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NCERT Solutions for Class 9th Science Chapter 1 The shared language of science: symbols and units — Threads of Curiosity

Book page 3 Updated on2026-09-08

Q1.
Why is the speed of light denoted by ‘c’?
Answer

Because c comes from the Latin word celeritas, meaning speed — not from the first letter of any English word.

  • Scientific symbols usually come from history and from international agreement. They are not convenient abbreviations invented by each writer.
  • That is why the symbols look odd in English: I for current (from the French intensité de courant), q for charge, Fe for iron (Latin ferrum).
  • Today the speed of light is one of the defined physical constants. Its value is fixed to be exactly 299792458 m/s — not measured, defined.
Why it happens: a symbol has to mean the same thing to a student in Kerala, a researcher in Japan and a textbook printed in Brazil. If everyone abbreviated in their own language, an equation could not be read across borders. Once a symbol is agreed internationally, changing it would break every book and paper already written — so the historical choice survives even when it no longer matches the language being used.
Did you know? Because c is now a defined number, it is the metre that depends on it, not the other way round: one metre is the distance light travels in vacuum in 1/299792458 of a second. So an experiment can no longer measure c more accurately — any improvement simply makes the metre more accurate.
Q2.
Why is a kilogram used everywhere?
Answer

So that one kilogram means exactly the same amount of matter in every shop, every laboratory and every country.

  • Measurements are based on agreed international standards, not on local objects or on someone's opinion of what a “seer” or a “handful” should be.
  • Standard units let scientific results be compared: a reaction that needs 5 g of a substance needs the same 5 g in Delhi and in Berlin.
  • They also make daily trade fair. When you buy a kilogram of rice or vegetables, you expect the same quantity from every seller.
Why it happens: a measurement is really a comparison — you are asking “how many times does the standard fit into this?” If two people use different standards, their numbers cannot be compared at all, even though both may be perfectly careful. The whole point of the SI system is to fix one standard for each quantity so that the number carries information rather than confusion. The aircraft in this chapter ran out of fuel for exactly this reason: the crew computed a density in pounds per litre and used it as though it were kilograms per litre, leaving the plane about 15,000 litres short of the 22,300 kg it needed.
Did you know? Until 2019 the kilogram was defined by a metal cylinder kept near Paris — a “local object” of exactly the kind this box warns against, and it was slowly losing mass. Today the kilogram is defined from a fixed constant of nature instead, so no country needs to hold the standard.
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