Question Details

For a uniform cylinder of length L and radius R the moment of inertia is I₁. Now for similar situation but length L/2 and radius R/2 moment of inertia is I₂.

Find I₁ / I₂.



Options

A

32


B

8


C

1/4


D

16

Show Answer

Correct Answer :

Option A

32


32

Solution :

The correct answer is 32.

Step-by-step Explanation:
Let us consider a uniform solid cylinder of mass M, length L, and radius R. As observed in the provided image, the axis of rotation for both cylinders is perpendicular to their length and passes through their respective centers of mass.

The moment of inertia of a uniform solid cylinder about an axis perpendicular to its length and passing through its center is given by the formula:
I = M ( R 2 4 + L 2 12 )

Since both cylinders are made of the same uniform material, they have the same mass density ρ. The mass of a cylinder is equal to its density multiplied by its volume:
M = ρ V = ρ π R 2 L

1. For the first cylinder:
Length = L, Radius = R
The mass is:
M 1 = ρ π R 2 L
The moment of inertia is:
I 1 = M 1 ( R 2 4 + L 2 12 )

2. For the second cylinder:
Length = L2, Radius = R2
The mass is:
M 2 = ρ π ( R 2 ) 2 ( L 2 ) = ρ π R 2 4 · L 2 = ρ π R 2 L 8 = M 1 8
The moment of inertia is:
I 2 = M 2 [ ( R / 2 ) 2 4 + ( L / 2 ) 2 12 ]

Substitute M2=M18 into the equation:
I 2 = M 1 8 [ R 2 16 + L 2 48 ]

Factor out 14 from the terms inside the brackets:
I 2 = M 1 8 · 1 4 [ R 2 4 + L 2 12 ]

I 2 = 1 32 M 1 [ R 2 4 + L 2 12 ] = I 1 32

3. Finding the ratio:
I 1 I 2 = 32

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