Physics-
General
Easy

Question

A solid sphere of radius R has moment of inertia Iabout its geometrical axis. If it is melted into a disc of radius r and thickness t. If its moment of inertia about the tangential axis (which is perpendicular to plane of the disc), is also equal to I, then the value of r is equal to

  1. fraction numerator 2 over denominator square root of 15 end fraction R    
  2. fraction numerator 2 over denominator square root of 5 end fraction R    
  3. fraction numerator 3 over denominator square root of 15 end fraction R    
  4. fraction numerator square root of 3 over denominator square root of 15 end fraction R    

The correct answer is: fraction numerator 2 over denominator square root of 15 end fraction R


    fraction numerator 2 over denominator 5 end fraction M R to the power of 2 end exponent equals fraction numerator 1 over denominator 2 end fraction M r to the power of 2 end exponent plus M r to the power of 2 end exponent
    or fraction numerator 2 over denominator 5 end fraction M R to the power of 2 end exponent equals fraction numerator 3 over denominator 2 end fraction M r to the power of 2 end exponent
    therefore r equals fraction numerator 2 over denominator square root of 15 end fraction R

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