PROBLEM \( 3.112 \) A 50-mm-diameter cylinder is made of a brass for which the stress-strain diagram is as shown. Knowing that the angle of twist is \( 5^{\circ} \) in a \( 725-\mathrm{mm} \) length, determine by approximate means the magnitude \( T \) of torque applied to the shaft SOLUTION \( (a) \) \[ \begin{aligned} \varphi & =5^{\circ}=87.266 \times 10^{-3} \mathrm{rad} \\ c & =\frac{1}{2} d=0.025 \mathrm{~m} \quad L=0.725 \mathrm{~m} \\ \gamma_{\max } & =\frac{c \varphi}{L}=\frac{(0.025)\left(87.266 \times 10^{-3}\right)}{0.725}=0.00301 \end{aligned} \] Let \( \quad z=\frac{\gamma}{\gamma_{\max }}=\frac{\rho}{c} \) \( T=2 \pi \int_{0}^{c} \rho^{2} \tau d \rho=2 \pi c_{2}^{3} \int_{0}^{1} z^{2} \tau d z=2 \pi c_{2}^{3} I \quad \) where the integral \( I \) is given by \( I=\int_{1 / 3}^{1} z^{2} \tau d z \) Evaluate \( I \) using a method of numerical integration. If Simpson's rule is used, the integration formula is \[ I=\frac{\Delta z}{3} \sum w z^{2} \tau \] where \( w \) is a weighting factor. Using \( \Delta z=0.25 \), we get the values given in the table below. \[ \begin{array}{l} I=\frac{(0.25)\left(283.75 \times 10^{6}\right)}{3}=23.65 \times 10^{6} \mathrm{~Pa} \\ T=2 \pi c^{3} I=2 \pi(0.025)^{3}\left(23.65 \times 10^{6}\right)=2.32 \times 10^{3} \mathrm{~N} \cdot \mathrm{m} \quad T=2.32 \mathrm{kN} \cdot \mathrm{m} \end{array} \]
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(a) Let us indicate the forces and their points of application for the cylinder. Choosing the positive direction for $x$ and $\varphi$ as shown in the figure, we write the equation of motion of the cylinder axis and the equation of moments in the C.M. frame relative to that axis i.e. from equation $F_{x}=m w_{c}$ and $N_{z}=I_{c} \beta_{z}$ As there is no slipping of thread on the cylinder $w_{c}=\beta R$ From these three equations $T=\frac{m g}{6}=13 \mathrm{~N}, \beta=\frac{2}{5} \frac{g}{R}=5 \times 10^{2} \mathrm{rad} / \mathrm{s}^{2}$ (b) we have $\beta=\frac{2}{3} \frac{g}{R}$ So, $w_{c}=\frac{2}{3} g>0$ or, in vector form $\vec{w}_{c}=\frac{2}{3} \vec{g}$ $P=\vec{F} \cdot \vec{v}=\vec{F} \cdot\left(\vec{w}_{c} t\right)$ $=m \vec{g} \cdot\left(\frac{2}{3} \overrightarrow{g t}\right)=\frac{2}{3} m g^{2} t$
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Let us use the equation $\frac{d M_{z}}{d t}=N_{z}$ relative to the axis through $O$ For this purpose, let us find the angular momentum of the system $M_{z}$ about the given rotation axis and the corresponding torque $N_{z}$. The angular momentum is $$ M_{z}=I \omega+m v R=\left(\frac{m_{0}}{2}+m\right) R^{2} \omega $$ [where $I=\frac{m_{0}}{2} R^{2}$ and $v=\omega R$ (no cord slipping)] So, $$ \frac{d M_{z}}{d t}=\left(\frac{M R^{2}}{2}+m R^{2}\right) \beta_{z} $$ The downward pull of gravity on the overhanging part is the only external force, which exerts a torque about the $z$ -axis, passing through $O$ and is given by, $$ N_{z}=\left(\frac{m}{l}\right) x g R $$ Hence from the equation $\quad \frac{d M_{z}}{d t}=N_{z}$ $$ \left(\frac{M R^{2}}{2}+m R^{2}\right) \beta_{z}=\frac{m}{l} x g R $$ Thus, $$ \beta_{z}=\frac{2 m g x}{l R(M+2 m)}>0 $$ Note: We may solve this problem using conservation of mechanical energy of the system (cylinder $+$ thread) in the uniform field of gravity.
A glass cylinder of radius $R,$ length $l,$ and density $\rho$ has a 10 turn coil of wire wrapped lengthwise, as seen in FIGURE P33.71. The cylinder is placed on a ramp tilted at angle $\theta$ with the edge of the coil parallel to the ramp. A uniform magnetic field of strength $B$ points upward. a. For what loop current $I$ will the cylinder rest on the ramp in static equilibrium? Assume that static friction is large enough to keep the cylinder from simply sliding down the ramp without rotating. b. Is this feasible? To find out, cvaluate $I$ for a 5.0 -cm-diameter, 10-cm-long cylinder of density $2500 \mathrm{kg} / \mathrm{m}^{3}$ on a $10^{\circ}$ slope in a 0.25 T magnetic field. (FIGURE CAN'T COPY)
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