كاتب الموضوع :
دلوعه بكيفي
المنتدى :
قهوة ليلاس
قوته نهي نهي
لا لا
نونو
يا حبيبتي وش الفايده اذا كتبتيها جنب بعض
استعملي الورقد الكبيره حقت الطابعه مو لازم ورق الدفاتر
بعدين اذا ما كفت
اكتبي
ذا الكلام
1.1 Standards of Length, Mass, and Time
The laws of physics are expressed as mathematical relationships among physical quantities
that we will introduce and discuss throughout the book. Most of these quantities
are derived quantities, in that they can be expressed as combinations of a small number
of basic quantities. In mechanics, the three basic quantities are length, mass, and time.
All other quantities in mechanics can be expressed in terms of these three.
If we are to report the results of a measurement to someone who wishes to reproduce
this measurement, a standard must be defined. It would be meaningless if a visitor
from another planet were to talk to us about a length of 8 “glitches” if we do not know
the meaning of the unit glitch. On the other hand, if someone familiar with our system
of measurement reports that a wall is 2 meters high and our unit of length is defined
to be 1 meter, we know that the ****** of the wall is twice our basic length unit. Likewise,
if we are told that a person has a mass of 75 kilograms and our unit of mass is defined
to be 1 kilogram, then that person is 75 times as massive as our basic unit.1 Whatever
is chosen as a standard must be readily accessible and possess some property that
can be measured reliably. Measurements taken by different people in different places
must yield the same result.
In 1960, an international committee established a set of standards for the fundamental
quantities of science. It is called the SI (Système International), and its units of length,
mass, and time are the meter, kilogram, and second, respectively. Other SI standards established
by the committee are those for temperature (the kelvin), electric current (the
ampere), luminous intensity (the candela), and the amount of substance (the mole).
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