speed of light, or of objects being affected by a strong gravitational field a method that typically begins with an observation and question that
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