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RE-NEET2024Physics-Circular Motion

RE-NEET 2024 Physics Centripetal Force MCQ Question

Type: MCQ-numerical-Medium-Class 11

A bob is whirled in a horizontal circle by means of a string at an initial speed of 10 rpm. If the tension in the string is quadrupled while keeping the radius constant, the new speed is:

A

20 rpm

B

40 rpm

C

5 rpm

D

10 rpm

Correct Answer

Option A

Detailed Explanation

Chapter: Motion in a Plane — Class 11 Physics Topic: Centripetal Force / Circular Motion Difficulty: 🟡 Medium

✅ Ans: A — 20 rpm

For horizontal circular motion, tension provides centripetal force:

T=mv2r=mω2rT=\frac{mv^2}{r}=m\omega^2r

Since mm and rr are constant:

T∝ω2T\propto \omega^2

Given:

T2=4T1T_2=4T_1

Therefore,

ω22ω12=4\frac{\omega_2^2}{\omega_1^2}=4 ω2=2ω1\omega_2=2\omega_1

Initial speed =10=10 rpm:

ω2=20 rpm\boxed{\omega_2=20\text{ rpm}}

📌 NCERT: Centripetal force Fc=mω2r=mv2rF_c=m\omega^2r=\frac{mv^2}{r}. (CloudFront)

🧠 NEET Trick: Tension ×4 → speed ×√4 = ×2. So 10 → 20 rpm.

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