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The inertia tensor for an ellipsoid with semi-axes \( a, b \), and \( c \), and mass \( m \) rotated about the center of mass is \( \left[\begin{array}{lc}\frac{1}{5} m\left(b^{2}+c^{2}\right) \\ 0 & \frac{1}{5} m\left(a^{2}+c^{2}\right) \\ 0 & 0 \\ \text { axes if } a=4, b=7, c=8 \text {, and } m=1 \text { ? } \\ \text { Enter the moments of inertia from smallest to largest. } \\ \text { First moment }=\text { Ex: } 1.23 \\ \text { Second moment }=\text { Ex: 1.23 } \\ \text { Third moment }=\text { Ex: 1.23 }\end{array}\right. \)

Ask by Washington Rodriguez. in the United States
Mar 20,2025

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Answer

First moment = 13 Second moment = 16 Third moment = 22.6

Solution

To find the moments of inertia for the ellipsoid with semi-axes \( a = 4 \), \( b = 7 \), \( c = 8 \), and mass \( m = 1 \), we will use the formula for the inertia tensor: \[ I = \begin{bmatrix} \frac{1}{5} m (b^2 + c^2) & 0 & 0 \\ 0 & \frac{1}{5} m (a^2 + c^2) & 0 \\ 0 & 0 & \frac{1}{5} m (a^2 + b^2) \end{bmatrix} \] Now, we will calculate each moment of inertia step by step. 1. **Calculate \( I_{xx} = \frac{1}{5} m (b^2 + c^2) \)**: \[ I_{xx} = \frac{1}{5} \cdot 1 \cdot (7^2 + 8^2) \] 2. **Calculate \( I_{yy} = \frac{1}{5} m (a^2 + c^2) \)**: \[ I_{yy} = \frac{1}{5} \cdot 1 \cdot (4^2 + 8^2) \] 3. **Calculate \( I_{zz} = \frac{1}{5} m (a^2 + b^2) \)**: \[ I_{zz} = \frac{1}{5} \cdot 1 \cdot (4^2 + 7^2) \] Now, let's perform these calculations. ### Step 1: Calculate \( I_{xx} \) \[ I_{xx} = \frac{1}{5} \cdot (49 + 64) = \frac{1}{5} \cdot 113 \] ### Step 2: Calculate \( I_{yy} \) \[ I_{yy} = \frac{1}{5} \cdot (16 + 64) = \frac{1}{5} \cdot 80 \] ### Step 3: Calculate \( I_{zz} \) \[ I_{zz} = \frac{1}{5} \cdot (16 + 49) = \frac{1}{5} \cdot 65 \] Now, let's compute these values. 1. \( I_{xx} = \frac{113}{5} = 22.6 \) 2. \( I_{yy} = \frac{80}{5} = 16 \) 3. \( I_{zz} = \frac{65}{5} = 13 \) Now we have the moments of inertia: - \( I_{xx} = 22.6 \) - \( I_{yy} = 16 \) - \( I_{zz} = 13 \) Finally, we will sort these values from smallest to largest: - \( I_{zz} = 13 \) - \( I_{yy} = 16 \) - \( I_{xx} = 22.6 \) Thus, the moments of inertia from smallest to largest are: 1. **First moment**: \( 13 \) 2. **Second moment**: \( 16 \) 3. **Third moment**: \( 22.6 \) So the final answer is: - First moment = 13 - Second moment = 16 - Third moment = 22.6

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To find the moments of inertia for the ellipsoid with given dimensions \( a = 4 \), \( b = 7 \), \( c = 8 \), and mass \( m = 1 \), we can calculate each moment as follows: 1. **First Moment** (about x-axis): \( \frac{1}{5} m(b^{2} + c^{2}) = \frac{1}{5} \times 1 \times (7^{2} + 8^{2}) = \frac{1}{5} \times 1 \times (49 + 64) = \frac{113}{5} = 22.6 \). 2. **Second Moment** (about y-axis): \( \frac{1}{5} m(a^{2} + c^{2}) = \frac{1}{5} \times 1 \times (4^{2} + 8^{2}) = \frac{1}{5} \times 1 \times (16 + 64) = \frac{80}{5} = 16 \). 3. **Third Moment** (about z-axis): \( \frac{1}{5} m(a^{2} + b^{2}) = \frac{1}{5} \times 1 \times (4^{2} + 7^{2}) = \frac{1}{5} \times 1 \times (16 + 49) = \frac{65}{5} = 13 \). Putting these moments in order from smallest to largest: **First moment** = 13.0 **Second moment** = 16.0 **Third moment** = 22.6

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