Engineering Mechanics: Statics & Dynamics (14th Edition)

Published by Pearson
ISBN 10: 0133915425
ISBN 13: 978-0-13391-542-6

Chapter 4 - Force System Resultants - Section 4.5 - Moment of a Force about a Specified Axis - Problems - Page 153: 66

Answer

$$ \begin{aligned} & M_a=4.37 \mathrm{~N} \cdot \mathrm{m} \\ & \alpha=33.7^{\circ} \\ & \beta=90^{\circ} \\ & \gamma=56.3^{\circ} \\ & M=5.41 \mathrm{~N} \cdot \mathrm{m} \end{aligned} $$

Work Step by Step

$$ \begin{aligned} \mathbf{F} & =30\left(\cos 60^{\circ} \mathbf{i}+\cos 60^{\circ} \mathbf{j}+\cos 45^{\circ} \mathbf{k}\right) \\ & =\{15 \mathbf{i}+15 \mathbf{j}+21.21 \mathbf{k}\} \mathrm{N} \\ \mathbf{r} & =\{-0.1 \mathbf{i}+0.15 \mathbf{k}\} \mathrm{m} \\ \mathbf{u} & =\mathbf{j} \\ M_\alpha & =\left|\begin{array}{ccc} 0 & 1 & 0 \\ -0.1 & 0 & 0.15 \\ 15 & 15 & 21.21 \end{array}\right|=4.37 \mathrm{~N} \cdot \mathrm{m} \end{aligned} $$ F must be perpendicular to $\mathbf{u}$ and $\mathbf{r}$. $$ \begin{aligned} \mathbf{u}_f & =\frac{0.15}{0.1803} \mathbf{i}+\frac{0.1}{0.1803} \mathbf{k} \\ & =0.8321 \mathbf{i}+0.5547 \mathbf{k} \\ \alpha & =\cos ^{-1} 0.8321=33.7^{\circ} \\ \beta & =\cos ^{-1} 0=90^{\circ} \\ \gamma & =\cos ^{-1} 0.5547=56.3^{\circ} \\ M & =30(0.1803)\\&=5.41 \mathrm{~N} \cdot \mathrm{m} \end{aligned} $$
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