This article stills in the draft and will be finished soon!
all data will be added
 |
| Tools to measure the length. |
I- the aim of physics:
 |
| Experiment. |
II- theories and reality:
when there is a mismatch between prediction of a theory and the experimental results, new theories are developed to avoid this mismatch. also many theories are applied under some conditions. for example Newton's laws are applied only at normal speed that are much relatively lower than speed of light, so when something goes fast at a speed close to speed of light the experimental results differ from the predictions of Newtonian mechanics!, so another theory is developed by Albert Einstein called relativity which describes the experimental results fairly well.
III- Standards of Length, Mass, and Time:
To describe natural phenomena and understand them, we must make measurements of factors affecting them. The laws of physics define the mathematical relationship among physical quantities. for example we measure length in meters and time in seconds to describe the speed of a moving car in terms of meter/second which is the unit of speed. In classical physics we have three fundamental quantities used to express any other quantity: Length, mass, and time.
It's very important to express physical quantities in terms of standard units so people can understand each other unless our numbers will be meaningless.
In 1960, a standard system of measurements was established, it is known as SI (Système International), the standard units of length, mass, and time are meter, kilogram, and second, respectively. There are more SI fundamental units such as kelvin for temperature, ampere for electric current, candela for luminous intensity, and mole for the amount of substance.
Length:
 |
| How length is measured |
Table 1.1 P41
Mass:
 |
| Mass is measured by comparing. |
Table 1.2 p41
Time:
 |
| Time is measured in seconds |
Below table presents values of time intervals. Table1.3 p41
IV- U.S customary system:
There is another system of measurement called U.S customary system which is used in the United States, but the SI system is more universal and more accepted in science. Units of length, mass, and time are foot (ft), slug, and second. We may use this system but not often.
You can use other units to express the fundamental units such as millimeter, nanosecond, and so on, notice that these units are not different, it's just the prefixes (mille-, nano- and so on) which are added to the unit. These prefixes denote multipliers of the basic units based on different powers of ten. For example 1 millimeter = 10-3 meter.
V- Prefixes of units:
<<<>>>>>
There are many other quantities called derived quantities that can be expressed as a combination of fundamental quantities such as speed which is expressed as m/s, i.e. meter per second, and density which is expressed as kg/m3.