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A disc of metal is melted to recast in the form of a solid sphere. The moment of inertias about a vertical axis passing through the centre would -
  • a)
     decrease
  • b)
    increase
  • c)
     remains same
  • d)
    nothing can be said
Correct answer is option 'A'. Can you explain this answer?
Most Upvoted Answer
A disc of metal is melted to recast in the form of a solid sphere. The...
Moment of inertia will decrease, because
Id=1/2mr2 and Is=2/5mr2,
 the radius of sphere formed on recasting the disc will also decrease.
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Community Answer
A disc of metal is melted to recast in the form of a solid sphere. The...
Moment of Inertia:
The moment of inertia is a measure of an object's resistance to changes in its rotational motion. It depends on the mass distribution of the object and the axis of rotation. For a solid sphere, the moment of inertia can be calculated using the formula:

I = (2/5) * m * r^2

Where I is the moment of inertia, m is the mass of the sphere, and r is the radius of the sphere.

Moment of Inertia of a Disc:
The moment of inertia of a disc can be calculated using the formula:

I = (1/4) * m * r^2

Where I is the moment of inertia, m is the mass of the disc, and r is the radius of the disc.

Conversion of Disc to Sphere:
When the disc is melted and recast into the form of a solid sphere, the mass remains the same. However, the mass distribution changes as the disc is transformed into a sphere.

Effect on Moment of Inertia:
The moment of inertia of a sphere is given by the formula:

I = (2/5) * m * r^2

Comparing this with the moment of inertia of the disc, we can see that the moment of inertia of the sphere is greater.

Therefore, by recasting the disc into a solid sphere, the moment of inertia about a vertical axis passing through the center increases.

Explanation:
When the disc is melted and recast into a solid sphere, the mass remains the same. However, the distribution of mass changes. In the case of a disc, the mass is concentrated towards the outer edge, resulting in a smaller moment of inertia. On the other hand, in the case of a sphere, the mass is distributed more evenly, resulting in a larger moment of inertia.

So, by recasting the disc into a solid sphere, the moment of inertia about a vertical axis passing through the center increases.

Conclusion:
The moment of inertia about a vertical axis passing through the center would increase when a disc of metal is melted and recast in the form of a solid sphere.
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