Units and Measurements — Class 11 MCQs with Answers

Units and Measurements — important questions & MCQs with answers (Class 11 Physics)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   The SI unit of force is:

   ANewtonBDyneCJouleDPascal
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   Answer: Newton

   Explanation: Newton's second law fixes force = mass × acceleration, so its SI unit is the newton (1 N = 1 kg·m·s⁻²). Dyne is the CGS unit — a common mix-up to avoid.

2. Q2 of 6Easy

   How many significant figures are there in 0.00450?

   A2B3C4D5
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   Answer: 3

   Explanation: In 0.00450, the leading zeros before 4 only fix the decimal point and are not significant; 4, 5, and the trailing 0 count, giving 3 significant figures, not 5.

3. Q3 of 6Medium

   Dimensions of Planck's constant h:

   A[M L² T⁻¹]B[M L T⁻²]C[M L² T⁻²]D[M L T⁻¹]
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   Answer: [M L² T⁻¹]

   Explanation: From E = hν, h = E/ν, so its dimensions are [M L² T⁻²]/[T⁻¹] = [M L² T⁻¹]. Forgetting to invert frequency's T⁻¹ gives the wrong [M L² T⁻²].

4. Q4 of 6Medium

   Result of (5.00 + 0.0023) has how many decimal places?

   A1B4C3D2
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   Answer: 2

   Explanation: When adding, the sum is rounded to the fewest decimal places among the terms; 5.00 has only 2 decimal places, so 5.00 + 0.0023 must be reported to 2 decimals.

5. Q5 of 6Hard

   Which of the following could be a dimensionally correct formula for the period T of a simple pendulum (length L, gravity g)?

   AT = 2π√(L/g)BT = 2π√(g/L)CT = 2π·L·gDT = 2π·L/g
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   Answer: T = 2π√(L/g)

   Explanation: Checking T = 2π√(L/g): [L]/[L T⁻²] = [T²], and √[T²] = [T], matching time. Inverting to √(g/L) instead gives [T⁻²], which fails dimensionally.

6. Q6 of 6Hard

   If the percentage error in measuring the radius of a sphere is 2%, the percentage error in its volume is:

   A2%B4%C8%D6%
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   Answer: 6%

   Explanation: V = (4/3)πr³, so a relative-error rule gives ΔV/V = 3×(Δr/r) = 3×2% = 6%. Forgetting the power-of-3 multiplier and just doubling to 4% is the usual slip.

Units and Measurements notes: key concepts (Class 11 Physics)

Every physical measurement carries a unit and an uncertainty, and this chapter builds the machinery for both: the seven SI base units, dimensional formulae and the homogeneity check, significant figures, and the rules for how errors combine when measured quantities are added, multiplied or raised to powers.

SI base units

Seven independent units — metre, kilogram, second, ampere, kelvin, mole and candela. Every other unit in physics is derived from these by multiplication or division.

Derived units

Units built from the base set, such as the newton (kg m s⁻²) and the pascal (N m⁻²). Recognising a derived unit's build is often faster than recalling its formula.

SI prefixes

Standard multipliers attached to any unit — kilo for 10³, milli for 10⁻³, micro for 10⁻⁶, nano for 10⁻⁹ and giga for 10⁹ — which keep numbers readable across huge ranges.

Dimensional formula

A quantity written as powers of the base dimensions M, L, T (and A, K, mol, cd). Pressure is [M L⁻¹ T⁻²]; energy and work share [M L² T⁻²].

Principle of homogeneity

Every term that is added, subtracted or equated in a physical equation must carry identical dimensions. This is what makes a dimensional check possible at all.

Limits of dimensional analysis

It can verify an equation and convert units, but can never supply a dimensionless constant such as ½ or 2π, and it cannot distinguish quantities sharing dimensions.

Significant figures

All digits known with certainty plus one estimated digit. Leading zeros never count; zeros trapped between digits always do; trailing zeros count only after a decimal point.

Rounding rules in calculation

A product or quotient keeps the least number of significant figures among the inputs; a sum or difference keeps the least number of DECIMAL PLACES, not significant figures.

Systematic error

A reproducible bias from instrument zero error, faulty calibration or flawed technique. It always pushes readings the same way, so repeating the measurement cannot remove it.

Random error

Unpredictable scatter from uncontrolled fluctuations in the measurement. Averaging a large number of readings does reduce it, since the deviations tend to cancel.

Absolute and relative error

Absolute error is the magnitude of the deviation from the true (or mean) value; relative error is that divided by the mean, and percentage error is the relative error times 100.

Combination of errors

For sums and differences the ABSOLUTE errors add; for products, quotients and powers the RELATIVE errors add, with a power multiplying its relative error by the exponent.

Least count

The smallest value an instrument can resolve, which sets its absolute error. Vernier callipers read to 0.01 cm and a screw gauge to 0.001 cm.

Accuracy versus precision

Accuracy is closeness to the true value; precision is repeatability. An instrument with a zero error is highly precise and consistently inaccurate.

Order of magnitude

The nearest power of ten to a quantity, used to estimate quickly and to sanity-check whether a calculated answer is physically plausible.