Meden Agan
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- Homework Statement
- A homogeneous cord rests on a pair of connected, rigid slopes with adjustable inclination angles, as shown in the figure. The coefficient of friction between the slopes and the cord is ##\mu##. How should we choose the inclination angles ##\alpha## and ##\beta##, and how should we deposit the cord to make the length of its hanging middle section maximal?
- Relevant Equations
- Newton's ##2^{\text{nd}}## Law of motion, some Calculus.
Let us take an infinitesimal piece of cord, long ##\mathrm ds##, resting on a plane inclined at an angle ##\theta## to the horizontal. The weight of the elements is ##\lambda g \, \mathrm ds##, where ##\lambda## is the linear density of the cord. The component parallel to the plane (directed downwards) is ##\lambda g \sin \theta \, \mathrm ds##. The normal reaction is ##\lambda g \cos \theta \, \mathrm ds##, so the maximum static friction available (directed upwards) is ##\mu \, \lambda g \cos \theta \, \mathrm ds##. Writing the axial equilibrium (positive upward direction) according to Newton's ##2^{\text{nd}}## Law of motion: $$T(s+ \mathrm ds) - T(s) = \lambda g \, \mathrm ds \left(\mu \cos \theta - \sin \theta \right), \tag{1}$$ and taking the limit ##\mathrm ds \to 0## on the LHS, we have ##T(s+ \mathrm ds) - T(s) \to \mathrm dT##; so, ##(1)## becomes $$\mathrm dT = \lambda g \, \mathrm ds \left(\mu \cos \theta - \sin \theta \right). \tag{2}$$ Integrating ##(2)## from the top end of the rope ##(T_{\text{top}} = 0, \, s = 0)## to the point where the rope detaches from the slope ##(T_0, \, s)## gives $$T_0 = \lambda g \, s \left(\sin \theta - \mu \cos \theta \right). \tag{3}$$
Now, let's work with two inclined planes (angles ##\alpha## and ##\beta## with respect to the horizontal plane).
If we call the two tensions ##T_{\alpha}## and ##T_{\beta}## for the values of ##T_0## at the two sides (they will not be equal) and ##s_{\alpha}## and ##s_{\beta}## as the lengths in contact with the slope at the two sides, then equation ##(3)## tells us the relationship between ##T_{\alpha}## and ##s_{\alpha}##, and also between ##T_{\beta}## and ##s_{\beta}##. We have
$$T_{\alpha} = \lambda g \, s_{\alpha} \left(\sin \alpha- \mu \cos \alpha \right) \tag{A}$$ and
$$T_{\beta} = \lambda g \, s_{\beta} \left(\sin \beta- \mu \cos \beta \right). \tag{B}$$
We have other constraints as well, because
- the net horizontal force on the rope must be zero, so we get $$T_{\alpha} \cos \alpha - T_{\beta} \cos \beta = 0 \iff T_{\alpha} \cos \alpha = T_{\beta} \cos \beta. \tag{C}$$
- if we suppose the length of the unsupported rope is ##\ell##, then its weight is ##\lambda g \, \ell## has to be supported by the force at the two ends of the unsupported section, i.e. by the vertical component of the tension ##T_0##. So, the total upwards force must equal the weight of the unsupported rope, and we get $$T_{\alpha} \sin \alpha + T_{\beta} \sin \beta = \lambda g \, \ell. \tag{D}$$
From ##(\text{C})##, we have ##T_{\beta} = T_{\alpha} \, \dfrac{\cos \alpha}{\cos \beta}##. Plugging the expression ##(\text{A})## for ##T_{\alpha}## into the previous equation, we get $$T_{\beta} = \lambda g \, s_{\alpha} \left(\sin \alpha- \mu \cos \alpha \right) \dfrac{\cos \alpha}{\cos \beta}. \tag{B1}$$
Plugging the expressions for ##(\text{A})## and ##(\text{B1})## into ##(\text{D})##, we get $$\cancel{\lambda g} \, s_{\alpha} \left(\sin \alpha- \mu \cos \alpha \right) \sin \alpha + \cancel{\lambda g} \, s_{\alpha} \left(\sin \alpha- \mu \cos \alpha \right) \tan \beta \cos \alpha = \cancel{\lambda g} \, \ell,$$ or, equivalently, $$\boxed{\frac{\ell}{s_{\alpha}} = \left(\sin \alpha- \mu \cos \alpha \right) \left(\sin \alpha + \tan \beta \cos \alpha \right)}.$$
Here I am stuck, because I can't understand how to find the maximum angles ##\alpha## and ##\beta## that maximize ##\ell##.
Now, let's work with two inclined planes (angles ##\alpha## and ##\beta## with respect to the horizontal plane).
If we call the two tensions ##T_{\alpha}## and ##T_{\beta}## for the values of ##T_0## at the two sides (they will not be equal) and ##s_{\alpha}## and ##s_{\beta}## as the lengths in contact with the slope at the two sides, then equation ##(3)## tells us the relationship between ##T_{\alpha}## and ##s_{\alpha}##, and also between ##T_{\beta}## and ##s_{\beta}##. We have
$$T_{\alpha} = \lambda g \, s_{\alpha} \left(\sin \alpha- \mu \cos \alpha \right) \tag{A}$$ and
$$T_{\beta} = \lambda g \, s_{\beta} \left(\sin \beta- \mu \cos \beta \right). \tag{B}$$
We have other constraints as well, because
- the net horizontal force on the rope must be zero, so we get $$T_{\alpha} \cos \alpha - T_{\beta} \cos \beta = 0 \iff T_{\alpha} \cos \alpha = T_{\beta} \cos \beta. \tag{C}$$
- if we suppose the length of the unsupported rope is ##\ell##, then its weight is ##\lambda g \, \ell## has to be supported by the force at the two ends of the unsupported section, i.e. by the vertical component of the tension ##T_0##. So, the total upwards force must equal the weight of the unsupported rope, and we get $$T_{\alpha} \sin \alpha + T_{\beta} \sin \beta = \lambda g \, \ell. \tag{D}$$
From ##(\text{C})##, we have ##T_{\beta} = T_{\alpha} \, \dfrac{\cos \alpha}{\cos \beta}##. Plugging the expression ##(\text{A})## for ##T_{\alpha}## into the previous equation, we get $$T_{\beta} = \lambda g \, s_{\alpha} \left(\sin \alpha- \mu \cos \alpha \right) \dfrac{\cos \alpha}{\cos \beta}. \tag{B1}$$
Plugging the expressions for ##(\text{A})## and ##(\text{B1})## into ##(\text{D})##, we get $$\cancel{\lambda g} \, s_{\alpha} \left(\sin \alpha- \mu \cos \alpha \right) \sin \alpha + \cancel{\lambda g} \, s_{\alpha} \left(\sin \alpha- \mu \cos \alpha \right) \tan \beta \cos \alpha = \cancel{\lambda g} \, \ell,$$ or, equivalently, $$\boxed{\frac{\ell}{s_{\alpha}} = \left(\sin \alpha- \mu \cos \alpha \right) \left(\sin \alpha + \tan \beta \cos \alpha \right)}.$$
Here I am stuck, because I can't understand how to find the maximum angles ##\alpha## and ##\beta## that maximize ##\ell##.