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ralqs
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Homework Statement
Suppose we have a system with scleronomic constraints. Is the condition that [itex]\frac{\partial V}{\partial q_j}=0[/itex] for generalized coordinates qj a necessary condition for equilibrium? A sufficient condition?
Homework Equations
[tex]-\frac{\partial V}{\partial q_j}=Q_j= \sum_i \vec{F}_i \cdot \frac{\partial \vec{r}_i}{\partial q_j}[/tex]
where Qj is the generalized for associates with qj.
The Attempt at a Solution
I think I managed to prove that the above condition is necessary and sufficient for any type of holonomic constaint, sclerenomic or rheonomic. This makes be believe I made a mistake, so I'd appreciate it if someone could check my work.
System is in equilibrium iff [itex]\vec{F}_i=0[/itex], where [itex]\vec{F}_i[/itex] is the total force on the ith particle.
[itex]Q_j=\sum_i \vec{F}_i\cdot\frac{\partial \vec{r}_i}{\partial q_j}[/itex], where Qj is the generalized force associated with the jth generalized coordinate. So, if [itex]\vec{F}_i=0[/itex], then Qj = 0. But [itex]Q_j=-\frac{\partial V}{\partial q_j}[/itex], so [itex]\frac{\partial V}{\partial q_j}=0[/itex] is a necessary condition for equilibrium.
Now we prove that it is a sufficient condition. To do this, we find the [itex]\vec{F}_i[/itex]'s as a function of the Qjs by making virtual displacements [itex]\delta q_j[/itex]to the generalized coordinates. The the virtual work is
[itex]\delta W = \sum_j Q_j \delta q_j = \sum_i \vec{F}_i \cdot \delta \vec{r}_i[/itex]. Writing [itex]\delta q_j = \sum_i \nabla_i q_j\cdot\delta \vec{r}_i[/itex] (we've tacitly expressed the generalized coordinates as functions of the ri's; [itex]\nabla_i q_j[/itex] stands for [itex]\hat{x}_i\frac{\partial q_j}{\partial x_i}+\hat{y}_i\frac{\partial q_j}{\partial y_i}+\hat{z}_i\frac{\partial q_j}{\partial z_i}[/itex]).
From this, it follows that [itex]\sum_i \vec{F}_i\cdot\delta \vec{r}_i = \sum_i (\sum_j Q_j \nabla_i q_j)\cdot \delta \vec{r}_i[/itex], implying that [itex]\vec{F}_i=\sum_j Q_j \nabla_i q_j[/itex] Therefore, if Q_j = 0, system is in equilibrium. QED.
Now, as far as I can tell I haven't used the assumption that the constraints are scleronomic, but maybe the assumption sneaked in there somewhere.