Physics · Class 11

SI unit builder

A physical quantity combines a number and a unit. SI has seven base units: metre, kilogram, second, ampere, kelvin, mole and candela. Derived units combine them: a newton is kg m s⁻². A named derived unit is not an eighth base unit. A unit system chooses its standards: SI mechanics uses metre, kilogram and second; CGS uses centimetre, gram and second. Measurement lets us compare a quantity with a shared standard.

Open the interactive experiment →

Build a unit from the seven SI bases.

இயற்பியல் அளவு என்பது எண் மற்றும் அலகு சேர்ந்தது. SI முறையில் ஏழு அடிப்படை அலகுகள் உள்ளன. நியூட்டன் போன்ற வழி அலகுகள் அடிப்படை அலகுகளின் சேர்க்கையால் உருவாகின்றன.

Key notes, relationships and model limits

Key relationship

F = ma: 1 N = 1 kg m s⁻²; 1 J = 1 kg m² s⁻²; 1 C = 1 A s.

Remember

The seven SI base units are m, kg, s, A, K, mol and cd. Named derived units combine them.

Common trap

Kilogram is the SI base unit of mass. Gram is not the base unit.

Model limits

A numerical value needs its unit. Changing the unit system leaves the physical quantity unchanged.

What you will learn

Name all seven base units; distinguish base from derived units; build force, energy and electric charge.

Try the experiment

Choose a target quantity. Set each base-unit exponent until the dimensions match.

Think of it this way

A recipe uses a small set of ingredients to make many dishes. Derived units likewise combine a small set of base units.

Worked example

F = ma: kg × (m/s²) = kg m s⁻² = N. The seven SI base units form a coherent system.

Why this matters

Using consistent units makes results comparable across laboratories and countries.

Before you start

Decimals · fractions · powers of ten

Check your understanding

Which is an SI base unit?

Newton
Kilogram
Joule

Read the explanation

Kilogram is a base unit. Newton and joule are derived units: kg m s⁻² and kg m² s⁻².

Course connection: CBSE XI Chapter 1; NEET/Main Unit 1; JEE Advanced General. Depth and exam wording differ.