Q.25 Physics ⭐ 360° Option Elimination Active

NEET 2025 Physics Question 25 Solution

Topic: Units and Measurements · Official NTA Booklet Code 45

Question

Consider the diameter of a spherical object being measured with the help of a Vernier callipers. Suppose its 10 Vernier Scale Divisions (V.S.D.) are equal to its 9 Main Scale Divisions (M.S.D.). The least division in the M.S. is 0.1 cm and the zero of V.S. is at x = 0.1 cm when the jaws of Vernier callipers are closed. If the main scale reading for the diameter is M = 5 cm and the number of coinciding vernier division is 8, the measured diameter after zero error correction, is

✓

Verified Against NTA Final Answer Key

Option (3) : 4.98 cm

📚 Units and Measurements Easy

📋 Given Parameters & Problem Setup

Vernier Division Relation 10 V.S.D. = 9 M.S.D.

Main Scale Division (MSD) 0.1 cm (1 mm)

Zero Error Position Zero of Vernier scale is at x = +0.1 cm when jaws are closed

Observed Measurement Main Scale Reading MSR = 5 cm, Coinciding Vernier Division VSR = 8

⚡

The 30-Second Exam Hall Shortcut: ⚡ Vernier Zero-Correction Gold Standard

⏱️ Save ~40 seconds

LC = 0.1 / 10 = 0.01 cm. Observed = 5 + (8 × 0.01) = 5.08 cm. Positive Zero Error = +0.1 cm ⟹ Corrected = 5.08 - 0.10 = 4.98 cm!

1 Calculate Least Count: LC = 1 MSD - 1 VSD = 0.1 cm - (9/10 × 0.1 cm) = 0.01 cm.

2 Compute Observed Reading: Observed = MSR + (VSR × LC) = 5 cm + (8 × 0.01 cm) = 5.08 cm.

3 Identify Zero Error: Vernier zero is at +0.1 cm (ahead to the right) ⟹ Positive Zero Error = +0.1 cm.

4 Apply Zero Correction: Corrected Reading = Observed - (Zero Error) = 5.08 - 0.10 = 4.98 cm.

5 Match Option (3) in under 15 seconds!

🛡️

Why You Can 100% Trust This Shortcut: Dimensional Analysis & Error Propagation (NCERT Physics · Units and Measurements).

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Calculate Least Count (Vernier Constant)

The least count (LC) of a vernier calliper is the difference between one main scale division and one vernier scale division:

10\text{ VSD} = 9\text{ MSD} \implies 1\text{ VSD} = 0.9\text{ MSD} = 0.9 \times 0.1\text{ cm} = 0.09\text{ cm}

• LC = 1 MSD - 1 VSD = 0.1 cm - 0.09 cm = 0.01 cm.

Step 2

Calculate Observed Reading

The total observed reading before zero error correction is given by the main scale reading plus the vernier scale contribution:

\text{Observed Reading} = \text{MSR} + (\text{VSR} \times \text{LC}) = 5\text{ cm} + (8 \times 0.01\text{ cm}) = 5.08\text{ cm}

• Raw measurement on the scales reads 5.08 cm.

Step 3

Apply Zero Error Correction

When the jaws are closed, the zero mark of the vernier scale lies at +0.1 cm to the right of the main scale zero. This is a positive zero error (+0.1 cm). The true reading is obtained by subtracting the zero error:

\text{Corrected Reading} = \text{Observed Reading} - \text{Zero Error} = 5.08\text{ cm} - (+0.10\text{ cm}) = 4.98\text{ cm}

• Matches Option (3) with absolute precision.

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 A

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Adds the zero error instead of subtracting it (5.08 + 0.10 = 5.18 cm).

⚠️

Student Fallacy (Why candidates pick this trap): Mistakenly adding positive zero error instead of subtracting.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. This is the uncorrected observed reading, ignoring the zero error entirely.

⚠️

Student Fallacy (Why candidates pick this trap): Forgetting to apply zero error correction.

3 C

✓ 100% Correct

Verdict & Scientific Reason:

100% correct. Corrected = 5.08 cm - 0.10 cm = 4.98 cm.

💡

Why this is airtight: None; textbook experimental physics.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Omits the vernier scale division contribution (5 - 0 + 0).

⚠️

Student Fallacy (Why candidates pick this trap): Ignoring VSR reading.

⚠️

NTA Trap Alert: Adding vs. Subtracting Zero Error Trap

Option (1) (5.18 cm) is the deliberate trap for students who add the positive zero error. Remember the golden rule of experimental physics: TRUE VALUE = OBSERVED - (ZERO ERROR). If error is positive, you SUBTRACT it!

🛡️ How to avoid the -1 mark: Always write the formula: True = Observed - Error. A positive error must be subtracted!

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Core NEET Syllabus Standard

NEET Physics Core Syllabus · Units and Measurements Standard Benchmark

Core Scientific Principle Tested:

"Corrected reading = MSR + (VSR × LC) - (Zero Error)."

Syllabus Focus: Measurement of Length · Vernier Callipers & Zero Error

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Positive Error means instrument reads TOO HIGH ⟹ SUBTRACT it: 5.08 - 0.10 = 4.98 cm!

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NEET 2025 Physics Question 25 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Vernier Zero-Correction Gold Standard

Step-by-Step Rigorous Workflow & Mathematical Proof

Calculate Least Count (Vernier Constant)

Calculate Observed Reading

Apply Zero Error Correction

360° Option Elimination Engine Key Feature

NTA Trap Alert: Adding vs. Subtracting Zero Error Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Units and Measurements

Vernier Calliper Negative Zero Error & Full Marks Bonus Audit

Dimensional Analysis of Combined Thermal Radiation Constants (sigma_s * k_B^-1 * b)

Vernier Constant of Vernier Calipers with Variable Divisions

Dimensionless Physical Quantities Matching Solid Angle

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