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## Main Question or Discussion Point

I am stuck at this part page 1,

$$\frac{\partial{L}}{\partial{\dot{φ}}}=\mu{r^2}\dot{φ}=const=l------->\dot{φ}=\frac{l}{\mu{r^2}}...........................(8)$$

Why is this a constant? Isn't r and dφ/dt variables of time?

Source: http://www.pha.jhu.edu/~kknizhni/Mechanics/Derivation_of_Planetary_Orbit_Equation.pdf [Broken]

$$\frac{\partial{L}}{\partial{\dot{φ}}}=\mu{r^2}\dot{φ}=const=l------->\dot{φ}=\frac{l}{\mu{r^2}}...........................(8)$$

Why is this a constant? Isn't r and dφ/dt variables of time?

Source: http://www.pha.jhu.edu/~kknizhni/Mechanics/Derivation_of_Planetary_Orbit_Equation.pdf [Broken]

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