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Q.22 Physics · Section A ⭐ 360° Option Elimination Active
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NEET 2024 Physics Question 22 Solution

Topic: Laws of Motion · Official NTA Booklet Code T1
NEET 2024 Physics Question 22: Tension in a Whirled String with Scaled Angular Velocity

Question

A bob is whirled in a horizontal plane by means of a string with an initial speed of ω rpm. The tension in the string is T. If speed becomes 2ω while keeping the same radius, the tension in the string becomes:

✓
Verified Against NTA Final Answer Key
Option (4) : 4T
📚 Laws of Motion Easy
📋 Given Parameters & Problem Setup
Initial angular velocity ω rpm
Initial string tension T
New angular velocity 2ω rpm (doubled)
Radius of circle Same radius r
⚡

The 30-Second Exam Hall Shortcut: ⚡ Centripetal Force Square Rule

⏱️ Save ~40 seconds

T = m ω² r -> T ∝ ω² -> When ω is doubled (2ω), tension quadruples: (2)² = 4T!

1 In a horizontal circle, tension provides the centripetal force: T = m ω² r.
2 Radius r and mass m remain constant.
3 Tension is directly proportional to the square of angular velocity: T ∝ ω².
4 Doubling the angular speed: T_new = m (2ω)² r = 4 (m ω² r) = 4T.
5 Option (4) in under 3 seconds!
🛡️
Why You Can 100% Trust This Shortcut: Centripetal Force Dynamics in Horizontal Circular Motion (F_c = m ω² r).
📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Equation of Motion in Horizontal Plane

For a mass m whirled in a horizontal plane of radius r on a frictionless surface, the tension T in the string provides the necessary centripetal force directed toward the center of the circle:

T=mω2rT = m \omega^2 r
Step 2

Scaling with New Angular Velocity

When the speed is increased to ω_new = 2ω while keeping the radius r unchanged, the new tension T_new becomes:

Tnew=mωnew2r=m(2ω)2rT_{new} = m \omega_{new}^2 r = m (2\omega)^2 r
Step 3

Expansion and Substitution

Expanding the squared term:

Tnew=m(4ω2)r=4(mω2r)=4TT_{new} = m (4\omega^2) r = 4 (m \omega^2 r) = 4 T
⭐

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:

Inverts the relationship (would occur if speed were halved).

⚠️
Student Fallacy (Why candidates pick this trap): Dividing by 4 instead of multiplying.
2 (2)
✗ Incorrect
Verdict & Scientific Reason:

Incorrect exponent; occurs if T ∝ √ω was used.

⚠️
Student Fallacy (Why candidates pick this trap): Confusing period formula with centripetal force formula.
3 (3)
✗ Incorrect
Verdict & Scientific Reason:

Ignores speed increase.

⚠️
Student Fallacy (Why candidates pick this trap): Assuming tension is independent of speed.
4 (4)
✓ 100% Correct
Verdict & Scientific Reason:

Direct consequence of T ∝ ω²: (2)² = 4.

💡
Why this is airtight: None.
⚠️

NTA Trap Alert: Linear vs Quadratic Scaling

Students in a rush sometimes write T ∝ ω and pick 2T (though 2T was not among the options here, keeping it foolproof).

🛡️ How to avoid the -1 mark: Always remember: centripetal acceleration has velocity squared: a = v²/r = ω²r.
📖

Core NEET Syllabus Standard

Physics Core Mechanics · Laws of Motion Standard Benchmark
Core Scientific Principle Tested:

"Centripetal force is directly proportional to the square of angular velocity (F_c = m ω² r)."

Syllabus Focus: Circular Motion and Centripetal Force
✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗
Double the spin, square the pull: 2 squared is 4, tension is full!

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⚖️ Legal & Fair Dealing Notice: Sourced from publicly released NEET 2024 question papers and NEET answer keys for private study, research, and non-commercial educational practice under Fair Dealing (Section 52(1)(a) of the Indian Copyright Act, 1957). Not affiliated with or endorsed by NTA or NMC.

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