Phys205A_Lecture2

Physics for Scientists & Engineers with Modern Physics (4th Edition)

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Lecture 2 Measuremetns, Standards & Units Measurements are the major tool to get new knowledge and test the current (or newly proposed) theory Measurements deal with physical quantities and thus require unified and well defined standards for this quantities. z Characteristics of standards for measurements z Readily accessible z Possess some property that can be measured reliably z Must yield the same results when used by anyone anywhere z Cannot change with time
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Why it is important?
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Standards of Fundamental Quantities z Standardized systems z Agreed upon by some authority, usually a governmental body z SI – Systéme International z Agreed to in 1960 by an international committee z Main system used in this text Gaithersburg, MD Boulder, CO
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Fundamental Quantities and Their Units Quantity SI Unit Length meter Mass kilogram Time second Temperature Kelvin Electric Current Ampere Luminous Intensity Candela Amount of Substance mole
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Quantities Used in Mechanics z In mechanics, three basic quantities are used z Length z Mass z Time z Will also use derived quantities z These are other quantities that can be expressed in terms of the basic quantities z Example: Area is the product of two lengths ± Area is a derived quantity ± Length is the fundamental quantity
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Length z Length is the distance between two points in space z Units z SI – meter, m z Most recent definition of a meter – the distance traveled by light in a vacuum during a time of 1/299 792 458 second. NIST
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Table 1-1, p. 4
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Time z Units z seconds, s z Defined in terms of the oscillation of radiation from a cesium atom ( atomic clock )
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Table 1-3, p. 5
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Mass z Units z SI – kilogram, kg z Defined in terms of a kilogram, based on a specific cylinder kept at the International Bureau of Standards (did not change since 1887)
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Table 1-2, p. 5
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Prefixes z Prefixes correspond to powers of 10 z Each prefix has a specific name z Each prefix has a specific abbreviation
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z The prefixes can be used with any basic units z They are multipliers of the basic unit z Examples: z 1 mm = 10 -3 m z 1 mg = 10 -3 g
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US Customary System z Still used in the US, but text will use SI Quantity Unit Length 1 foot Mass 1 slug Time 1 second Conversion 0.38 meter 14.56 kg 1 second Do not mix the units from different systems while solving problems!!!
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General tools for solving problems 1. Build the model of the process 2. Formulate the task in mathematical relations between
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This document was uploaded on 09/27/2011.

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Phys205A_Lecture2 - Lecture 2 Measuremetns, Standards &...

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