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It is an intriguing fact that some physical quantities are more fundamental than others and that the most fundamental physical quantities can be defined only in terms of the procedure used to measure them. The units in which they are measured are thus called fundamental units    . In this textbook, the fundamental physical quantities are taken to be length, mass, time, and electric current. (Note that electric current will not be introduced until much later in this text.) All other physical quantities, such as force and electric charge, can be expressed as algebraic combinations of length, mass, time, and current (for example, speed is length divided by time); these units are called derived units    .

Units of time, length, and mass: the second, meter, and kilogram

The second

The SI unit for time, the second    (abbreviated s), has a long history. For many years it was defined as 1/86,400 of a mean solar day. More recently, a new standard was adopted to gain greater accuracy and to define the second in terms of a non-varying, or constant, physical phenomenon (because the solar day is getting longer due to very gradual slowing of the Earth’s rotation). Cesium atoms can be made to vibrate in a very steady way, and these vibrations can be readily observed and counted. In 1967 the second was redefined as the time required for 9,192,631,770 of these vibrations. (See [link] .) Accuracy in the fundamental units is essential, because all measurements are ultimately expressed in terms of fundamental units and can be no more accurate than are the fundamental units themselves.

A top view of an atomic fountain is shown. It measures time using the vibration of the cesium atom.
An atomic clock such as this one uses the vibrations of cesium atoms to keep time to a precision of better than a microsecond per year. The fundamental unit of time, the second, is based on such clocks. This image is looking down from the top of an atomic fountain nearly 30 feet tall! (credit: Steve Jurvetson/Flickr)

The meter

The SI unit for length is the meter    (abbreviated m); its definition has also changed over time to become more accurate and precise. The meter was first defined in 1791 as 1/10,000,000 of the distance from the equator to the North Pole. This measurement was improved in 1889 by redefining the meter to be the distance between two engraved lines on a platinum-iridium bar now kept near Paris. By 1960, it had become possible to define the meter even more accurately in terms of the wavelength of light, so it was again redefined as 1,650,763.73 wavelengths of orange light emitted by krypton atoms. In 1983, the meter was given its present definition (partly for greater accuracy) as the distance light travels in a vacuum in 1/299,792,458 of a second. (See [link] .) This change defines the speed of light to be exactly 299,792,458 meters per second. The length of the meter will change if the speed of light is someday measured with greater accuracy.

The kilogram

The SI unit for mass is the kilogram    (abbreviated kg); it is defined to be the mass of a platinum-iridium cylinder kept with the old meter standard at the International Bureau of Weights and Measures near Paris. Exact replicas of the standard kilogram are also kept at the United States’ National Institute of Standards and Technology, or NIST, located in Gaithersburg, Maryland outside of Washington D.C., and at other locations around the world. The determination of all other masses can be ultimately traced to a comparison with the standard mass.

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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