What is the equation for a body's trajectory in a central force field?

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SUMMARY

In a central force field defined by the equation \(\vec{F}(\vec{r})=f(r)\frac{\vec{r}}{r}\), the trajectory of a body can be expressed in terms of \(u=\frac{1}{r}\) as \(\ddot{u}+u=-\frac{mf(\frac{1}{u})}{L^{2}u^{2}}\). Here, \(m\) represents the mass of the body and \(L\) denotes the angular momentum. The transformation of Newton's equations for \(\ddot{r}\) leads to a differential equation involving \(\ddot{u}\) and \(u\), but the challenge lies in eliminating \(\dot{u}\) from the equation.

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Homework Statement



Show that in a central force field \vec{F}(\vec{r})=f(r)\frac{\vec{r}}{r} the equation for a body's trajectory in terms of u=\frac{1}{r} is given by:
\ddot{u}+u=-\frac{mf(\frac{1}{u})}{L^{2}u^{2}}

where m is the body's mass and L is the angular momentum


Homework Equations





The Attempt at a Solution



Transforming Newton's equations for \ddot{r} leads to ab equation for \ddot{u} and u where unfortunately \dot{u} also appears and I don't know how to eliminate it from the equation.
 
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can you show us how you did it?
 
\ddot u=\frac{d^2u}{d\theta^2}
:smile:
 

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