Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (3rd Edition)

Published by Pearson
ISBN 10: 0321740904
ISBN 13: 978-0-32174-090-8

Chapter 12 - Rotation of a Rigid Body - Exercises and Problems - Page 350: 55

Answer

$I= \dfrac{M}{ 3 L} \left[ (L-d)^3+d^3 \right]$

Work Step by Step

We know that the moment of inertia is given by $$I=\int x^2 dm\tag 1$$ Since the mass of uniformly distributed through the length of the rod, then its density is given by $$\rho =\dfrac{M}{V}=\dfrac{dm}{dV}$$ Hence, $$ \dfrac{M}{ \color{red}{\bf\not}AL}=\dfrac{dm}{ \color{red}{\bf\not}Adx}$$ where $A$ is the cross-sectional area of the rod. $$ \dfrac{Mdx}{ L}= dm $$ Plugging into (1); $$I=\int x^2 \dfrac{Mdx}{ L} $$ As seen in the figure below, we chose our origin to be the black dot. Thus, $$I=\dfrac{M}{ L} \int_{-d}^{L-d} x^2dx $$ $$I=\dfrac{M}{ L} \left[ \dfrac{x^3}{3}\right]\bigg|_{-d}^{L-d}=\dfrac{M}{ 3 L} \left[ (L-d)^3-(-d)^3 \right] $$ Thus, $$I= \dfrac{M}{ 3 L} \left[ (L-d)^3-(-d)^3 \right] $$ $$\boxed{I= \dfrac{M}{ 3 L} \left[ (L-d)^3+d^3 \right] } $$ ________________________________________________ When $d=0$, $$I= \dfrac{M}{ 3 L} \left[ (L-0)^3+0^3 \right]= \dfrac{M}{ 3 L} L^3$$ $$\boxed{I= \dfrac{ML^2}{ 3 }} $$ which is the same result as Table 12.2. ________________________________________________ And when $d=\frac{L}{2}$, $$I= \dfrac{M}{ 3 L} \left[ \left(L-\frac{L}{2}\right)^3+\left(\frac{L}{2}\right)^3 \right]= \dfrac{M}{ 3 L} \left[ \left( \frac{L}{2}\right)^3+\left(\frac{L}{2}\right)^3 \right]$$ $$I= \dfrac{2M}{ 3 L} \left[ \frac{L^3}{2^3} \right]$$ $$\boxed{I= \dfrac{ML^2}{ 12}} $$ which is the same result as Table 12.2.
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