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Physics · Work, Power & Energy

JEE Main 2026 — 23 January, Morning Shift — Question 27

A small both A of mass m is attached to a massless rigid rod of length 1 m pivoted at point P and kept at an angle of 60∘60^{\circ} with vertical as shown in figure. At distance of 1 m below point P , an identical bob B is kept at rest on a smooth horizontal surface that extends to a circular track of radius R as shown in figure. If bob B just manages to complete the circular path of radius R upto a point Q after being hit elastically by bob A , then radius R is ____\_\_\_\_ m.

Question figure
  1. Option A:

    35\frac{3}{5}

  2. Option B:

    15\frac{1}{5}

    Correct
  3. Option C:

    2+35\frac{2+\sqrt{3}}{5}

  4. Option D:

    2−35\frac{2-\sqrt{3}}{5}

Answer: B

Step-by-step solution

VA\mathrm{V}_{\mathrm{A}} at lowest point VA=2 gℓ(1−cos⁡θ)\mathrm{V}_{\mathrm{A}}=\sqrt{2 \mathrm{~g} \ell(1-\cos \theta)} VA=2×10×1(1−12)=10\mathrm{V}_{\mathrm{A}}=\sqrt{2 \times 10 \times 1\left(1-\frac{1}{2}\right)}=\sqrt{10} After collision velocity of B becomes, VA=10=VB\mathrm{V}_{\mathrm{A}}=\sqrt{10}=\mathrm{V}_{\mathrm{B}} (Same mass) Now to complete circular motion VB=5gR\mathrm{V}_{\mathrm{B}}=\sqrt{5 \mathrm{gR}} R=15\mathrm{R}=\frac{1}{5}

Solution figure

Answer key and solution verified before publishing.

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Exam
JEE Main 2026
Paper
23 January, Morning Shift
Subject
Physics
Chapter
Work, Power & Energy
Topic
Vertical Circular Motion
A small both A of mass m is attached to a massless rigid rod of… | JEE Main 2026 PYQ with Solution · DhiX AI