NCERT Notes Class 11 Physics Chapter 1 Units and Measurements

These Class 11 Physics Notes Chapter 1 Units and Measurements pull together every unit, dimensional formula, significant-figure rule, and error method that the Boards, JEE Main, JEE Advanced, NEET, CUET and NDA papers actually test in 2026-27. Use them to revise the whole chapter fast, with formulas, definitions and worked rules in one place.

This is the first chapter of the NCERT textbook, and the measurement skills here carry into every numerical you solve later in the year.

• CBSE Weightage: 4 to 6 marks, usually one short answer plus one numerical on dimensions or errors.
• Topics covered: SI base and derived units, dimensional analysis, significant figures, error analysis, and order of magnitude.
• Key formulas: percentage error, combination of errors, and the dimensional formulae of common quantities.

These Class 11 Physics Notes Chapter 1 Units and Measurements are curated by subject experts, based on the 2026-27 NCERT textbook, and checked against the last five years of CBSE Board, JEE Main and NEET papers.

Topic-by-Topic Summary of Units and Measurements

The chapter builds measurement from the ground up. It starts with fixed standards, then teaches you to check equations, count digits, and state the error in every result. Here is the quick map of what each topic gives you.

• Units: the seven SI base units and the derived units built from them.
• Dimensional analysis: writing quantities in terms of mass, length and time, then using the principle of homogeneity to check formulas.
• Significant figures: the digits in a measured value that carry real meaning, plus the rounding rules.
• Error analysis: absolute, relative and percentage error, and how errors combine.
• Measurement and estimation: length, mass and time standards, plus order of magnitude.

Revise the topics in this order, because each one uses the one before it. Get the units right first, and the numericals become far easier. These Class 11 Physics Notes Chapter 1 Units and Measurements follow the same sequence as the NCERT textbook.

SI Base Units and the International System of Units

Measurement starts with a fixed set of standards. Physics is built on the seven SI base units, and every other unit, called a derived unit, is made by combining them. Getting these right is the first step in any numerical of the chapter.

A derived unit comes from combining base units. Speed is metre per second, force is the newton (kg m s^-2), and energy is the joule (kg m² s^-2). Always convert every quantity to SI before you substitute numbers. This one habit prevents the most common mark loss in the whole chapter.

Dimensional Analysis and the Principle of Homogeneity

The dimensional formula of a quantity shows how it is built from mass (M), length (L) and time (T). Force has the dimensional formula [M L T^-2] and energy has [M L² T^-2]. The chapter uses these formulas for three jobs.

• Checking an equation: by the principle of homogeneity, every term on both sides must have the same dimensions.
• Deriving a relation: guess how one quantity depends on others, then match dimensions to find the powers.
• Converting units: use the dimensional formula to move a value from one system to another.

A dimensionally correct equation may still be physically wrong, but a dimensionally wrong equation is always wrong. Remember this limit so you do not over-trust the method in the exam. Dimensions cannot find pure numbers like 2 or π, and they cannot handle equations with sums of unlike terms.

Significant Figures and Rounding Rules in Measurement

Significant figures are the digits in a measured value that carry real meaning. They tell the reader how precise a measurement is. The chapter gives a fixed set of rules, so the final answer is never written with false precision.

• All non-zero digits are significant.
• Zeros between non-zero digits are significant.
• Leading zeros are not significant; trailing zeros after a decimal point are significant.
• In multiplication or division, the answer keeps as many significant figures as the least precise value.
• In addition or subtraction, the answer keeps as many decimal places as the least precise value.

Round only at the last step, never in the middle of a calculation, because early rounding builds up error. When the digit dropped is 5, round to make the preceding digit even. Keeping full precision through the working and rounding the boxed answer at the end is exactly what CBSE expects.

Error Analysis: Absolute, Relative and Percentage Error

No measurement is exact, so the chapter teaches you to state the error in every result. Three ideas do most of the work, and they appear in every error numerical of the chapter.

• Absolute error: the size of the difference between a measured value and the true value, written as Δa.
• Relative error: the absolute error divided by the true value, Δa/a.
• Percentage error: the relative error written as a percent, (Δa/a) × 100%.

When two measured quantities are combined, the errors combine by fixed rules. The table below is the one students use most in the exam.

For a quantity raised to a power, multiply its relative error by that power before adding. This single step is the most common slip in the chapter, so write it on its own line in the exam. Percentage error is the version markers look for in the final answer.

All Formulas for Units and Measurements

Every formula you need for the chapter sits in one table below, with its meaning and its SI unit. Learn the error-combination rows first, since those carry the most marks in both Boards and entrance papers.

Carry the SI unit on every line of your working. Losing the unit is a silent way to drop the final mark even when the number is right. Keep this table open while you solve the back-exercise numericals.

Key Definitions and Derivations for Units and Measurements

Boards short-answer questions often ask for a clean definition in one or two lines. Learn these word-for-word, because a vague definition loses easy marks. Each one also sets up a derivation you can be asked to show.

A common derivation asks you to use dimensions to find how the time period of a simple pendulum depends on its length and gravity. Match the dimensions of each side and solve for the powers to get T ∝ √(l/g). The method finds the form of the relation but not the number in front.

Measurement of Length, Mass, Time and Order of Magnitude

The chapter also covers how very large and very small quantities are measured. These methods appear in short-answer and fill-in-the-blank questions, so keep the standard for each one ready.

• Length: the parallax method measures the distance to a far object, such as a star, using the shift in its apparent position from two points.
• Mass: large masses use a physical balance; atomic masses use the unified atomic mass unit (u).
• Time: an atomic clock, based on the vibration of a caesium atom, gives the modern standard of the second.

The order of magnitude of a number is the power of ten nearest to it. Write the number in scientific notation and round the coefficient to the nearest power of ten. This tool lets you compare sizes quickly, from the radius of an atom to the distance of a galaxy, and it is a favourite in objective papers for JEE Main and NEET.

Common Mistakes Students Make in Units and Measurements

These slips happen while writing or calculating, not because the concept is unclear. Each one costs 1 to 3 marks in the paper, so watch for them at the exact step.

Units and Measurements Weightage in CBSE Boards, JEE and NEET

This chapter is small but reliable. It rarely carries a long-answer question, yet it shows up every year as a short answer plus an objective numerical. Here is how the marks split across the main exams for 2026-27.

The error-combination formula is the single most tested idea from this chapter across all four exams. Master it first, then significant figures, then dimensional analysis, in that order of return on effort.

How to Revise Units and Measurements Quickly

Use these Class 11 Physics Notes Chapter 1 Units and Measurements for a fast, ordered recap the night before a test. The checklist below takes about 30 minutes and hits every marks-heavy idea.

• First 10 minutes: write the seven base units and the dimensional formulae of force, energy, pressure and density from memory.
• Next 10 minutes: redo one error-combination numerical for a sum, a product, and a power.
• Last 10 minutes: run through the significant-figure rules and round three sample answers correctly.

Close the loop by checking one equation with the principle of homogeneity. If you can do all four blocks without notes, the chapter is exam-ready. Keep the All Formulas table beside you for the first pass only, then try it closed-book.

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