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Beam of light from a flashlight is represented by an arrow pointing right, traveling the length of a meter stick.
The meter is defined to be the distance light travels in 1/299,792,458 of a second in a vacuum. Distance traveled is speed multiplied by time.

Electric current and its accompanying unit, the ampere, will be introduced in Introduction to Electric Current, Resistance, and Ohm's Law when electricity and magnetism are covered. The initial modules in this textbook are concerned with mechanics, fluids, heat, and waves. In these subjects all pertinent physical quantities can be expressed in terms of the fundamental units of length, mass, and time.

Metric prefixes

SI units are part of the metric system    . The metric system is convenient for scientific and engineering calculations because the units are categorized by factors of 10. [link] gives metric prefixes and symbols used to denote various factors of 10.

Metric systems have the advantage that conversions of units involve only powers of 10. There are 100 centimeters in a meter, 1000 meters in a kilometer, and so on. In nonmetric systems, such as the system of U.S. customary units, the relationships are not as simple—there are 12 inches in a foot, 5280 feet in a mile, and so on. Another advantage of the metric system is that the same unit can be used over extremely large ranges of values simply by using an appropriate metric prefix. For example, distances in meters are suitable in construction, while distances in kilometers are appropriate for air travel, and the tiny measure of nanometers are convenient in optical design. With the metric system there is no need to invent new units for particular applications.

The term order of magnitude    refers to the scale of a value expressed in the metric system. Each power of 10 size 12{"10"} in the metric system represents a different order of magnitude. For example, 10 1 , 10 2 , 10 3 size 12{"10" rSup { size 8{1} } ,`"10" rSup { size 8{2} } ,`"10" rSup { size 8{3} } } {} , and so forth are all different orders of magnitude. All quantities that can be expressed as a product of a specific power of 10 size 12{"10"} are said to be of the same order of magnitude. For example, the number 800 size 12{"800"} can be written as 8 × 10 2 , and the number 450 can be written as 4. 5 × 10 2 . Thus, the numbers 800 and 450 are of the same order of magnitude: 10 2 . Order of magnitude can be thought of as a ballpark estimate for the scale of a value. The diameter of an atom is on the order of 10 9  m, while the diameter of the Sun is on the order of 10 9  m.

The quest for microscopic standards for basic units

The fundamental units described in this chapter are those that produce the greatest accuracy and precision in measurement. There is a sense among physicists that, because there is an underlying microscopic substructure to matter, it would be most satisfying to base our standards of measurement on microscopic objects and fundamental physical phenomena such as the speed of light. A microscopic standard has been accomplished for the standard of time, which is based on the oscillations of the cesium atom.

The standard for length was once based on the wavelength of light (a small-scale length) emitted by a certain type of atom, but it has been supplanted by the more precise measurement of the speed of light. If it becomes possible to measure the mass of atoms or a particular arrangement of atoms such as a silicon sphere to greater precision than the kilogram standard, it may become possible to base mass measurements on the small scale. There are also possibilities that electrical phenomena on the small scale may someday allow us to base a unit of charge on the charge of electrons and protons, but at present current and charge are related to large-scale currents and forces between wires.

Questions & Answers

wave is a disturbance that travels through a medium and transfer energy from one place to another without causing any permanent displacement of the medium itself
Bello Reply
what is wave
atiba Reply
wave is a disturbance that travels through a medium and transfer energy from one place to another without causing any permanent displacement of the medium itself
Bello
measurements of the period of a simple pendulum in an experiment are 20 oscillations for a give length of the pendulum 30.50secs
Praise Reply
patient with a pacemaker is mistakenly being scanned for an MRI image. A 10.0-cm-long section of pacemaker wire moves at a speed of 10.0 cm/s perpendicular to the MRI unit’s magnetic field and a 20.0-mV Hall voltage is induced. What is the magnetic field strength
Muhammad Reply
fundamental and derived quantities
Kadijatu Reply
is any thing that has weight or mass and can occupy space
Kadijatu
explain why the earth is an exact shere
Akinyemi Reply
because the pull of gravity is equal on all sides
Paul
how long did sun take to rich the earth
Billy
A tennis ball is projected vertically up with a velocity of 100m/s the top of a tower 50m high. Determine the a). height reach from the ground. b). Time to reach the hmax.
Luca Reply
100×50=5000
Muhammad
what are the formula and calculation
Brianca Reply
what are the formula for resistance, ohm's law, and electric current
Brianca
I=V/R and so R=V/I
Akiiza
what is gamma rays
Uche Reply
gamma ray is an energetic ray .it does not have acharge
Merkew
what is wave
Faruq Reply
What happens to the mass of water in a pot when it cools, assuming no molecules escape or are added? Is this observable in practice? Explain.
Angel Reply
please explain to me,coz I have no idea
Brianca
what is a magnetic field
Jedaiah Reply
magnetic field is the area around a magnet where the magnetic force is exerted or felt.
Paul
what is the first Faraday's law
Abba
Formula for Newton 1st law
Maryam Reply
what is physics
Isa Reply
Physics is a way to see and solve things around us
Prajukta
what is physics
Naomi Reply
physics is the measurement of mass and weight
Jamila
physics is a situation where by measurement include length, time and mass
Muhammad

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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