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Ichigo449
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Homework Statement
A bead of mass m slides in a frictionless hollow open-ended tube of length L which is held at an angle of β to the vertical and rotated by a motor at an angular velocity ω. The apparatus is in a vertical gravitational field.
a) Find the bead's equations of motion
b) Find the equilibrium position of the bead.
c) What is the minimum angular velocity so that the bead remains at the equilibrium position?
d) Suppose the bead is displaced from rest a distance δ from the equilibrium position and at the exact same time the motor is shut off and the tube can freely spin without friction about its lower endpoint. Find the position of the bead as a function of time while it is still in the tube.
Homework Equations
Lagrange's equations
The Attempt at a Solution
a) Using spherical coordinates to describe the position of the bead only the distance along the rod, r, is a degree of freedom. θ is fixed in the problem to be β, and $\dot θ =ω$. So substituting these constraints into the kinetic energy expressed in spherical coordinates gives:
$L = T - V = \frac{1}{2}m[\dot{r}^{2}+r^{2}ω^{2}sin^{2}β] - mgrcosβ$
From which the equations of motion easily follow as:
$\frac{d}{dt}\frac{\partial L}{\partial \dot{r}} -frac{\partial L}{\partial r} = 0$,
$\ddot{r} -rsin^{2}βω^{2} = -gcosβ$
b) At equilibrium $\ddot{r} = 0$, implying that $r= \frac{gcosβ}{sin^{2}βω^{2}}$.
c) Linearizing the equation of motion gives:
$\ddot{r} -rβ^{2}ω^{2} = -g$, so if ω^{2} > \frac{g}{rβ^{2}}, then the equilibrium position may be stable.
But something seems odd here since normal mode analysis implies that perturbations of this system about equilibrium grow exponentially so stability of the equilibrium position should be impossible.
d) I don't really know where to begin with this part of the problem. The azimuthal angle φ is now a degree of freedom with the motor shut off but I don't know how to relate the two situations.