Lesson 1 of 10 · 7 min

Units: base, derived and systems

NCERT §1.1

Riya writes on her card that the pendulum's thread is 0.994 long. A visitor asks: 0.994 of what? Metres, feet and yards would give three very different pendulums.

The story this chapter follows: Riya's science-fair pendulum

Riya builds a pendulum for her school science fair: a small brass cube hung on a thread from a wooden stand with a base plate. With a metre scale, a stopwatch and a kitchen balance she sets out to measure g, and to show her visitors exactly how far each digit of her answer can be trusted.

The lesson in notes

In short

To measure a physical quantity is to compare it with a reference standard that has been chosen arbitrarily and agreed internationally; that standard is the unit.

A measured result always has two parts, a numerical value and a unit: a pendulum string is 0.994 m long, never just 0.994.

There are very many physical quantities, but they are linked to each other by equations, so a small set of units is enough to express all of them.

The quantities chosen as the starting set are the fundamental or base quantities, and their units are the base units (for example metre, kilogram, second).

Every other unit is built from the base units by multiplying and dividing them; these are derived units, such as m s⁻² for acceleration or kg m s⁻² (the newton) for force.

The base units together with all the derived units made from them form a system of units.