Q.

A uniform rod AB of mass M is attached to a hinge at one end A, and released from the horizontal position. The rod rotates about A, and when it reaches the vertical position the rod strikes a sphere of mass m and radius r initially at rest on the smooth horizontal surface as shown in the adjacent figure. The impact is along the horizontal direction and perfectly elastic.
If at the moment of impact, the lowest end of the rod is very close to the smooth horizontal surface. After the impact, the sphere moves along the horizontal and the rod, subsequently rises to a maximum of 600 with the vertical. Choose the correct statement(s) from the following, taking into account the information given above. The length of the rod equals 2r.r=6210m(take g =10 m/s2  )
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a

The ratio M/m is 3/2

b

The ratio m/M is 9/2  

c

The ratio m/M is 3/2

d

The ratio m/M is 2/9

answer is A.

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Detailed Solution

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Using conservation of mechanical energy

mgL2=12ML23ω02  ω0=3gL,  ω=3g2L=3g22rL=2r

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(Lr)ω0=vω(Lr)v=(Lr)(ω0+ω)=(Lr)ω(2+1)=rω=3gr2r=6210mv=3×10×6×222×10=3m/s 

Using COAM

lω0=lω+mv(Lr)l(ω0ω)=m(ω0+ω)(Lr)2ML23(ω0ω)=m(ω0+ω)(Lr)2Mm=3(Lr)2L2(ω0ω)=32

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