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Bob B of mass m at rest is hanging vertically from the ceiling via a massless string of length 10 m, as shown in the figure. Point mass A of mass m travelling horizontally with speed 10 m s⁻¹ hits bob B elastically. The bob B rises h meter after the collision. Taking the acceleration due to gravity g=10 m s⁻² and neglecting the size of the bob, the value of h is :

Asked in Re-NEET 2026 · Velocity exchange and mass ratios

• (1) 8
• (2) 7
• (3) 5
• (4) 2.5

Step-by-step solution

Why the other options are wrong

• (1) No single slip gives 8 m; B leaves with 10 m/s and (v²)/(2g)=5 m.
• (2) No single slip gives 7 m; B leaves with 10 m/s and (v²)/(2g)=5 m.
• (4) No single slip gives 2.5 m; even a perfectly inelastic collision (5 m/s) would give 1.25 m, and here B takes all of A's speed.

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