# Physics, Measurements and System of Units

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Physics, Measurements and System of Units

What is Physics? Physics is a fundamental science that seeks to explain our physical environment (the universe). Physicists aim to formulate a few laws (statements) that apply to the whole universe and explain all observable phenomena. Physics is both experimental and theoretical. All theories are tested by experiments or observations.

What else is Physics? Physical theories seek to explain observations and predict new results. If two theories can explain observations, physicists will accept the simpler theory. Physicists use both inductive and deductive methods.

Physics is an experimental science
It is based on observations with our senses or instruments. Physicists measure quantities (quantitative) as precisely as possible for comparison with theory and other experiments. Requires universal, accessible standards of measurement for reproducibility, comparability and accuracy of results.

Standards: Definitions of Fundamental or Base Quantities
Fundamental quantities are agreed upon by convention. The minimum number of base quantities and their corresponding units are defined for simplicity and maintainability. Choice and definitions of standards and their corresponding units are based on accessibility, accuracy and universality. Example: time (second)

Standards: Definitions of Fundamental or Base Quantities
Derived quantities are combinations of fundamental quantities. (Composite units) Units of derived quantities are usually chosen to honor a prestigious person or scientist. Example: Force (Newton, N) The Newton is a combination of kilograms, meters, and seconds

System of Units A collection of fundamental quantities and their associated units is called a system of units. The SI (Système International) is now used throughout the world except for the USA. Other system of units are the CGS, Imperial, and customary American systems.

The SI System (metric system)
the 7 fundamental quantities Quantity Unit Symbol length meter m mass kilogram kg time second s temperature degrees Celsius C electric current ampere A luminous intensity candela cd amount of substance mole mol Two Examples of Derived Quantities Newton: 1 N = 1 kg•m/s2 Pascal: 1 Pa = 1 N/m2

Two ways to deal with very small or very large quantities
Use scientific notation electron rest mass = 9.11 x kg speed of light in vacuum = x 108 m/s Use SI prefixes a very short laser pulse width = 100 fs radius of earth = 6.38 Gm The above methods also have the advantage of indicating clearly the number of significant figures.

SI Prefixes Prefix Abbreviation Value Link to more exa E 1018
peta P tera T giga G 109 mega M 106 kilo k 103 hecto h 102 deka da 101 deci d centi c milli m micro m nano n pico p femto f atto a Link to more

Dimension The term “dimension” has two meanings in physics.
It is used to describe the nature of space (3D space with length, width, height). It is also used to mean the “quality” or “type” of the physical quantity. (length, time, mass, force, etc.) We use the second meaning when we speak of dimensional consistency of physical equations.

Physics Equations Have Consistent Dimensions or Units
Left and right side terms of equations must agree in terms of dimension (or units after conversion to common units) Algebraic operations also apply to units. Quantities to be added or subtracted must have the same units. Unit consistency or conversion of units follow the rules of algebra.

Two Examples of Equations with Dimensional Consistency

Significant Figures express the uncertainty of measured quantities
Ex. (3 significant figures) 1.04 cm without an explicit plus/minus is usually interpreted as a length somewhere between and cm. least significant digit is the least reliable digit known only to a certain plus/minus value Ex ± 0.03 kg (4 significant figures)

Significant Figures cont...
the number of significant figures of the result of algebraic operations (except for addition and subtraction) is determined by the least reliable input value. the decimal place of the result of addition/subtraction is the same as the least number of decimal places of the addends or subtrahends.

Significant Figures cont...
the decimal place of the result of addition/subtraction is the same as the least number of decimal places of the addends or subtrahends. Number of significant figures of numbers added or subtracted are not used for rounding the result. 4 sig. figs. hundredths dec. place 3 sig. figs. hundredths dec. place 4 sig. figs. thousandths dec. place 2 sig. figs. hundredths dec. place

Significant Figures cont ...
In this course we will not strictly enforce the rounding rules for addition. Usually, physics problems usually involve a series of calculations requiring additions, subtractions, multiplications, divisions, raising to a power, exponentiation, trigonometric functions, etc. Please keep the full number of decimal places of intermediate steps in your calculator and round only the answers to specific questions. There should not be any ‘calculator answers’