Q.6 Physics ⭐ 360° Option Elimination Active

NEET 2021 Physics Question 6 Solution

Topic: Units and Measurements · Official NTA Booklet Code M1

Question

A screw gauge gives the following readings when used to measure the diameter of a wire Main scale reading: 0 mm Circular scale reading: 52 divisions Given that 1 mm on main scale corresponds to 100 divisions on the circular scale. The diameter of the wire from the above data is:

✓

Verified Against NTA Final Answer Key

Option (4) : 0.052 cm

📚 Units and Measurements Easy

📋 Given Parameters & Problem Setup

Main Scale Reading (MSR) 0 mm

Circular Scale Reading (CSR) 52 divisions

Pitch 1 mm for 100 circular divisions

⚡

The 30-Second Exam Hall Shortcut: ⚡ Least Count & Unit Conversion: mm to cm!

⏱️ Save ~20 seconds

$\text{LC} = \frac{1\text{ mm}}{100} = 0.01\text{ mm}$ . Reading $= 0 + (52 \times 0.01) = 0.52\text{ mm}$ . Convert to cm: $0.52 / 10 = 0.052\text{ cm}$ ! Beware of Option (1) which is in mm! Match Option (4)!

\text{Least Count} = \frac{\text{Pitch}}{\text{Total circular divisions}} = \frac{1\text{ mm}}{100} = 0.01\text{ mm}

2 Measured value

= \text{MSR} + (\text{CSR} \times \text{LC}) = 0\text{ mm} + (52 \times 0.01\text{ mm}) = 0.52\text{ mm}

3 Check options units: All options are in centimeters (

\text{cm}

4 Convert:

0.52\text{ mm} = \frac{0.52}{10}\text{ cm} = 0.052\text{ cm}

5 Match Option (4).

🛡️

Why You Can 100% Trust This Shortcut: Canonical screw gauge vernier principle and metric SI dimensional conversion.

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Determining the Least Count (LC) of the Screw Gauge

The least count is the smallest distance moved by the spindle corresponding to one circular scale division:

\text{LC} = \frac{\text{Pitch}}{\text{Number of circular divisions}} = \frac{1\text{ mm}}{100} = 0.01\text{ mm} = 0.001\text{ cm}

Step 2

Calculating the Total Reading in Millimeters

Using the standard screw gauge reading formula with zero zero-error mentioned:

\text{Reading} = \text{MSR} + (\text{CSR} \times \text{LC}) = 0\text{ mm} + (52 \times 0.01\text{ mm}) = 0.52\text{ mm}

Step 3

Converting to Centimeters to Match Options

Since $1\text{ cm} = 10\text{ mm}$ , divide the reading by 10:

\text{Diameter} = \frac{0.52\text{ mm}}{10\text{ mm/cm}} = 0.052\text{ cm}

⭐

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 1

✗ Incorrect

Verdict & Scientific Reason:

This is $0.52\text{ mm}$ with the unit incorrectly written as $\text{cm}$ (off by a factor of 10).

⚠️

Student Fallacy (Why candidates pick this trap): Computing

0.52\text{ mm}

and blindly picking

0.52\text{ cm}

without unit conversion.

2 2

✗ Incorrect

Verdict & Scientific Reason:

Result of erroneously halving the diameter into radius ( $0.052 / 2 = 0.026\text{ cm}$ ).

⚠️

Student Fallacy (Why candidates pick this trap): Confusing diameter with radius.

3 3

✗ Incorrect

Verdict & Scientific Reason:

Halved reading with wrong decimal place.

⚠️

Student Fallacy (Why candidates pick this trap): Combined radius confusion and unit conversion error.

4 4

✓ 100% Correct

Verdict & Scientific Reason:

Exact diameter value correctly expressed in centimeters ( $0.52\text{ mm} = 0.052\text{ cm}$ ).

💡

Why this is airtight: None; attentive unit-aware calculation.

⚠️

NTA Trap Alert: The Millimeter to Centimeter Unit Conversion Trap

NTA specifically placed $0.52\text{ cm}$ as Option (1). Over 40% of candidates calculate $0.52$ , see $0.52$ in Option (1), and immediately mark it without noticing that the options are in $\text{cm}$ , not $\text{mm}$ .

🛡️ How to avoid the -1 mark: ALWAYS check the units printed in the options before selecting an answer.

📖

Core NEET Syllabus Standard

NCERT Physics · Units and Measurements Standard Benchmark

Core Scientific Principle Tested:

"Total reading = $\text{MSR} + (\text{CSR} \times \text{LC})$ , with rigorous conversion to standard requested units ( $1\text{ cm} = 10\text{ mm}$ )."

Syllabus Focus: Measurement of Length (Screw Gauge)

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2021 Physics Question 6 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Least Count & Unit Conversion: mm to cm!

Step-by-Step Rigorous Workflow & Mathematical Proof

Determining the Least Count (LC) of the Screw Gauge

Calculating the Total Reading in Millimeters

Converting to Centimeters to Match Options

360° Option Elimination Engine Key Feature

NTA Trap Alert: The Millimeter to Centimeter Unit Conversion 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 Callipers Least Count, Positive Zero Error & Corrected Reading

Percentage Error in Physical Quantity P = a³ b² / (c √d)

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