A circular disc of radius R and thickness R/6 has moment of inertia I ...
Understanding Moment of Inertia
The moment of inertia (I) of an object quantifies its resistance to rotational motion. For a circular disc and a solid sphere, the formulas differ based on their geometry.
Step 1: Moment of Inertia of the Disc
- A circular disc of radius R and thickness R/6 has a moment of inertia about its center (I) given by the formula:
I = (1/2) * M * R², where M is the mass of the disc.
- The volume (V) of the disc can be calculated as:
V = π * R² * (R/6) = (π/6) * R³.
- The density (ρ) of the disc is:
ρ = M/V.
- From density, we can express mass (M) in terms of volume and density.
Step 2: Melting and Recasting
- When the disc is melted and recast into a solid sphere, the mass remains the same.
- The volume of the sphere (V_sphere) is:
V_sphere = (4/3) * π * r³, where r is the radius of the sphere.
- Setting the volumes equal gives:
(π/6) * R³ = (4/3) * π * r³.
- From this, we can find the radius (r) of the sphere in terms of R.
Step 3: Moment of Inertia of the Sphere
- The moment of inertia (I_sphere) of a solid sphere about its diameter is given by:
I_sphere = (2/5) * M * r².
- Since the mass (M) is the same for both the disc and the sphere, substituting r into this formula allows us to express I_sphere in terms of I.
Final Calculation
- After substituting and simplifying, we find that:
I_sphere = I/5.
Thus, the correct answer is option 'C'.
A circular disc of radius R and thickness R/6 has moment of inertia I ...
Moment of inertia of a disc,
I=1/2MR
2Disc is melted and recasted into a solid sphere.
