Two discs have moments of intertia I 1 and I 2 about their respective axes perpendicular to the plane and…

Two discs have moments of intertia I1 and I2 about their respective axes perpendicular to the plane and passing through the centre. They are rotating with angular speeds, ω1 and ω2 respectively and are brought into contact face to face with their axes of rotation coaxial. The loss in kinetic energy of the system in the process is given by:
  1. I1I2I1+I2ω1-ω22
  2. ω1-ω222I1+I2
  3. I1I22I1+I2ω1-ω22
  4. I1-I22ω1ω22I1+I2

Solution

Angular momentum conservation

I1ω1+I2ω2=I1+I2ω

ω=I1ω1+I2ω2I1+I2

Loss =12I1ω12+12I2ω22-12I1+I2ω2

=12I1ω12+12I2ω22-12I1+I2I1ω1+I2ω2I1+I22

=12I1I2I1+I2ω1-ω22

Ei-Ef=I1I2ω1-ω222I1+I2

Asked in: JEE Main 2021 (27 Aug Shift 2)

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