Problems

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Solution:

1. \begin{align*} \text{N2}(\uparrow): && T \cos \theta + F -W &= 0 \\ && W &= T\cos \theta + \mu R \tag{1} \\ \text{N2}(\rightarrow): && R-T\sin \theta &= 0 \\ && R &= T \sin \theta \tag{2}\\ \\ (1)+(2): && W&=(\cos \theta + \mu \sin \theta)T \tag{3} \\ \overset{\curvearrowright}{A}: && 0 &= W(b\sin \phi + a \cos \phi) - Td\sin(\phi+\theta) \tag{4} \\ \\ (3)+(4): && 0 &= (\cos \theta + \mu \sin \theta)(b\sin \phi + a \cos \phi)-d\sin(\phi+\theta) \\ \Rightarrow && d\sin(\phi+\theta) &= (\cos \theta + \mu \sin \theta)(b\sin \phi + a \cos \phi) \end{align*} as required.
2. If \(F\) is operating downwards, it's equivalent to \(-\mu\), ie: \[d\sin(\phi+\theta) = (\cos \theta - \mu \sin \theta)(b\sin \phi + a \cos \phi)\]
3. For the frictional force to be acting upwards, we need \begin{align*} && d\sin(\phi+\theta) &\geq \cos \theta(b\sin \phi + a \cos \phi) \\ \Rightarrow && d &\geq \frac{\cos \theta(b\sin \phi + a \cos \phi)}{\sin(\phi + \theta)} \\ &&&= \frac{\cos \theta(b\sin \phi + a \cos \phi)}{\sin\phi \cos\theta+\cos\phi\sin \theta)}\\ &&&= \frac{(b\sin \phi + a \cos \phi)}{\sin\phi+\cos \phi \tan \theta)}\\ &&&= \frac{a+b\tan \phi}{\tan\theta+\tan\phi }\\ \end{align*} We know that \(d < 2b\), so \begin{align*} && 2b &>\frac{a+b\tan \phi}{\tan\theta+\tan\phi }\\ \Rightarrow && 2b \tan \theta + 2b \tan \phi &> a + b \tan \phi \\ \Rightarrow &&b(2 \tan \theta + \tan \phi) &> a\\ \end{align*} Therefore we will have problems if the inequality is reversed!

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Solution:

The horizontal distance to \(X\) is \(a\cos \alpha\). The horizontal distance to \(G\) from \(X\) is \(b \sin \alpha\), therefore the centre of mass is a distance \(a \cos \alpha + b \sin \alpha\) from the wall. \begin{align*} \text{lim eq}: && F_W &= \mu R_W \\ && F_G &= \mu R_G\\ \text{N2}(\rightarrow): && \mu R _G &= R_W \\ \text{N2}(\uparrow): && \mu R_W + R_G &= W \\ \Rightarrow && (1+\mu^2)R_G &= W \\ \overset{\curvearrowleft}{Y}: && R_G 2a \cos \alpha - F_G 2a \sin \alpha - W (a \cos \alpha + b \sin \alpha) &= 0 \\ \Leftrightarrow && 2a R_G \cos \alpha -2a \mu R_G \sin \alpha - (1+\mu^2)R_G(a \cos \alpha + b \sin \alpha) &= 0 \\ \Leftrightarrow && a(1-\mu^2)\cos \alpha - (b(1+\mu^2)+2a\mu) \sin \alpha &= 0 \\ \Leftrightarrow && a(1-\tan^2 \lambda )\cos \alpha - (b(1-\tan^2 \lambda)+2a\tan \lambda) \sin \alpha &= 0 \\ \Leftrightarrow&& a(2-\sec^2 \lambda) \cos \alpha - (b\sec^2 \lambda+2a\mu) \sin \alpha &= 0 \\ \Leftrightarrow && a (2\cos \lambda - 1)\cos \alpha - 2a \sin \lambda \cos \lambda \sin \alpha &= b \sin \alpha \\ \Leftrightarrow && a\cos 2 \lambda \cos \alpha - a\sin 2 \lambda \sin \alpha &= b \sin \alpha \\ \Leftrightarrow && a\cos (2 \lambda +\alpha) &= b \sin \alpha \end{align*} as required. If the lamina is a square, \(a = b\), so \begin{align*} && \cos(2\lambda + \alpha) &= \sin \alpha \\ \Rightarrow && 0 &= \cos(2\lambda + \alpha) -\sin \alpha \\ &&&= \sin \left (\frac{\pi}{2} - 2 \lambda - \alpha \right )-\sin \alpha \\ &&&= 2 \cos\left ( \frac{\frac{\pi}{2} - 2 \lambda - \alpha +\alpha}{2} \right) \sin\left ( \frac{\frac{\pi}{2} - 2 \lambda - \alpha -\alpha}{2} \right) \\ &&&= 2 \cos\left ( \frac{\pi}4 -\lambda\right) \sin\left ( \frac{\pi}4 -\lambda-\alpha \right) \\ \Rightarrow && \lambda -\frac{\pi}{4} = -\frac{\pi}{2} & \text{ or } \frac{\pi}{4} - \lambda - \alpha = 0 \\ \Rightarrow && \alpha &= \frac{\pi}{4}-\lambda \end{align*}