Question

Three identical sperical shells, each of mass m and radius r are placed as shown in figure. Consider an axis XX' which is touching to two shells and passing through diameter of third shell. Moment of inertia of the system consisting of these three spherical shells about XX' axis is:

• A. $3m{r}^2$
• B. $\frac{16}{5}m{r}^2$
• C. $4m{r}^2$
• D. $\frac{11}{5}m{r}^2$

Answer 1

The correct option is C $4m{r}^2$

Moment of inertia of shell about its diameter, $I_1=\frac{2}{3}m{r}^2$
$\therefore I_2=I_3=I_1+m{r}^2=\frac{2}{3}m{r}^2+m{r}^2=5/3m{r}^2$
$\therefore I=2\times \frac{5}{3}m{r}^2+\frac{2}{3}m{r}^2=4m{r}^2$