Units and Measurements Class 11 Physics NCERT Solutions | Step-by-Step Answers, Short Notes & Mnemonics for NEET

- Dr.Sanjaykumar pawar 

Units and Measurements Class 11 Physics – Easy NCERT Solutions, Short Notes, Formulas, and NEET Revision Tricks

 Internal Links

Physics Class 11 Chapter 1 Physical World Notes

Measurement Errors Complete Notes

Significant Figures Explained

Vernier Calipers Complete Guide

Screw Gauge Complete Guide

Least Count Formula Explained

Dimensional Analysis Notes

NCERT Class 11 Physics Solutions

NEET Physics Short Notes

NEET Physics Formula Handbook

Class 11 Physics Important Questions

Physics MCQs with Solutions

NCERT Exemplar Physics Solutions

JEE Physics Revision Notes

Measurement Instruments Comparison

UNITS AND MEASUREMENTS

Question (a)

You are given a thread and a metre scale. How will you estimate the diameter of the thread?

Step-by-Step Answer

Step 1: Take a pencil or a cylindrical rod.

Step 2: Wind the thread closely around the pencil without leaving any gap.

Step 3: Make 20–50 turns of the thread.

Step 4: Measure the total length (L) of all turns using the metre scale.

Step 5: Count the total number of turns (N).

Diameter of Thread = Total Length (L) / Number of Turns (N)

Answer: Divide the total length of all turns by the number of turns to obtain the diameter of the thread.

Question (b)

A screw gauge has a pitch of 1.0 mm and 200 divisions on the circular scale. Do you think it is possible to increase the accuracy of the screw gauge arbitrarily by increasing the number of divisions on the circular scale?

Step-by-Step Answer

Step 1: Use the formula

Least Count = Pitch / Circular Scale Divisions

Step 2:

Least Count = 1.0 / 200 = 0.005 mm

Step 3: Increasing the number of divisions decreases the least count.

Step 4: However, accuracy cannot increase forever because of:

• Human error
• Instrument error
• Manufacturing limitations
• Backlash error

Final Answer:

No. Accuracy cannot be increased arbitrarily by increasing the number of divisions.

Question (c)

The mean diameter of a thin brass rod is to be measured using Vernier Calipers. Why is a set of 100 measurements more reliable than only 5 measurements?

Step-by-Step Answer

Step 1: Every measurement contains small random errors.

Step 2: With only 5 readings, random errors affect the average more.

Step 3: With 100 readings, positive and negative errors cancel each other.

Step 4: Therefore, the mean value becomes closer to the true value.

More Measurements → Less Random Error → Better Mean Value

Short Notes (Quick Revision)

Topic	Key Point
Thread Diameter	Wind the thread around a pencil and use Length ÷ Turns.
Screw Gauge	Measures very small thickness like wire diameter.
Least Count	Least Count = Pitch ÷ Circular Divisions.
Vernier Calipers	Measures external diameter, internal diameter and depth.
Repeated Measurements	More readings reduce random errors.

Remember:

• More Readings = More Accuracy
• Smaller Least Count = Better Precision
• Screw Gauge is more accurate than Vernier Calipers.

Mnemonics (Easy Memory Tricks)

1. Instrument Order

Mnemonic:

Meter → Vernier → Screw

Sentence:

"My Very Smart Friend"

Word	Meaning
My	Meter Scale
Very	Vernier Calipers
Smart	Screw Gauge

2. Accuracy Order

Mnemonic:

Meter < Vernier < Screw

Sentence:

"Accuracy Climbs Up"

Instrument	Accuracy
Meter Scale	Low
Vernier Calipers	Medium
Screw Gauge	Highest

3. Formula Memory Trick

L ÷ N = Diameter

Mnemonic:

"Length Needs Number"

Formula	Meaning
L	Total Length
N	Number of Turns
L ÷ N	Diameter

4. NEET MCQ Trick

If Question Says	Remember
More Readings	Better Mean Value
Increase Circular Divisions	Accuracy improves only up to practical limits.
Least Count	Smaller LC → Better Precision