I'm a bit unclear about the description of orbital motion in a plane by using the polar coordinates [itex](r,\theta)[/itex]. This coordinate system changes its orientation in the inertial reference frame, that it is rotating as the orbiting object moves along its path. In the derivation of the equations of motion the radial part comes to(adsbygoogle = window.adsbygoogle || []).push({});

[tex]F(r)=m(\ddot r\ -\ r\dot \theta^2)[/tex]

My problem is that in a rotating reference system it is normally necessary to introduce a centrifugal force [itex]F_C[/itex], which sorts of explains the second term in the equation above since the centrifugal force is given as

[tex]F_C=mr\dot \theta^2[/tex]

Why is the term then negative in the top equation?

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# Homework Help: Orbital motion

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