**Introduction**

The topic ‘units of measurement’ deals with the quantities used as a standard of measurements. It explains the comparison and measurement of certain physical quantities. We need some fixed quantities as standard units for the comparison and measurement. For example, an elephant’s weight is more than a horse’s, but how do we ascertain this? Rahul is taller than Rony, but how tall is he? To find answers to such questions and make comparisons and measurements, we need standard units for different quantities. For example, length, weight, time, capacity, etc., have their standard units for measurement.

**Understanding Units of Measurement**

To understand units of measurement**, **we first need to understand the meaning of units. To measure any quantity or compare any two quantities, we need an internationally accepted/acquired unit standard. The measurement of any physical quantity is expressed in terms of a number and a specific unit. Measurement = quantity x unit

**Examples**

If Aakriti studies for 4 hours, then “4” is the quantity, and “hour” is the unit of time.

If Shubham weighs 79 kilograms or kg, then “79” is the quantity and “kg” is the unit of weight.

If Isha is 16 cm taller than Akash, “16” represents the quantity, and “cm” is a unit of length.

**How are Units Decided?**

Two important points have to be kept in mind while choosing a unit.

- The units should be internationally acquired.
- The body named ‘General Conference on Weights and Measures’ has the right to decide and mention units. The units mentioned or passed by the body are internationally accepted.

**Why Do We Need Standard Units and Measurements?**

Imagine a condition where a boy wants to purchase a blue-colored t-shirt, so he goes to a shop. The shopkeeper speaks only in English. The boy says, “pólót akarok” and the shopkeeper is not able to comprehend what it is that the boy wants. This is because the boy asked for a blue-colored t-shirt, but he said it in Hungarian. The shop owner could not understand anything as Hungarian is not a standard language.

Similarly, many systems and units are in place for calculating various quantities like area, mass, length, volume, etc. For example, an acre is a traditional way of measuring size in India. One acre is equivalent to 4046 square meters according to the metric system but this form of measurement is not universal. Now you can assume how difficult it will be to measure quantities without universally accepted standards and units.

**An international System of Units **

An internationally acquired/accepted coherent system of physical units is a scientific method of expressing the magnitudes or quantities of important natural phenomena. This international system of units was formerly called the *meter-kilogram-second (MKS)* system.

**SI Unit Prefixes **

SI unit prefixes are a specifier or mnemonic that is prepended to units of measurement to indicate multiples or fractions of the units. For example, the prefixes such as *kilo *and *milli *represent multiplication by the power of ten. There are 20 SI unit prefixes that are used to form decimal multiples and submultiples of SI units.

**Fundamental and Derived Quantities **

There are a large number of physical quantities that need to be measured and given a specific unit for it to be unique. Remember that these quantities are interdependent. For example, the unit of area can be defined (metre × metre) if that length unit is pre-defined.

Example 1

If we make a square with a side of 2 units, we can get a standard area unit and compare other areas with our standard unit.

Example 2

If a body of any mass travels unit length in unit time, then we can define its unit velocity.

**Fundamental Quantities**

The fundamental quantities do not depend on other physical quantities of measurement. We also refer to these quantities as base quantities**. **There are seven fundamental quantities, and the rest of the physical quantities are known as derived quantities. The following are the seven fundamental quantities:

**Kilogram**

Kilogram is the unit mass. For example, when the mass of a cylinder made up of platinum-iridium alloy is measured in the International Bureau of Weights and Measures, the result is defined as one kg.

**Second**

Second is the unit of time. Cesium-133 atoms release electromagnetic radiation of several wavelengths. Particular radiation is selected corresponding to the transition involved between two hyperfine levels of the ground state of Cs-133. Each radiation has a time period.

**Kelvin**

Kelvin is the unit of temperature. 1 kelvin is defined as 1/273.16th part of thermodynamic temperature of the triple point of water.

**Ampere**

Ampere is the standard unit for measuring current. 1 ampere is defined as the current flowing in the same direction placed parallel to each other and attracting each other with a force of 2 × 10-7 newton/metre, where newton is the unit of force.

**Candela**

1 cd is the standard unit for luminous intensity. In the International System of Units, candela is the base unit of luminous intensity.

**Metre**

Metre is the unit of length.

**Mole**

The amount of substances is measured in moles.

**Derived quantity**

Derived quantities are those which depend on the other quantities for their measurements. Derived units are those which determine derived quantities.

These quantities are calculated from two or more measurements, like Area, Volume, and Density. For example, the area of a rectangle is calculated by multiplying its length with width. Similarly, the volume of a rectangle is calculated by multiplying its length, width, and height.

**Conclusion**

In this topic, units of measurement, we have learned that we need fixed standard units to do measurements and comparisons. These standard units are accepted worldwide. The body named ‘General Conference on Weights and Measures’ has the right to decide and mention units. There are two types of quantities: fundamental and derived quantity. There are seven fundamental quantities: *kilogram, second, kelvin, ampere, candela, metre, and mole. *Derived quantities are those which depend on the other quantities for their measurements. To study units of measurement, we need to understand the topic of SI unit prefixes. There are 20 SI unit prefixes that are used to form decimal multiples and submultiples of SI units.