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Units and Measurements for NEET UG

Every number in physics carries a unit and an uncertainty. This chapter teaches you to check formulas with dimensions and to report results honestly.

PhysicsClass 1167 MCQs3 lessons12 flashcards4 memory tricks

Practise Units and Measurements free

What you'll learn

  1. Dimensions: checking a formula without solving it
    Write the dimensional formula of a quantity and use it to test whether an equation can be right.
  2. Significant figures and rounding
    Count significant figures and give answers to the right precision after multiplying, dividing, adding or subtracting.
  3. Errors: how sure is a measurement?
    Find absolute, relative and percentage errors, and combine errors in products, quotients and powers.

Key formulas

Absolute errorΔa = |a_measured − a_mean|
Relative errorΔa / a
Percentage error(Δa / a) × 100%

Memory tricks

SI base units (metre, kilogram, second, ampere, kelvin, mole, candela)Many Kind Souls Are Keeping Mother Calm

First letters: Metre, Kilogram, Second, Ampere, Kelvin, Mole, Candela.

Dimensions of force, work and powerForce → Work → Power: add an L, then add a T⁻¹. Like getting off one local-train stop at a time.

Force = MLT⁻². Work (energy) = force × distance = ML²T⁻². Power = work ÷ time = ML²T⁻³. Each step changes just one thing.

Which zeros are significantLeading zeros are like relatives who come only for the photo: they don't count. Sandwiched zeros do. Trailing zeros count only if there's a decimal point.

0.0045 has 2 significant figures; 4005 has 4; 4.500 has 4; 4500 (no decimal point) is ambiguous and is usually taken as 2.

Errors in products and powersPercentage errors are like everyone's share of the restaurant bill: they always add, never cancel. Squaring something doubles its share.

For A = x²y/z, %error in A = 2(%error in x) + %error in y + %error in z. Errors add even for division.

Try 3 NEET-style questions

Q1. Using dimensional analysis, the time period T of a simple pendulum of length l in a gravitational field g is proportional to

  1. √(g/l)
  2. √(l/g)
  3. l/g
  4. l·g
Show answer

Answer: (2) √(l/g)

Let T ∝ lᵃgᵇ. Then [T] = [L]ᵃ[LT⁻²]ᵇ, so a + b = 0 and −2b = 1. Hence b = −½, a = ½, and T ∝ √(l/g).

Q2. The dimensions of 1/√(μ₀ε₀) are the same as those of

  1. [L²T⁻²]
  2. [LT⁻¹]
  3. [T]
  4. [MLT⁻²]
Show answer

Answer: (2) [LT⁻¹]

1/√(μ₀ε₀) equals the speed of light in vacuum, so its dimensions are those of velocity, [LT⁻¹].

Q3. Which of the following is NOT an SI unit?

  1. metre
  2. angstrom
  3. second
  4. kilogram
Show answer

Answer: (2) angstrom

The angstrom (10⁻¹⁰ m) is accepted for use with SI but is not an SI unit. The metre, second and kilogram are SI units.

Practise all 67 questions Free · explanations for every option · see where you go wrong

More Physics chapters (Class 11)

Motion in a Straight Line · Motion in a Plane · Laws of Motion · Work, Energy and Power · System of Particles and Rotational Motion · Gravitation · Mechanical Properties of Solids · Mechanical Properties of Fluids · Thermal Properties of Matter · Thermodynamics · Kinetic Theory of Gases · Oscillations · Waves