Question

Two circular discs A and B are of equal masses and thicknesses but made of metal with densities $d_A$ and $d_B$ ($d_A$ $>$ $d_B$). If their moments of inertia about an axis passing through their centres and perpendicular to circular faces are $I_A$ and $I_B$

• A. $I_A=I_B$
• B. $I_A>I_B$
• C. $I_A<I_B$
• D. $I_A\ge I_B$

Answer 1

The correct option is C $I_A<I_B$

Since the masses of the two discs are equal we see that for the disc A which has a higher density will have smaller radius to keep the product of volume x density equal.
Thus $r_A<r_B$, thus we get $I_A<I_B$