Inspired By Mehul Arora

As Time period oscillation of rigid body is $2 \pi \times\sqrt {\dfrac {l}{g}}$

where $l=\frac{k^{2}}{l_{com}}+l_{com}$ where $k$ is radius of gyration and $l_{com}$ represents distance of $COM$ from point of suspension . And $Mk^{2}=I_{com}$ where is $M$ is mass of rigid body and $I_{com}$ is moment of inertia about axis passing through $COM$ .

Initially Sphere can be considered as uniform solid sphere whose $I_{com}=\frac{2}{5}MR^{2}$ ( $R$ is radius of sphere)

Finally it is hollow sphere whose $I_{com}=\frac{2}{3}MR^{2}$

Clearly $l$ is more for hollow sphere So time period of hollow sphere is more than solid sphere

i.e $T_{1}$ < $T_{2}$ .

So people might think that time period will remain same but which is approximately but exactly it will not same.