Q.
Point masses m1 and m2 are placed at the opposite ends of a rigid rod of length L, and negligible mass. The rod is to be set rotating about an axis perpendicular to it. The position of point P on this rod through which the axis should pass so that the work required to set the rod rotating with angular velocity ω0 is minimum, is given by
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a
x=m2m1L
b
x=m2Lm1+m2
c
x=m1Lm1+m2
d
x=m1m2L
answer is B.
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Detailed Solution
Moment of inertia of the system about the axis of rotation (through point P ) isI=m1x2+m2(L-x)2 By work energy theorem, Work done to set the rod rotating with angular velocity ω0= Increase in rotational kinetic energy W=12Iω02=12m1x2+m2(L-x)2ω02 For W to be minimum, dWdx=0 i.e. 122m1x+2m2(L-x)(-1)ω02=0 or m1x-m2(L-x)=0 ∵ω0≠0 or m1+m2x=m2L or x=m2Lm1+m2
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