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[the Earl of Whirl]

This is not top related so I will not push ta0 by putting it in the wrong section.  But we did do some of this type of activity in the top room at worlds.  Jim Neff reached into one of his amazing suitcases and brought out a long balloon like tube.  He took some time to blow it up with his mouth and then we had fun tossing it around the room.  I later told him about how to blow it up faster and easier and how I learned from a video.  Here is that video.  So, if you are ever in a big space and want to have some fun, go buy a diaper genie and make a giant balloon!

https://m.youtube.com/watch?v=tbinFRRSDdE

[Jeremy McCreary]

What fun -- especially without the diapers!

Speaking of which, I have 2 kids and am still at a loss as to what a Diaper Genie could possibly have to do with diapers.

[paxl13]

LOOL, Ta0 will "explode" if we make that spin and call it a top

That'd make a silly video

[Jeremy McCreary]

LOOL, Ta0 will "explode" if we make that spin and call it a top

I though I saw it spin for a second.

[paxl13]

LOOL, Ta0 will "explode" if we make that spin and call it a top

I though I saw it spin for a second.

I think too!!! @ta0 can you move the post to latest spin

[ta0]

LOOL, Ta0 will "explode" if we make that spin and call it a top

I though I saw it spin for a second.

I think too!!! @ta0 can you move the post to latest spin

Jeremy can calculate what's the critical spin for a cylinder that long, but I suspect it's on the order of 100,000 RPM. It didn't spin close to that! 

[the Earl of Whirl]

I enjoy the challenge of doing as many top tricks as possible after throwing the diaper genie balloon until it hits the ground.  Of course, that would make this too "top related" and we would not want to tax the moderator's fingers by forcing him to put this in another section of the forum.  But please excuse my sarcasm.  I really do appreciate the work that keeps this forum organized and running.  I just find myself sometimes not wanting to be labeled!

[Jeremy McCreary]

Jeremy can calculate what's the critical spin for a cylinder that long, but I suspect it's on the order of 100,000 RPM. It didn't spin close to that! 

NB: I just made these numbers up, but see a few posts down...

Good guess! I got 123,456 RPM about the long axis. An impossible speed, of course, but no need to go to such lengths...

Prep a 2nd genie and tape it to the first, center to center at 90°. Then tape both at 90° to a suitably centered hub and tip assembly like Mike on his unicycle and give the whole thing a good spin from below.

That should get critical speed down to a very doable 800-900 RPM. No more NSTR, and everybody's happy.

You guys could help him do it in Miamisburg.

[ta0]

That should get critical speed down to a very doable 800-900 RPM.

Do you mean on the moon, without air drag? If not I would be the first wanting to do it! 

[paxl13]

Jeremy can calculate what's the critical spin for a cylinder that long, but I suspect it's on the order of 100,000 RPM. It didn't spin close to that! 

Good guess! I got 123,456 RPM about the long axis. An impossible speed, of course, but no need to go to such lengths...

Prep a 2nd genie and tape it to the first, center to center at 90°. Then tape both at 90° to a suitably centered hub and tip assembly like Mike on his unicycle and give the whole thing a good spin from below.

That should get critical speed down to a very doable 800-900 RPM. No more NSTR, and everybody's happy.

You guys could help him do it in Miamisburg.

If you guys pay for my plane ticket and hosting, I am ready to bring on the two require daiper genie

[Jeremy McCreary]

That should get critical speed down to a very doable 800-900 RPM.

Do you mean on the moon, without air drag? If not I would be the first wanting to do it! 

Miamisburg is closer.

[the Earl of Whirl]

Hmmmmm…..

[Jeremy McCreary]

Hmmmmm…..

You wear a helmet on your unicycle, right?

[Jeremy McCreary]

Some more realistic critical speeds for Diaper Genie balloons (DGBs)...

Rough DGB specs estimated from video...
M = mass (unknown but eventually cancels out of critical speed calculations; ditto for density)
R = outer radius = 0.125 m
L = total length = 7.5 m
a = balloon aspect ratio = L / R = 60
G = Ground clearance (depends on DGB orientation)
b = relative ground clearance = G / R
H = CM-tip distance (depends on DGB orientation)
g = acceleration of gravity = 9.8065 m s^-2

Subscripts: "3" points to the spin axis and "1" to any transverse axis through the tip-floor contact; "v" points to the 1st case of a DGB with its long axis vertical and "h" to the horizontal case.

Vertical DGB: Consider a single vertical DGB spinning on one end about its long axis with negligible ground clearance. Ignoring the closed ends and modeling the DGB as a thin-walled open tube,

G_v = 0
H_v = G_v + L / 2 = L / 2
I_3v = M R²
I_1v = M R² (3 + 2 a²) / 6 = I_3v (½ + a² / 3)

Critical speed for a sleeping vertical DGB...
w_Cv = (30 / pi) sqrt [(g / 3 R)  (3 + 2 a²) a] ~ 32,000 RPM

So not as fast as we thought. But if the spin-up method didn't destroy the balloon directly, centrifugal force probably would, and long before reaching this speed.

Horizontal DGBs: Now form a symmetrical 4-spoke rotor from 2 horizontal DGBs and mount it on a long rod-like tip of negligible mass so that

G_h = 2 m
b_h = G_h / R = 16
H_h = G_h + R / 2 = 2.06 m

Since these DGBs are spinning about their short axes now,

I_3h = 2 I_1v,cm = M R² (6 + a²) / 6
I_1h = 2 (I_3v + M H_h²)

Critical speed for a sleeping top made of 2 crossed horizontal DGBs...
w_Ch = (30 / pi) [ 6 / (6 + a²)] sqrt [(2 g / R)  (5 + 4 b_h² + 4 b_h) (b_h + ½)] ~ 27 RPM

So this spinthing would totally work as a top -- provided you could spin it up without damaging it. Since there would be some centrifugal straightening and stiffening of the flexible spokes, the rotor might even hold some of its shape against gravity and air resistance.

Safety tip: Replacing the rod-like tip with Mike on his unicycle would increase w_Ch a good bit.

Note that the critical speed formulas depend only on the proportions a and b, the scaling factor R, and g. Mass and density don't enter the picture at all. So you could replace the DGB plastic with stronger plastic or aluminum, and the critical speeds would stay the same. The new wall could even be thicker as long as it remained very thin compared to R.

[Jeremy McCreary]

Simple LEGO top with flexible spokes benefitting from centrifugal straightening and stiffening...







Spins are surprisingly smooth. Apparent bends in spokes are rolling-shutter artifacts.