Q42 / 122 · NTA Abhyas (JEE Main) 2020 · Physics · Rotational Motion

A thin circular metal disc of the radius 500 mm is set rotating about a central axis normal to its plane. Upon raising in temperature gradually, the radius increases to 507.5 mm. The percentage change in the rotational kinetic energy will be

  • A
    1.5%
  • B
    -1.5%
  • C
    3%
  • Correct answer
    D
    -3%

Solution

Given   dRR×100=507.5-500.0500.0×100=1.5%

Now,   K=12Iω2=12IIω2=L22I

or,   K=L22MR22=L2MR2

Taking log and differentiating, dK K = - 2 dR R

or, the percentage change in K = dK × 1 0 0 K = - 2 dR R × 1 0 0 = - 3 %

Hence, the correct answer is option: 'D'.

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