Answer the following in detail.
Question 1. What are fundamental physical quantities? Name any three fundamental physical quantities.
Answer: Basic physical quantities that do not depend upon other quantities are called fundamental physical quantities. There are seven fundamental quantities – length, mass, temperature, time, electric current, luminous intensity and amount of substance.
Question 2. Explain by giving two examples why the measurement of a physical quantity is expressed as a combination of a numeral and a unit.
Answer: To measure a physical quantity, we need to compare it with a known fixed physical quantity of the same kind, i.e., a unit. Hence, the measurement of a physical quantity is always written as a combination of a numeral along with the unit. The numeral specifies the number of times the unit is repeated. Example:
- Using a centimetre scale, the length of pencil box is found to be 20 centimetres
(cm). 20 cm simply means that the length is 20 times a centimetre. (The centimetre forms the unit of length in a centimetre scale.) Here, the number 20 is the numeral (magnitude) and cm is the unit.
- Using a weighing (kilogram) scale, the weight of the box is found to be 2
Kilograms (kg) 2 kg simply mean that the mass of the box is 2 times a kilogram. (The kilogram forms the unit of mass in a kilogram scale). Here, the number 2 is the numeral (magnitude) and kg is the unit.
Question 3. Explain in detail why there was a need to standardize units.
Answer: The traditional units were not uniform as the length of a cubit, foot and hand span varied from person to person according to their body size. Similarly, there was no certainty that all grains were exactly the same weight. So these units could not be used for scientific measurements where accuracy was a prime concern. The development of a large number of systems of measurement also made it very difficult to conduct trade and commerce between different societies. Therefore, people felt the need to have standard units which could be used for accurate measurement and were accepted universally.
Question 4. Why are multiples and submultiples of SI units required?
Answer: Sometimes, the size of the SI unit is either too small or too big to measure a certain quantity. For example, a meter is too small a unit to measure the distance between two cities and too big a unit to measure the thickness of a wire. Hence, multiples and submultiples of units are required. Multiples are factors used to create larger forms whereas submultiples are factors used to create smaller forms of the SI units. For example, a centimeter is a submultiple and kilometer is a multiple of a meter.