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## Homework Statement

- What is the moment of inertia of a spinning object of radius 0.5 m and mass 6 kg moving at 5 m/s, if it has a kinetic energy of 100 J?

- 1) 1 kgm2

- 2) 2 kgm2

- 3) 4 kgm2

- 4) 8 kgm2

- 5) 20 kgm2

- 1) 1 kgm2

## Homework Equations

K.E. = Kinetic energy of rotation + kinetic energy of translation.

## The Attempt at a Solution

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Hi everyone. I keep trying to solve this and I get the answer to be 0.5 kgm^2.

Obviously, this is not an answer. They say that the answer is 2).

Here's what I did:

(1 / 2) * m * v^2 + (1 /2) * I * omega^2 = 100.

(1 / 2) * 6 * 5^2 + (1 /2) * I * (v^2 / r^2)^2 = 100.

Rearrange to get I = (r^2 / v^2 ) * 50 = 0.5

Why do I not stack up with what the authors have written here? Thanks in advance for the help!