Energy is usually more relevant than momentum, for inflicting damage.
For m = 500 g and v = 1 m/s, I have d[sub]ouch[/sub] = 2.5 cm … we need someone to try 2 m/s now …
Every body should just take an art class…and feel the happy happy swirl/galaxy/hurricane shape!
One thing to keep in mind is that the structure of galaxies is preserved only because the outer parts of the disk are rotating faster than they ought to be. If they weren’t, those beautiful trailing arms would have smeared into nothingness long ago.
The best explanation for this is dark matter, but its description smacks of Handwavium* to me.
*“There is this stuff we need to have around to explain some phenomena we see, but we have no way of detecting it except its gravitational effects. Oh, yeah, we need to have five times of the stuff as there is of ordinary matter to make it work.”
The similarities of spiral galaxies and hurricanes are only superficial. The structure of spiral galaxies is not completely understood.