A spring mass system (mass ‘m’, spring constant ‘k’ and natural length l0 ) rest in equilibrium on a horizontal disc. The free end of the spring is fixed at the centre of disc. If 5g mass of the body attached to the spring having spring constant 20Nm−1. If the disc together with spring mass system, rotates about its axis with an angular velocity of 5 rad.s−1 . k>>mω2 , then the percentage change in the length of the spring is
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a
6.25
b
0.625
c
0.0625
d
62.5
answer is B.
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Detailed Solution
kΔl←m→ml0+Δlω2 from free body diagram, forces acting on mass m are,(k=force constant), restoring force of spring F=-kx=-kΔl, towards the centre here Δl is extension in length of spring and centrifugal force F=mw2l0+Δl, away from the centre, l0=unstretched length of spring equating the above two equations kΔl=mw2l0+Δl kΔl-mw2Δl=mw2l0 Δl=mw2l0k-mw2 Δll0=mw2k-mw2≃mw2k 100(Δll0)=(mw2k)100 Δll0%=(5x10-3x5220)100 Δll0%=0.625