(adsbygoogle = window.adsbygoogle || []).push({}); In short:

Why are galaxies flat?

In more detail:

A very large mass M is at rest in vacuum, rotating at point O. Its axis of rotation is the xz-plane with angular momentum [itex]\vec{L}[/itex] directed along the positive y-axis. What forces do the following masses experience?

(Units of distance are arbitrary and only given for orientation. Please explain if the actual distance in a given orientation is important.)

1) m_{1}is placed at rest with position [itex]\vec{r}[/itex] = [itex]\hat{i}[/itex].

2) m_{2}is placed at rest with position [itex]\vec{r}[/itex] = [itex]\hat{j}[/itex].

3) m_{3}is placed at rest with position [itex]\vec{r}[/itex] = [itex]\hat{i}[/itex] + [itex]\hat{j}[/itex]

4) m_{4}is set into circular orbit in the xz-plane, initial position [itex]\vec{r}[/itex] = [itex]\hat{i}[/itex].

5) m_{5}is set into circular orbit in the yz-plane, initial position [itex]\vec{r}[/itex] = [itex]\hat{j}[/itex].

6) m_{6}is set into circular orbit, at angle ∅ = 45° with the x-axis, initial position [itex]\vec{r}[/itex] = [itex]\hat{i}[/itex] + [itex]\hat{j}[/itex].

Further, a large mass M undergoes rotation. This implies that particles in M are moving with angular velocityw= rv. However, in the case of black holes, what is the radius r? If black holes are not considered a point, then what keeps them from becoming a point?

Thank you very much for any help in understanding these confusing things!

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# Conceptual Q: Gravity, Angular Momentum, and Galaxies

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