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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

what is fraction
Helen Reply
what is matter
Yahaya Reply
An electron excited to another level E2= -2.42 × 10^-19J falls to the ground level Eo = -21.8×10^-19J. Calculate the frequency
Teniola Reply
What is physics
Onaji Reply
Is the study of energy in relation to matter
physics is a sciecnce it consists two major parts those are theoretical and experimental (practical ) physics
if A= 2i + 3j + 2k and B= I + 4j +2k find A+B
ibrahim Reply
what is matter
hi how does a force affect a moving body with fast deceleration
Oni Reply
0 is lesser than tita lesser than 180
Olabamerun Reply
how can u find an acute angle of a projectile motion
Kamsi Reply
Explain law of thermodynamics
Jamila Reply
Young’s double slit experiment breaks a single light beam into two sources. Would the same pattern be obtained for two independent sources of light, such as the headlights of a distant car? Explain
Jairuz Reply
The earth has more rotational kinetic energy now than did the cloud of gas and dust from which is formed. Where did this energy come from
Anjali Reply
what is circular motion?
Godwin Reply
This is the movement of a body or object in a circular path. An example is the rotation of a fan
this is the circular movement of a body
this is the movement of an object or a body in a circular path or way
What is temperature
Banjo Reply
This is degree of the hotness or coldness of the body
what is physics
Tiamiyu Reply
physics is the study of matter,time and space.
what is Zeroth law of thermodynamics?
Esther Reply

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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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