A thin disc of mass M and radius R has mass per unit area σ r = k r 2 where r is the distance from its…

A thin disc of mass M and radius R has mass per unit area σr=kr2 where r is the distance from its centre. Its moment inertia about an axis going through its centre of mass and perpendicular to its plane is:
  1. MR23
  2. MR22
  3. MR26
  4. 2MR23

Solution


dm=σdA=σ2πrdr
=2πkr3dr
M=dm=2πk0Rr3dr
=2πkR44=πkR42
dI=dmr2=2πkr5dr
I=2πk0Rr5dr=2πkR66=2πkR63×2

since,  M=πkR42

so,
I=2MR23

Asked in: JEE Main 2019 (10 Apr Shift 1)

Practice more Rotational Motion questions on Aicharya