measurements and unit (part 1)

 😜read

Measurement And Units

Physics is based on measurement of physical quantities.Certain physical quantities have been chosen as base quantities (such as length, time, and mass); each has been defined in terms of a standard and given a unit of measure (such as meter,second, and kilogram). Other physical quantities are defined in terms of the base quantities and their standards and units.

 

Measuring Things

We discover physics by learning how to measure the quantities involved in physics. Among these quantities are length, time, mass, temperature, pressure,and electric current.

We measure each physical quantity in its own units, by comparison with a standard. The unit is a unique name we assign to measures of that quantity—for example, meter (m) for the quantity length. The standard corresponds to exactly 1.0 unit of the quantity. As you will see, the standard for length, which corresponds.

Classification of physical quantities(derived and fundamental or base quantities)

Base Quantities. There are so many physical quantities that it is a problem to organize them. Fortunately, they are not all independent; for example, speed is the ratio of a length to a time. Thus, what we do is pick out—by international agreement—a small number of physical quantities, such as length and time, and assign standards to them alone. We then define all other physical quantities in terms of these base quantities and their standards (called base standards). Speed, for example,is defined in terms of the base quantities length and time and their base standards.

Base standards must be both accessible and invariable. If we define the length standard as the distance between one’s nose and the index finger on an outstretched arm, we certainly have an accessible standard—but it will, of course,vary from person to person. The demand for precision in science and engineering pushes us to aim first for invariability. We then exert great effort to make duplicates of the base standards that are accessible to those who need them.

 

Fundamental(base quantities)

Fundamental quantities have basic quantities that are not dependent upon others and cannot be defined in terms of other quantities.Their units are called fundamental units.

fundamental quantities and their units

Quantities    Units Name       Units Symbol

Mass             kilogram                   kg

Time              Second                      s

Length            Meter                      m

The International System of Units

In 1971, the 14th General Conference on Weights and Measures picked seven

quantities as base quantities, thereby forming the basis of the International

System of Units, abbreviated SI from its French name and popularly known as

the metric system.These units were defined to be on a “human scale.”

Many SI derived units are defined in terms of these base units. For example,

the SI unit for power, called the watt (W), is defined in terms of the base units

for mass, length, and time. Thus, as you will see in Chapter 7,

1 watt = 1 W = 1 kg *m^2/s^3.

where the last collection of unit symbols is read as kilogram-meter squared per

second cubed.

Now see the new table for base quantities.

Quantities      Units Name Units            Symbol

Mass                     kilogram                            kg

Time                      Second                               s

Length                    Meter                               m

Current                  Ampere                              a

Temperature         kelvin                                  k

light intensity       candela                              cd

 

Prefixes for SI Units

Factor                                 Prefix                                      Symbol

deka                                     10^1                                         da

hecto                                    10^2                                          h

kilo                                       10^3                                          k

mega                                    10^6                                          M

giga                                      10^9                                          G

centi                                     10^-2                                         c

milli                                     10^-3                                         m

micro                                   10^-6                                        meu

nano                                    10^-9                                          n

pico                                     10^-12                                        p

To express the very large and very small quantities we often run into in

physics, we use scientific notation, which employs powers of 10. In this notation,

and

3560000000 m = 3.56 * 10 ^9 m

0.000000492 s =4.92 * 10 ^-7 s.

Scientific notation on computers sometimes takes on an even briefer look,where E stands for “exponent of ten.” It is briefer still on

some calculators, where E is replaced with an empty space.

As a further convenience when dealing with very large or very small measurements, As you can see, each prefix represents a certain power of 10, to be used as a multiplication factor. Attaching a prefix to an SI unit has the effect of multiplying by the associated factor. Thus,we can express a particular electric power as: 

           1.27 *10^9 watts =1.27 gigawatts = 1.27 G