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 4.4 - Principle of Moments - Problems - Page 143: 42

Answer

$$ \begin{aligned} & M_O=4.27 \mathrm{~N} \cdot \mathrm{m} \\ & \alpha=95.2^{\circ} \\ & \beta=110^{\circ} \\ & \gamma=20.6^{\circ} \end{aligned} $$

Work Step by Step

$$ \begin{aligned} & \mathbf{r}_A=0.2 \sin 15^{\circ} \mathbf{i}+0.2 \cos 15^{\circ} \mathbf{j}+0.075 \mathbf{k} \\ & =0.05176 \mathbf{i}+0.1932 \mathbf{j}+0.075 \mathbf{k} \\ & \mathbf{F}=-20 \cos 15^{\circ} \mathbf{i}+20 \sin 15^{\circ} \mathbf{j} \\ & =-19.32 \mathbf{i}+5.176 \mathbf{j} \\ & M_O=\mathbf{r}_A \times \mathbf{F}=\left|\begin{array}{ccc} \mathbf{i} & \mathbf{j} & \mathbf{k} \\ 0.05176 & 0.1932 & 0.075 \\ -19.32 & 5.176 & 0 \end{array}\right| \\ & =\{-0.3882 \mathbf{i}-1.449 \mathbf{j}+4.00 \mathbf{k}\} \mathrm{N} \cdot \mathrm{m} \\ & M_O=4.272=4.27 \mathrm{~N} \cdot \mathrm{m} \\ & \alpha=\cos ^{-1}\left(\frac{-0.3882}{4.272}\right)=95.2^{\circ} \\ & \beta=\cos ^{-1}\left(\frac{-1.449}{4.272}\right)=110^{\circ} \\ & \gamma=\cos ^{-1}\left(\frac{4}{4.272}\right)=20.6^{\circ} \\ & \end{aligned} $$
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