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justwild
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Homework Statement
Finding the time period of a conical pendulum by D'Alembert's principle. The string is of a constant length and all dissipations are to be ignored.
Homework Equations
The time period of a conical pendulum is [itex]2\pi \sqrt{\frac{r}{g\tan\theta}}[/itex]. I need to arrive at this result starting from the D'Alembert's principle.
The Attempt at a Solution
I assume a cylindrical coordinates system whose origin coincides with the fulcrum of the pendulum. Thus, for the conical pendulum with constant string length, the acceleration of the particle would be
[itex]\ddot{\vec{r}}=\ddot{\rho}\hat{\rho}+\rho\ddot{\phi}\hat{\phi}[/itex].
Now the virtual displacement can be given as
[itex]\delta\vec{r}=\rho\delta\phi\hat{\phi}[/itex].
And the force acting on the particle as [itex]\vec{F}=-mg\hat{z}[/itex].
Now if I substitute all these in the D'Alembert's principle, I won't be able to calculate the time period from there. This is obvious because the virtual displacement does not contain the [itex]\hat{z}[/itex] term and due to the dot product, the expression won't include any [itex]g[/itex] term which is necessary because the result does contain the [itex]g[/itex] term.
I am out of ideas for now and I would appreciate anyone from the PF helping me out.