A bob of simple pendulum executes shm in water


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The bob of a simple pendulum executes SHM in water with time period T, while its period of oscillation is T' in air. Given that density of the bob is 4000/3 kg m^(-3) . Find the relation between T andT' by equating torque.

Answered by admin @



Let   V be the volume of the bob.Thus mass of the bob       M=ρbV=43×1000VkgAcceleration due to gravity in air is   g.Effective weight of the bob in water       Mg′=Mg−B       where    buoyant force   B=(Vρwater)g43×1000Vg′=43×1000Vg−V×1000g                       (as ρwater=1000kg/m3)
43×1000Vg′=13×1000Vg  g′=g4   
Now  time period of the bob in air           to=2πlgTime period of the bob in water          t=2πlg′  =2π4lg    =2×2πlg 
⟹t=2to

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