## Who are you calling a unit?

To express a physical quantity, a unit of the same type is needed. For the measurement of each quantity, a standard value of the same quantity is chosen. This standard is called a unit. To reveal the measure of a quantity, two things must be mentioned:-

• Unit of amount: The physical quantity in which it is measured.
• Numerical Value: In which the magnitude of the quantity is expressed. This makes it possible to tell how many times its unit has been used in that amount. For example, if the length of the wire is '3 metres', it means that the unit for measuring length is 'meter' and the length of the wire is three times the chosen unit 'meter'.

#### Unit Type:

There are two types of units. (i) Basic Unit (ii) Derived Unit:

• Base unit: Basic units are those units which are independent of other units, that is, they cannot be interchanged or interchanged. For example, meter, second and kilogram are used for length, time and mass.
• Derived Units: The units obtained by putting appropriate powers on one or more basic units are called derived units.

#### International Value System or SI Method of Measurement:

Many quantities have to be measured in physics and if separate units are considered for each physical quantity, then the number of units will become so much that it will be impossible to remember them. That is why a method has been adopted to express all physical quantities, which is called the International System of Basic Units or it is called the SI system. According to this method, all the quantities coming in mechanics can be expressed in the units of length, mass, and time. The quantities used in heat, electricity and magnetism and optics are expressed in the parameters of temperature, electric current and light intensity. In 1971, by the International Committee on Measures and Weights, considering the quantity of a substance as the basic quantity, the mole has been determined as its basic unit.

##### Base and Derivative Units

For most units, a unit is necessary to communicate the values of that physical quantity. Imagine you need to buy some rope to tie something to the roof of a car. How do you tell the measurer how much rope you need without using some unit of measurement? However, not all quantities require a unit of their own. Using physical laws, units of quantities can be expressed as a combination of units of other quantities. Therefore, only a small set of units is required. These units are called base units, and other units are derived units. Derived units are a matter of convenience, as they can be expressed as basic units. Different systems of units are based on different choices of base units. The most commonly used system of units is the International System of Units or SI. There are seven SI base units, and all other SI units can be derived from these base units.

##### There are seven base SI units:

1. Length: m (meters)
2. Mass: kg (kg)
3. Time: s (seconds)
4. Electric current: A (ampere)
5. Thermodynamic Temperature: K (degree Kelvin)
6. Amount of Substance: mol (mol)
7. Luminous Intensity: cd (candela)

###### Values of units from one system to another:

 one mile 1.6 km one liter 1000 cubic centiliter one acre 104 square meters one angstrum 10 -10 m one nautical mile 1. 85 km one inch 2.54 cm one chain 20. 11 m one foot 30 cm one fathom 1.8 m one yard 91 cm
 one ounce 28. 35 kilograms one pound 4. 536 grams one yard 3 feet 370 Centigrade 98. 60 Fahrenheit Units of Mass: Unit Mass 1 teragram 109 kg 1 gigagram 106 kg 1 megagram 103 kg 1 ton 103 kg
 1 quintal 102 kg 1 picogram 10-15 kg 1 mg 10-6 kg 1 decigram 10-4 kg 1 slug 10.57 kg 1 metric ton 1000 kg 1 oz 28.35 g 1 pound 16 oz 453.52 g 1 kg 2.205 lbs 1 carat 205.3 mg
 1 megagram 1 ton 1 gram 10-3 kg Units of Time: Unit Time 1 picosecond 10-12 seconds 1 nanosecond 10-9 second 1 microsecond 10-6 seconds 1 microsecond 1-3 seconds Major units of length: Unit Length (in metres)
 1 Terameter (T) 1012 1 gigameter (G) 109 1 Megameter (M) 109 1 diameter 106 1 kilometer (K) 103 1 hectometer 102 1 decameter 10 1 decimeter (d) 1-Oct 1 centimeter (c) 2-Oct 1 millimeter (m) 3-Oct
 1 µM µ 6-Oct 1 ml µM mμ 9-Oct 1 Angtram (Å) 10-Oct 1 picometer (p) 12-Oct 1 X-unit 13-Oct 1 Fermimeter (f) 15-Oct 1 Automator 18-Oct

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