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A small bob 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° 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.

Asked in JEE Main 23rd Jan 1st Shift 2026 · Energy in vertical circular motion

• (1) (2-√3)/5
• (2) 1/5
• (3) (2+√3)/5
• (4) 3/5

Step-by-step solution

Why the other options are wrong

• (1) Comes from using 1-cos 30° for the drop of bob A; the rod makes 60° with the vertical.
• (3) Comes from using 1+cos 30° for the drop, an angle and a sign both wrong.
• (4) Comes from writing the drop as L(1+cos 60°) instead of L(1-cos 60°).

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