Question

A circular disc of radius R/3 is cut from a circular disc of radius R and mass 9 M as shown. The MOI of the remaining disc about O perpendicular to the disc is

• A. 4 MR²
• B. 9 MR²
• C. 37/9 MR²
• D. 40/9 MR²

Answer 1

Answer: (A)

4 MR²

Answer 2

I = $\frac { 9 \mathrm { MR } ^ { 2 } } { 2 } - \left[ \frac { \mathrm { M } } { 2 } \left( \frac { \mathrm { R } } { 3 } \right) ^ { 2 } + \mathrm { M } \left( \frac { 2 \mathrm { R } } { 3 } \right) ^ { 2 } \right]$ = 4MR²

mass of hole made = M = $\frac { 9 \mathrm { M } } { \pi \mathrm { R } ^ { 2 } } \left[ \pi \left( \frac { \mathrm { R } } { 3 } \right) ^ { 2 } \right]$