Physics turns observations into testable statements by measuring physical quantities. A physical quantity is a property that can be compared with an accepted standard. Every measurement has two parts: a numerical magnitude and a unit. Writing only “length = 5” is incomplete; “length = 5 m” communicates both size and standard.

Measurement means comparison. Saying a rod is 2 m long means its length is twice the distance represented by the standard metre.

Fundamental and derived quantities

Fundamental quantities are chosen to be independent. Derived quantities are defined through relations among them: velocity = displacement/time, density = mass/volume, and force = mass × acceleration. The choice of base quantities belongs to a system of units; nature itself does not label a quantity “base”.

Physical quantity
Has magnitude and a measurement standard.

Unit
A defined reference amount of the quantity.

Numerical value
Changes when the chosen unit changes.

Physical size
Does not change with the unit.

A useful standard unit must be well-defined, reproducible, invariable with place and time, and accessible. Modern standards use stable atomic phenomena because an ordinary physical object can change or be damaged.

NEET focus: distinguish a physical quantity from its unit, and a base quantity from a derived quantity. “Plane angle” is dimensionless but has the named SI unit radian.

NCERT alignment: Class XI Physics, Units and Measurements—introduction and systems of units.

A numerical value varies inversely with the chosen unit size: the physical length 2 m becomes 200 cm, not a larger rod. A base quantity is selected independently in an agreed unit system, whereas a derived quantity follows a relationship among base quantities. For instance momentum equals mass times velocity; its unit kg m s⁻¹ follows without introducing another independent base unit. Standards should be reproducible. State the measured quantity, magnitude and unit before interpreting a laboratory result. This reasoning also explains why two students can report different numerical values for the same physical object when one uses metres and the other centimetres.

Worked example: For a 2.0 m rod, write 200 cm; the size stays fixed while the number changes.

NCERT trap: A bare number is not a complete measured length.

NEET method: First identify the quantity, then its number and unit.

Reading estimate based on the lesson text and figure; allow 3–5 more minutes for each three-question check set.

Units and Measurements

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Check 2 of 3

Check 3 of 3

Topic checkpoint