Important Questions Class 11 Physics Chapter 2: Units and Measurement

Measurement compares a physical quantity with an accepted reference standard called a unit.
Dimensional analysis checks formula consistency, but it cannot find dimensionless constants.

Measurement forms the base of physics because every law needs reliable numerical values and accepted units. Important Questions Class 11 Physics Chapter 2 help students practise SI units, measurement of length, mass and time, errors, significant figures, dimensions, dimensional formulae and dimensional analysis. The CBSE 2026 chapter builds the foundation for numericals in mechanics, thermodynamics, waves and modern physics.

Key Takeaways

• SI System: The International System of Units uses seven base units for seven base quantities.
• Parallax Method: Large distances can be measured using angle subtended by a known baseline.
• Significant Figures: The number of significant figures shows the precision of a measured value.
• Dimensional Analysis: Dimensional equations check formula correctness using base dimensions.

Important Questions Class 11 Physics Chapter 2 Structure 2026

Important Questions Class 11 Physics Chapter 2 with Answers

Units and Measurement connects physical quantities with units, errors and dimensions.
Students should write units and significant figures clearly in every numerical answer.
These units and measurement class 11 important questions follow the NCERT 2026 flow.

1. What does Important Questions Class 11 Physics Chapter 2 mainly test?

Important Questions Class 11 Physics Chapter 2 mainly test SI units, measurement methods, errors, significant figures and dimensional analysis. The chapter checks calculation accuracy and formula logic.

1. Units Skill: Identify base and derived units.
2. Measurement Skill: Use least count and error rules.
3. Significant Figures Skill: Round answers correctly.
4. Dimensions Skill: Check equations using [M], [L] and [T].
5. Final Result: The chapter tests measurement accuracy and dimensional reasoning.

2. What is measurement in physics?

Measurement is comparison of a physical quantity with a fixed standard unit. It gives a numerical value with a unit.

1. Physical Quantity: Something measurable.
2. Standard Unit: Accepted reference quantity.
3. Result Format: Numerical value plus unit.
4. Final Result: Measurement expresses a quantity using a unit.

3. Why are units necessary in physics?

Units are necessary because numerical values have meaning only with a standard reference. A value like 5 can mean 5 m, 5 kg or 5 s.

1. Without Unit: Quantity stays unclear.
2. With Unit: Quantity becomes specific.
3. Example: 10 m and 10 kg describe different quantities.
4. Final Result: Units make measurements meaningful.

SI Units Class 11 Physics Questions

The SI system gives a common international language for measurement.
It uses base units and derived units to express physical quantities.
These SI units class 11 physics questions cover fundamental units, derived units and prefixes.

4. What is the International System of Units?

The International System of Units is the modern metric system used in science. It is abbreviated as SI.

1. Full Name: Système International d’Unités.
2. Use: Scientific and technical measurement.
3. Structure: Base units and derived units.
4. Final Result: SI is the accepted international system of units.

5. What are base quantities in physics?

Base quantities are independent physical quantities that cannot be expressed using other quantities. SI has seven base quantities.

1. Length: metre.
2. Mass: kilogram.
3. Time: second.
4. Electric Current: ampere.
5. Final Result: Base quantities form the foundation of all units.

6. Name the seven SI base units.

The seven SI base units are metre, kilogram, second, ampere, kelvin, mole and candela. Each corresponds to one base quantity.

1. Length: metre.
2. Mass: kilogram.
3. Time: second.
4. Temperature: kelvin.
5. Final Result: SI has seven base units.

7. What are derived units?

Derived units are units obtained from base units. They express quantities like velocity, force, pressure and energy.

1. Velocity: m/s.
2. Acceleration: m/s².
3. Force: kg m/s².
4. Final Result: Derived units come from base units.

8. What is the SI unit of force?

The SI unit of force is newton. One newton equals kg m s^-2.

1. Formula Used: F = ma.
2. Mass Unit: kg.
3. Acceleration Unit: m s^-2.
4. Final Result: 1 N = 1 kg m s^-2.

9. What is the SI unit of energy?

The SI unit of energy is joule. One joule equals kg m² s^-2.

1. Formula Used: Work = force × displacement.
2. Force Unit: kg m s^-2.
3. Displacement Unit: m.
4. Final Result: 1 J = 1 kg m² s^-2.

Measurement of Length Class 11 Questions

Length measurement changes with scale, from atomic distances to astronomical distances.
Different methods are used for small lengths, ordinary lengths and very large distances.
These measurement of length class 11 questions cover direct and indirect measurement methods.

10. Which instruments measure ordinary length?

A metre scale, vernier callipers and screw gauge measure ordinary laboratory lengths. The choice depends on required precision.

1. Metre Scale: Measures length up to millimetre precision.
2. Vernier Callipers: Measures internal and external diameters.
3. Screw Gauge: Measures very small thickness.
4. Final Result: Instrument choice depends on size and precision.

11. What is least count?

Least count is the smallest measurement an instrument can read. Smaller least count gives higher precision.

1. Metre Scale: Least count is usually 1 mm.
2. Vernier Callipers: Least count can be 0.1 mm.
3. Screw Gauge: Least count can be 0.01 mm.
4. Final Result: Least count decides instrument precision.

12. What is parallax?

Parallax is the apparent shift in position of an object when viewed from two different points. It helps measure large distances.

1. Observation Points: Two separated positions.
2. Apparent Shift: Object seems displaced.
3. Use: Distance measurement in astronomy.
4. Final Result: Parallax measures distance through angular shift.

13. How is distance found using parallax method?

Distance is found using the relation D = b/θ when θ is small in radians. Here b is the baseline.

1. Baseline: b.
2. Parallax Angle: θ.
3. Small Angle Formula: θ = b/D.
4. Final Result: D = b/θ.

14. Find distance if baseline is 2 m and parallax angle is 0.01 rad.

The distance is 200 m. Use D = b/θ.

1. Given Data:
   b = 2 m
   θ = 0.01 rad
2. Formula Used:
   D = b/θ
3. Calculation:
   D = 2/0.01
   D = 200 m
4. Final Result: Distance = 200 m.

Measurement of Mass Class 11 Questions

Mass measures the amount of matter in a body.
It remains different from weight because weight depends on gravity.
These measurement of mass class 11 questions cover units, balances and atomic mass scale.

15. What is the SI unit of mass?

The SI unit of mass is kilogram. Its symbol is kg.

1. Base Quantity: Mass.
2. SI Unit: Kilogram.
3. Symbol: kg.
4. Final Result: Mass is measured in kilogram.

16. How is mass different from weight?

Mass is the amount of matter, while weight is gravitational force on that mass. Weight changes with g.

1. Mass: Scalar quantity.
2. Weight: Force.
3. Formula: W = mg.
4. Final Result: Mass is measured in kg, weight in newton.

17. What instrument measures mass in a laboratory?

A balance measures mass in a laboratory. It compares an unknown mass with standard masses.

1. Common Instrument: Beam balance.
2. Principle: Comparison of masses.
3. Measured Quantity: Mass.
4. Final Result: A balance measures mass by comparison.

18. What is atomic mass unit?

Atomic mass unit is used for atomic and molecular masses. One atomic mass unit is nearly 1.66 × 10^-27 kg.

1. Symbol: u.
2. Use: Atomic-scale masses.
3. Value: 1 u = 1.66 × 10^-27 kg.
4. Final Result: Atomic masses are expressed in u.

Measurement of Time Class 11 Questions

Time measurement uses periodic events that repeat uniformly.
Clocks use regular oscillations to compare time intervals.
These measurement of time class 11 questions cover seconds, clocks and periodic motion.

19. What is the SI unit of time?

The SI unit of time is second. Its symbol is s.

1. Base Quantity: Time.
2. SI Unit: Second.
3. Symbol: s.
4. Final Result: Time is measured in seconds.

20. Why are periodic motions used to measure time?

Periodic motions are used because they repeat at regular intervals. This repeatability creates a reliable time standard.

1. Example: Pendulum oscillation.
2. Modern Example: Atomic transitions.
3. Requirement: Repetition must stay uniform.
4. Final Result: Regular periodic motion helps measure time.

21. What is a time period?

Time period is the time taken for one complete oscillation or cycle. Its SI unit is second.

1. One Cycle: Complete repeated motion.
2. Symbol: T.
3. Unit: Second.
4. Final Result: Time period measures one full repetition.

22. What is frequency?

Frequency is the number of cycles completed per second. Its SI unit is hertz.

1. Symbol: f.
2. Formula: f = 1/T.
3. Unit: Hz.
4. Final Result: Frequency equals cycles per second.

Accuracy and Precision Class 11 Questions

Accuracy and precision describe different aspects of measurement quality.
A measurement can be precise without being accurate if systematic error is present.
These accuracy and precision class 11 questions clarify common exam confusion.

23. What is accuracy in measurement?

Accuracy is the closeness of a measured value to the true value. A smaller error gives higher accuracy.

1. True Value: Accepted value.
2. Measured Value: Experimental reading.
3. Comparison: Closeness decides accuracy.
4. Final Result: Accuracy means closeness to true value.

24. What is precision in measurement?

Precision is the closeness of repeated measurements to each other. It depends on instrument resolution and repeatability.

1. Repeated Readings: Compared with each other.
2. Small Spread: High precision.
3. Instrument Role: Smaller least count improves precision.
4. Final Result: Precision means repeatability of readings.

25. Can a measurement be precise but inaccurate?

Yes, a measurement can be precise but inaccurate. This happens when readings cluster together away from the true value.

1. Repeated Values: Nearly same.
2. True Value: Different from measured cluster.
3. Cause: Systematic error.
4. Final Result: Precision does not guarantee accuracy.

26. What is least count error?

Least count error is the uncertainty caused by the smallest division of an instrument. It limits precision.

1. Instrument Division: Smallest readable value.
2. Reading Limit: Cannot read below least count.
3. Example: Metre scale has 1 mm least count.
4. Final Result: Least count error comes from instrument resolution.

Errors in Measurement Class 11 Questions

Every measurement contains some uncertainty.
Errors must be reported and combined correctly in physics calculations.
These errors in measurement class 11 questions cover absolute, relative and percentage errors.

27. What is absolute error?

Absolute error is the magnitude of difference between measured value and true value. It has the same unit as the measured quantity.

1. Measured Value: a.
2. True Value: atrue.
3. Formula: Δa = |a − atrue|.
4. Final Result: Absolute error measures actual deviation.

28. What is mean absolute error?

Mean absolute error is the average of absolute errors in repeated measurements. It estimates uncertainty in the mean value.

1. Repeated Measurements: a1, a2, a3.
2. Mean Value: amean.
3. Average Error: Add absolute errors and divide by number.
4. Final Result: Mean absolute error gives average uncertainty.

29. What is relative error?

Relative error is the ratio of absolute error to measured or mean value. It has no unit.

1. Absolute Error: Δa.
2. Mean Value: a.
3. Formula: Relative error = Δa/a.
4. Final Result: Relative error is dimensionless.

30. What is percentage error?

Percentage error is relative error multiplied by 100. It is written as a percentage.

1. Relative Error: Δa/a.
2. Formula Used: Percentage error = Δa/a × 100%.
3. Use: Compares errors across quantities.
4. Final Result: Percentage error expresses relative error in percent.

31. A length is measured as 20.0 cm with error 0.1 cm. Find percentage error.

The percentage error is 0.5%. Use percentage error = Δl/l × 100%.

1. Given Data:
   l = 20.0 cm
   Δl = 0.1 cm
2. Formula Used:
   Percentage error = Δl/l × 100%
3. Calculation:
   Percentage error = 0.1/20.0 × 100
   Percentage error = 0.5%
4. Final Result: Percentage error = 0.5%.

32. If A = x + y, what is absolute error in A?

The absolute error in A is ΔA = Δx + Δy. Errors add in addition and subtraction.

1. Quantity: A = x + y.
2. Error in x: Δx.
3. Error in y: Δy.
4. Final Result: Maximum absolute error = Δx + Δy.

33. If Z = AB, what is relative error in Z?

The relative error in Z is ΔZ/Z = ΔA/A + ΔB/B. Relative errors add in multiplication.

1. Quantity: Z = AB.
2. Error Rule: Add fractional errors.
3. Formula: ΔZ/Z = ΔA/A + ΔB/B.
4. Final Result: Product error uses relative errors.

34. If Z = A²B³, what is relative error in Z?

The relative error is ΔZ/Z = 2ΔA/A + 3ΔB/B. Powers multiply relative errors.

1. Quantity: Z = A²B³.
2. Power of A: 2.
3. Power of B: 3.
4. Final Result: Relative error = 2ΔA/A + 3ΔB/B.

Significant Figures Class 11 Questions

Significant figures show meaningful digits in a measured value.
They depend on instrument precision and measurement uncertainty.
These significant figures class 11 questions cover counting and rounding rules.

35. What are significant figures?

Significant figures are reliable digits in a measured value plus the first uncertain digit. They indicate measurement precision.

1. Reliable Digits: Certain digits.
2. Uncertain Digit: Last estimated digit.
3. Purpose: Shows precision.
4. Final Result: Significant figures express meaningful digits.

36. How many significant figures are in 0.00420?

The number 0.00420 has 3 significant figures. Leading zeros are not significant.

1. Leading Zeros: 0.00 are not significant.
2. Digits Counted: 4, 2 and final 0.
3. Reason: Final zero after decimal is significant.
4. Final Result: 0.00420 has 3 significant figures.

37. How many significant figures are in 2300?

The number 2300 can be ambiguous without decimal or scientific notation. It may have 2, 3 or 4 significant figures.

1. 2300: Usually 2 significant figures.
2. 2300.: Four significant figures.
3. 2.30 × 10³: Three significant figures.
4. Final Result: Scientific notation removes ambiguity.

38. What is the rule for multiplication with significant figures?

The final answer should have as many significant figures as the factor with the least significant figures. This rule applies to multiplication and division.

1. Factor 1: Has n significant figures.
2. Factor 2: Has m significant figures.
3. Answer: Use the smaller count.
4. Final Result: Least significant figure count decides the final answer.

39. Round 3.678 to three significant figures.

The rounded value is 3.68. The fourth digit is 8, so the third digit increases.

1. Original Number: 3.678.
2. First Three Significant Digits: 3, 6, 7.
3. Next Digit: 8.
4. Final Result: 3.678 becomes 3.68.

40. Calculate 2.5 × 3.42 with correct significant figures.

The result is 8.6. The factor 2.5 has the least significant figures.

1. Calculation: 2.5 × 3.42 = 8.55.
2. Significant Figures: 2.5 has 2 significant figures.
3. Rounded Answer: 8.6.
4. Final Result: 2.5 × 3.42 = 8.6.

Dimensions of Physical Quantities Class 11 Questions

Dimensions express physical quantities in terms of base quantities.
They reveal the nature of a quantity without depending on a unit system.
These dimensions of physical quantities class 11 questions cover [M], [L], [T] and derived dimensions.

41. What are dimensions of a physical quantity?

Dimensions show how a physical quantity depends on base quantities. They are written using symbols like [M], [L] and [T].

1. Mass Dimension: [M].
2. Length Dimension: [L].
3. Time Dimension: [T].
4. Final Result: Dimensions express physical nature of quantities.

42. What is dimensional formula?

Dimensional formula expresses a quantity in powers of base dimensions. It has the form [M^a L^b T^c].

1. Mass Power: a.
2. Length Power: b.
3. Time Power: c.
4. Final Result: Dimensional formula shows base-dimension powers.

43. What is the dimensional formula of velocity?

The dimensional formula of velocity is [L T^-1]. Velocity is displacement per unit time.

1. Formula Used: v = displacement/time.
2. Displacement Dimension: [L].
3. Time Dimension: [T].
4. Final Result: [v] = [L T^-1].

44. What is the dimensional formula of acceleration?

The dimensional formula of acceleration is [L T^-2]. Acceleration is change in velocity per unit time.

1. Velocity Dimension: [L T^-1].
2. Time Dimension: [T].
3. Formula Used: a = v/t.
4. Final Result: [a] = [L T^-2].

45. What is the dimensional formula of force?

The dimensional formula of force is [M L T^-2]. Use F = ma.

1. Mass Dimension: [M].
2. Acceleration Dimension: [L T^-2].
3. Formula Used: F = ma.
4. Final Result: [F] = [M L T^-2].

46. What is the dimensional formula of work?

The dimensional formula of work is [M L² T^-2]. Work equals force multiplied by displacement.

1. Force Dimension: [M L T^-2].
2. Displacement Dimension: [L].
3. Formula Used: W = F × s.
4. Final Result: [W] = [M L² T^-2].

Dimensional Formula Class 11 Questions

Dimensional formula questions help students derive units and compare physical quantities.
They also help detect incorrect equations in numerical problems.
These dimensional formula class 11 questions cover common formulas from mechanics.

47. Find dimensional formula of pressure.

The dimensional formula of pressure is [M L^-1 T^-2]. Pressure is force per unit area.

1. Formula Used: P = F/A.
2. Force Dimension: [M L T^-2].
3. Area Dimension: [L²].
4. Final Result: [P] = [M L^-1 T^-2].

48. Find dimensional formula of density.

The dimensional formula of density is [M L^-3]. Density is mass per unit volume.

1. Formula Used: ρ = m/V.
2. Mass Dimension: [M].
3. Volume Dimension: [L³].
4. Final Result: [ρ] = [M L^-3].

49. Find dimensional formula of momentum.

The dimensional formula of momentum is [M L T^-1]. Momentum equals mass multiplied by velocity.

1. Formula Used: p = mv.
2. Mass Dimension: [M].
3. Velocity Dimension: [L T^-1].
4. Final Result: [p] = [M L T^-1].

50. Find dimensional formula of power.

The dimensional formula of power is [M L² T^-3]. Power is work done per unit time.

1. Formula Used: P = W/t.
2. Work Dimension: [M L² T^-2].
3. Time Dimension: [T].
4. Final Result: [P] = [M L² T^-3].

51. Find dimensional formula of universal gravitational constant G.

The dimensional formula of G is [M^-1 L³ T^-2]. Use Newton’s law of gravitation.

1. Formula Used: F = GMm/r².
2. Rearrange: G = Fr²/(Mm).
3. Dimensions: [G] = [M L T^-2][L²]/[M²].
4. Final Result: [G] = [M^-1 L³ T^-2].

Dimensional Analysis Class 11 Questions

Dimensional analysis checks whether equations are dimensionally correct.
It can derive relations between quantities, but it cannot find numerical constants.
These dimensional analysis class 11 questions cover applications and limits.

52. What is dimensional analysis?

Dimensional analysis is the method of using dimensions to check or derive relations between physical quantities. It uses dimensional homogeneity.

1. Base Idea: Both sides of an equation must have the same dimensions.
2. Use 1: Check formula correctness.
3. Use 2: Convert units.
4. Final Result: Dimensional analysis tests dimensional consistency.

53. What is the principle of dimensional homogeneity?

The principle states that only quantities with the same dimensions can be added, subtracted or equated. Every valid physical equation follows it.

1. Addition: Terms must have same dimensions.
2. Subtraction: Terms must have same dimensions.
3. Equation: LHS and RHS must match.
4. Final Result: A valid equation is dimensionally homogeneous.

54. Check whether s = ut + 1/2 at² is dimensionally correct.

The equation is dimensionally correct. Every term has dimension of length.

1. s Dimension: [L].
2. ut Dimension: [L T^-1][T] = [L].
3. at² Dimension: [L T^-2][T²] = [L].
4. Final Result: All terms have dimension [L].

55. Can dimensional analysis find numerical constants?

No, dimensional analysis cannot find numerical constants. It cannot determine factors like 1/2, 2π or dimensionless numbers.

1. Dimensional Method: Uses powers of dimensions.
2. Numerical Constants: Dimensionless.
3. Example: It cannot derive 1/2 in s = ut + 1/2 at².
4. Final Result: Dimensional analysis cannot find dimensionless constants.

56. Can dimensional analysis distinguish scalar and vector quantities?

No, dimensional analysis cannot distinguish scalar and vector quantities. It only compares dimensions.

1. Scalar Example: Work has dimensions [M L² T^-2].
2. Vector Example: Torque also has [M L² T^-2].
3. Limitation: Direction information is absent.
4. Final Result: Dimensional analysis ignores vector nature.

57. Derive time period of simple pendulum using dimensions.

The time period depends on length l and acceleration due to gravity g. Dimensional analysis gives T ∝ √(l/g).

1. Assume: T ∝ l^a g^b.
2. Dimensions: [T] = [L]^a [L T^-2]^b.
3. Compare Powers:
   Length: a + b = 0
   Time: −2b = 1
4. Solve: b = −1/2 and a = 1/2.
5. Final Result: T ∝ √(l/g).

58. Derive dimensions of a and b in equation v = at + b/t.

The dimensions are [a] = [L T^-2] and [b] = [L]. Each term must have velocity dimensions.

1. Given Equation: v = at + b/t.
2. Velocity Dimension: [v] = [L T^-1].
3. For at: [a][T] = [L T^-1], so [a] = [L T^-2].
4. For b/t: [b]/[T] = [L T^-1], so [b] = [L].
5. Final Result: a has acceleration dimensions, and b has length dimensions.

Class 11 Physics Chapter 2 Numericals

Numericals in this chapter focus on errors, significant figures and dimensions.
Students should apply rounding rules only after completing calculations.
These Class 11 Physics Chapter 2 numericals support formula-based practice.

59. A quantity x = 10.0 ± 0.1 cm and y = 5.0 ± 0.1 cm. Find percentage error in xy.

The percentage error in xy is 3%. Add percentage errors for multiplication.

1. For x:
   Δx/x × 100 = 0.1/10.0 × 100 = 1%
2. For y:
   Δy/y × 100 = 0.1/5.0 × 100 = 2%
3. For xy:
   Percentage error = 1% + 2%
4. Final Result: Percentage error = 3%.

60. If radius r = 2.0 ± 0.1 cm, find percentage error in area of circle.

The percentage error in area is 10%. Area depends on r².

1. Formula Used: A = πr².
2. Relative Error: ΔA/A = 2Δr/r.
3. Calculation:
   Percentage error = 2 × 0.1/2.0 × 100
   Percentage error = 10%
4. Final Result: Percentage error in area = 10%.

61. If side of a cube is 4.0 ± 0.1 cm, find percentage error in volume.

The percentage error in volume is 7.5%. Volume depends on side cubed.

1. Formula Used: V = a³.
2. Relative Error: ΔV/V = 3Δa/a.
3. Calculation:
   Percentage error = 3 × 0.1/4.0 × 100
   Percentage error = 7.5%
4. Final Result: Percentage error in volume = 7.5%.

62. Express 0.000345 in scientific notation with three significant figures.

The number is 3.45 × 10^-4. Leading zeros are not significant.

1. Original Number: 0.000345.
2. Move Decimal: 3.45.
3. Power of 10: 10^-4.
4. Final Result: 0.000345 = 3.45 × 10^-4.

NCERT Class 11 Physics Chapter 2 Questions

NCERT questions test concepts through unit conversion, error estimation and dimensional checks.
Students should show units in every step to avoid dimensional mistakes.
These NCERT Class 11 Physics Chapter 2 questions follow the 2026 pattern.

63. Why can physical quantities with different dimensions not be added?

Physical quantities with different dimensions cannot be added because addition requires the same physical nature. Length and time cannot form one direct sum.

1. Valid Addition: 2 m + 3 m.
2. Invalid Addition: 2 m + 3 s.
3. Rule: Dimensions must match.
4. Final Result: Only quantities with same dimensions can be added.

64. Why are radians dimensionless?

Radians are dimensionless because angle equals arc length divided by radius. Both have the dimension of length.

1. Formula: θ = arc/radius.
2. Arc Dimension: [L].
3. Radius Dimension: [L].
4. Final Result: Radian has no dimension.

65. What is order of magnitude?

Order of magnitude is the nearest power of 10 to a quantity. It gives a rough scale of size.

1. Example: 2300 ≈ 10³.
2. Use: Quick comparison.
3. Meaning: Approximate power of ten.
4. Final Result: Order of magnitude gives scale.

66. Why should final answers not have too many digits?

Final answers should not have too many digits because measurements have limited precision. Extra digits create false accuracy.

1. Measured Data: Has limited significant figures.
2. Calculation: Cannot improve original precision.
3. Rule: Round according to significant figures.
4. Final Result: Final digits must match measurement precision.

CBSE Class 11 Physics Chapter-Wise Important Questions