NEET 2025 · PhysicsPrevious Year Question
A bob of heavy mass m is suspended by a light string of length l. The bob is given a horizontal velocity v₀ at the lowest point. If the string goes slack at some point P making an angle θ from the horizontal, the ratio of the speed v of the bob at P to its initial speed v₀ is
- A.√(cosθ/(2+3sinθ))
- B.√(sinθ/(2+3sinθ))✓
- C.½sinθ
- D.½√(1/(2+3sinθ))
Correct Answer
(B) √(sinθ/(2+3sinθ))
Solution & Explanation
Correct Answer (B): √(sinθ/(2+3sinθ)) Solution: At slack point tension=0: component of gravity supplies centripetal force, mgsinθ=mv²/l⇒ v²=glsinθ. Energy conservation from lowest point to P (height =l+lsinθ): ½ v₀²=½ v²+gl(1+sinθ). Solving, v₀²=glsinθ+2gl(1+sinθ)=gl(2+3sinθ), so v/v₀=√(sinθ/(2+3sinθ)).
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