Units and Measurements — Class 11 Physics NCERT Solutions (Free)

Free step-by-step NCERT solutions for Class 11 Physics chapter "Units and Measurements" — 8 important questions with detailed answers for CBSE board exam preparation.

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TL;DR: Free step-by-step NCERT solutions for Class 11 Physics chapter "Units and Measurements" — 8 important questions with detailed answers for CBSE board e…

Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated Jul 23, 2026

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Key Questions Covered:

Define the SI unit of length and explain how the metre is defined.
A student measures the thickness of a single sheet of paper using a screw gau…
Express the following in scientific notation with appropriate significant fig…
The radius of a circular disc is measured as 5.21 ± 0.05 cm. Calculate the ar…
What is the difference between accuracy and precision? Give an example of each.
Using Vernier callipers with least count 0.01 cm, the main scale reading is 2…
+ 2 more questions in the full chapter

Solutions Summary:

Question	Status
Define the SI unit of length and explain how the metre is…	✓ Solved
A student measures the thickness of a single sheet of pap…	✓ Solved
Express the following in scientific notation with appropr…	✓ Solved
The radius of a circular disc is measured as 5.21 ± 0.05 …	✓ Solved
What is the difference between accuracy and precision? Gi…	✓ Solved
Using Vernier callipers with least count 0.01 cm, the mai…	✓ Solved

Showing 6 of 8 questions

Q1: Define the SI unit of length and explain how the metre is defined.

The SI unit of length is the metre (m). Definition: The metre is defined as the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second. This definition was adopted at the 17th General Conference on Weights and Measures (CGPM) in 1983. It is based on the speed of light, which is a fundamental constant of nature (c = 299,792,458 m/s). Advantages of this definition: - It relates the metre to a fundamental physical constant - It is reproducible in any ...

Q2: A student measures the thickness of a single sheet of paper using a screw gauge. If the main scale reading is 0 mm and the circular scale reading is 45 divisions (least count = 0.01 mm), calculate the thickness of the paper.

Given: Main scale reading (MSR) = 0 mm Circular scale reading (CSR) = 45 divisions Least count (LC) = 0.01 mm Formula: Reading = MSR + (CSR × LC) Step 1: Identify the main scale reading MSR = 0 mm Step 2: Calculate the circular scale contribution CSR contribution = 45 × 0.01 mm = 0.45 mm Step 3: Add to get total reading Thickness = 0 + 0.45 = 0.45 mm Answer: The thickness of the paper is 0.45 mm or 4.5 × 10⁻⁴ m

Q3: Express the following in scientific notation with appropriate significant figures: (a) 0.000456 kg (b) 45000 m (c) 0.0050 A

Step 1: Identify significant figures and convert to scientific notation (a) 0.000456 kg - Significant figures: 3 (the zeros before 4 are not significant) - Scientific notation: 4.56 × 10⁻⁴ kg (b) 45000 m - Significant figures: 2 (assuming only 4 and 5 are significant; trailing zeros without decimal point are ambiguous) - Scientific notation: 4.5 × 10⁴ m - If all zeros are significant: 4.5000 × 10⁴ m (c) 0.0050 A - Significant figures: 2 (the two zeros before 5 are not significant; the zero af...

Q4: The radius of a circular disc is measured as 5.21 ± 0.05 cm. Calculate the area of the disc with proper uncertainty.

Given: Radius r = 5.21 ± 0.05 cm Formula: Area A = πr² Step 1: Calculate central value of area A = π × (5.21)² A = π × 27.1441 A = 85.26 cm² Step 2: Calculate relative error in radius Δr/r = 0.05/5.21 = 0.0096 or 0.96% Step 3: Calculate relative error in area For A = πr², we have ΔA/A = 2(Δr/r) ΔA/A = 2 × 0.0096 = 0.0192 or 1.92% Step 4: Calculate absolute error in area ΔA = A × (ΔA/A) = 85.26 × 0.0192 = 1.64 cm² Step 5: Round to appropriate significant figures ΔA ≈ 1.6 cm² or 2 cm² (round...

Q5: What is the difference between accuracy and precision? Give an example of each.

Accuracy and precision are related but different concepts in measurement: Accuracy: - It refers to how close a measured value is to the true or accepted value - It is a measure of correctness - It depends on the skill of the experimenter and the quality of the apparatus - Example: If the true length of a rod is 10.00 cm and your measurement gives 10.02 cm, your measurement is accurate (within 0.02 cm of true value) Precision: - It refers to how close repeated measurements are to each other (re...

Q6: Using Vernier callipers with least count 0.01 cm, the main scale reading is 2.3 cm and Vernier scale reading is 7. Calculate the measurement and state the number of significant figures.

Given: Main scale reading (MSR) = 2.3 cm Vernier scale reading (VSR) = 7 Least count (LC) = 0.01 cm Formula: Total reading = MSR + (VSR × LC) Step 1: Calculate Vernier contribution Vernier contribution = 7 × 0.01 = 0.07 cm Step 2: Add to main scale reading Total reading = 2.3 + 0.07 = 2.37 cm Step 3: Identify significant figures - The main scale reading 2.3 cm has 2 significant figures - The Vernier reading adds precision to the third decimal place - Total significant figures: 3 (2, 3, and 7...

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