**Measurement** is the basis of all important scientific study. It plays an important role in our daily life also. When finding your height, buying milk for your family, timing the race completed by your friend and so on, you need to be able to make measurements.

**The units** used to measure the fundamental quantities are called fundamental units and the units which are used to measure derived quantities are called derived units.

**Fortnight:** A fortnight is two weeks or 14 days.

**Moment:** If you ask someone to wait for a moment, you know it is a short period of time. But, how short? It is 1/40 th of an hour or 1.5 minutes.

**Atomus:** The smallest amount of time imaginable to us is a twinkling of the eye. This is called atomus. Do you know the value of this? It is 1/6.25 seconds or 160 milliseconds.

**Donkey Power: **You might have heard about horse power. But do you know donkey power? It is one third of a horse power. Its value is around 250 watt.

**Length **is defined as the distance between two points. The SI unit of length is metre. One metre is the distance travelled by light through vacuum in 1/29,97,92,458 second.

The nearest star alpha centauri is about 1.34 parsec from the sun. Most of the stars visible to the unaided eye in the night sky are within 500 parsec distance from the sun.

Light travels 3 × 10^8 m in one second or 3 lakhs kilometre in one second. In one year we have 365 days.

The total length of all the blood vessels in human body is 96,000 km. When born, a baby giraffe is 1.8 m (6Ft) tall. A chameleons tongue is twice the length of its body.

**1 AU** is equal to 14,95,97,871 km or approximately equal to 150 million km or 1,500 lakhs km.

**1 parsec** = 3.26 light year.

**1 TMC** is (thousand million cubic feet) hundred crore cubic feet. 1 TMC = 2.83 × 10^10 litre.

1 TMC is approximately 3000 crore litres.

**Pierre Vernier** (1580 – 1637) was a French government official. Vernier was taught mathematics and science by his father who was a lawyer and engineer.

**1. Metre [m] **The metre is the basic unit of length. It is the distance light travels, in a vacuum, in 1/299792458th of a second.

**2. Kilogram [kg] **The kilogram is the basic unit of mass. It is the mass of an international prototype in the form of a platinum-iridium cylinder kept at Sevres in France. It is now the only basic unit still defined in terms of a material object, and also the only one with a prefix [kilo] already in place.

**3. Second [s]** The second is the basic unit of time. It is the length of time taken for 9192631770 periods of vibration of the Caesium-133 atom to occur.

**4. Ampere [A]** The ampere is the basic unit of electric current. It is that current which produces a specified force between two parallel wires which are 1 metre apart in a vacuum.

**5. Kelvin [K] **The kelvin is the basic unit of temperature. It is 1/273.16th of the thermodynamic temperature of the triple point of water.

**6. Mole [mol]** The mole is the basic unit of a substance. It is the amount of the substance that contains as many elementary units as there are atoms in 0.012 kg of carbon-12.

**7. Candela [cd**] The candela is the basic unit of luminous intensity. It is the intensity of a source of light of a specified frequency, which gives a specified amount of power in a given direction.

**8. Farad [F]** The farad is the SI unit of the capacitance of an electrical system, that is, its capacity to store electricity. It is rather a large unit as defined and is more often used as a microfarad.

**9. Joule [J]** The joule is the SI unit of work or energy. One joule is the amount of work done when an applied force of 1 newton moves through a distance of 1 metre in the direction of the force.

**10. Newton [N] **The newton is the SI unit of force. One newton is the force required to give a mass of 1 kilogram an acceleration of 1 metre per second.

**11. Ohm** The ohm is the SI unit of resistance of an electrical conductor. Its symbol is the capital Greek letter ‘omega’.

**12. Pascal [Pa]** The pascal is the SI unit of pressure. One pascal is the pressure generated by a force of 1 newton acting on an area of 1 square metre. It is rather a small unit as defined and is more often used as a kilopascal [kPa].

**13. Volt [V]** The volt is the SI unit of electric potential. One volt is the difference of potential between two points of an electical conductor when a current of 1 ampere flowing between those points dissipates a power of 1 watt.

**14. Watt [W]** The watt is used to measure power or the rate of doing work. One watt is a power of 1 joule per second. Electrical power V × I = W.