Relation between position vector and velocity vector

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SUMMARY

The discussion clarifies the relationship between the position vector ##\vec{r}## and the velocity vector ##\vec{\dot{r}}## in the context of central forces. It establishes that while the acceleration vector for a central force is directed along the position vector, the velocity vector can point in any direction, indicating that ##\vec{\dot{r}}## is not necessarily parallel to ##\vec{r}##. The distinction between the magnitude of the velocity, ##\dot{r}##, and the velocity vector itself, ##|\vec{\dot{r}}|##, is emphasized as crucial for understanding motion in a central force field.

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The equation following (3.80) seems to suggest that the velocity vector ##\vec{\dot{r}}## must always be parallel to the position vector ##\vec{r}##. But clearly this is not true as a particle's velocity can be in any direction.

What's wrong?

image.png
 
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That looks like the acceleration vector for a central force, which must indeed be in the direction of the position vector (from the central point).

The last equation says "the component of the velocity in the radial direction". There is an angular component as well.
 
PeroK said:
That looks like the acceleration vector for a central force, which must indeed be in the direction of the position vector (from the central point).

The last equation says "the component of the velocity in the radial direction". There is an angular component as well.

##\vec{\dot{r}}##should be the velocity vector. The acceleration vector should be ##\vec{\ddot{r}}##.

I think I get it. ##\dot{r}\neq|\vec{\dot{r}}|##.
 

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