Topsy Turvy

[watchdog]

I was watching QI last week and this was looked at. You have asll probably heard the term "topsy turvy" but did you know it is derived from an actual top (as in spinning top ). This top looks like one of those hemispherical tap washers complete with the shaft. Both ends have a pointed bit to aid the spinning. Now if the shaft is held and spun like you normally would with a small top , something amazing happens. The top ends up inverting itself so that it is actually spinning on the end of the shaft BUT the direction of spin (clockwise/anti clockwise) DOES NOT CHANGE. So if it is spinning clockwise before AND after it inverts itself then the direction of spin changes but how why & when ? This was shown in slow motion video but not slow enough to see what takes place. Anyone got an explanation ?

[trash]

The spin direction doesn't change.
The top still spins the same direction relative to the floor.
In terms of the of the stem of the top it appears to spin the reverse direction.
It's just conservation of momentum.

If you roll a ball down a road, but then impart a smaller rotation sideways so that the left axis of the ball is now on the right side, the ball now appears to rotate backwards relative to what it was before, but the ball is still rolling down the road in the same direction.

The topsy turvy top is is actually just following entropy.
Normally entropy in the form of gravity keeps the heavy end of the top down.
When a tiny wobble develops the friction of the bottom of the top is amplified. You might also call this the butterfly effect.
This pushes the top further towards a tipping point. The energy for this comes from the spinning of the top itself.
The friction of the top pushes it over the potential hump and slows the spinning of the top down slightly.
It gets to a point where the gyroscopic force from the severe wobble kicks it over the potential hump and the gyroscopic force then stabalises it and the pointy end minimises the friction and it can return to behaving like a normal top under newton's first law.
Eventually gravity overwhelms the momentum of the top and it falls over heavy end down.

[Onefella]

This is what Gyro-copters do, just before they invert and drill the pilot, (safely strapped to the fuel-tank) into the ground.

[watchdog]

The spin direction doesn't change.
The top still spins the same direction relative to the floor.In terms of the of the stem of the top it appears to spin the reverse direction.
It's just conservation of momentum.

If you roll a ball down a road, but then impart a smaller rotation sideways so that the left axis of the ball is now on the right side, the ball now appears to rotate backwards relative to what it was before, but the ball is still rolling down the road in the same direction.

The topsy turvy top is is actually just following entropy.
Normally entropy in the form of gravity keeps the heavy end of the top down.
When a tiny wobble develops the friction of the bottom of the top is amplified. You might also call this the butterfly effect.
This pushes the top further towards a tipping point. The energy for this comes from the spinning of the top itself.
The friction of the top pushes it over the potential hump and slows the spinning of the top down slightly.
It gets to a point where the gyroscopic force from the severe wobble kicks it over the potential hump and the gyroscopic force then stabalises it and the pointy end minimises the friction and it can return to behaving like a normal top under newton's first law.
Eventually gravity overwhelms the momentum of the top and it falls over heavy end down.

On the show all the guests thought it was the inverting that was the oddity to be discussed but it wasn't. It was the fact that the top was still spinning in the same direction relative to the floor when (after inverting) it should've been spinning in the opposite. This is the real mystery.

[trash]

Nup, no mystery. Just simple conservation of moment.

Gyrocopters are one of the safest aircraft these days.
Training is greatly improved and airframes are high performance.

[Uncle Fester]

Your life hanging on one single bolt is my only concern.
For aircraft safety you need redundancy, but probably still safer than the average small chopper or other light aircraft.

[watchdog]

Nup, no mystery. Just simple conservation of moment.

]

No - conservation of energy would mean if it was turning clockwise relative to the floor initially then it would be turning anticlockwise relative to the floor after it inverted

[trash]

Conservation of momentum doesn't say which way it would turn. Only that it continues to turn.

m1v1 = m2v2 (no mention of phase).

I'm not sure why it is so baffling to some people.

See if I can either make it more baffling or easier to understand.
I'll use the two frames of reference.
The first is from the table.
The top spins clockwise and as friction destabilises it, it still spins clockwise.
The wobble of the offset top still spins clockwise and as it catches the stem and stands up it is still spinning clockwise.


The other frame of reference is the pin of the top itself.
So we spin the top clockwise.
What we see from the top's frame of reference is the table is spinning anti-clockwise (CCW).
As the friction of the top the table appears to revolve around the side of the top still anti-clockwise.
The spin of the table appears to slow down in the vertical Z axis (table turning CCW), and speed up in the X axis, and you guessed it, the rotation of the table in this axis is clockwise.
Eventually the top gets to the tipping point where the Z axis rotation stops and the X axis has all the rotational momentum.
But the top is still producing just enough asymmetric friction to pull the top back into the Z axis at the stem end.
When this happens, the top is over the kinetic hump and entropy and newton's first law in the form of a gyroscopic force kicks the top back vertical in the Z axis but now sitting on the stem.

So from the top's point of view, it is the universe that changes it's rotation and it's inverted.
From the universes point of view... the top is still rotating the same direction. (it's just upside down now).


Or you're on a roller coaster.... the train rolls down the hill and it turns left and then it turns right. No doubt, the passengers experience this too.
Now the train is inverted in a half barrel roll, and the track and the train turns left. But from the passengers perspective, they think they are turning right.
The train turns right inverted and the passengers on the train experience it turning left.

The people watching the inverted rollercoaster see it turning left and right respectively but their frame of reference is different from the inverted passengers.

[watchdog]

Clear as mud , I get the old baffle with bullshit feeling. Try this. Grab a pencil or something & hold it verticaly so you are looking down on it. Now start to turn it in a clockwise direction. At the same time (continue turning it) move it to above your head. You are now looking at the other end of the pencil & it is turning in an anticlockwise direction. This is because your point of perception has shifted. You are still turning it in the same direction as when you started. This is easily replicable and beyond dispute. Do the same thing but instead of moving it above your head , simply turn it over. Same result. It is turning in the OPPOSITE direction because your point of perception has changed. So a spinning object when inverted should "appear" (from your new perspective) to be going in the opposite direction. This is normal. BUT in the film clip on QI , after the top inverted , and from the new perspective , the top appeared to spin in the same direction as before it was inverted. This is not normal.

[Tiny]

OK, you peaked my interest & I had to find out why this thing flips. I found several explanations & a couple of great videos.
You can draw your own conclusions, as I don't pretend to have enough understanding to explain it clearly.

Topsy Turvy also known as Tippe Top & Tippy Top & Tipse Top.

This is a fascinating spinning top with an interesting behaviour.
The 'Tippe Top' inverts itself when spinning at sufficient speed. It does this because its centre of mass does not coincide with the geometrical centre of sphere. This means that when spinning the bulbous top slides over the surface on which it is spinning and this friction and motion causes enough force to 'drive' the spinning top up and over until it is spinning on the stalk. Once on the stalk there are far less sliding forces and the top can now spin stably until it no longer has enough energy to sustain its speed and spin.

Accompanying video in normal speed.


Now the following video in slow motion done by Harvard Natural Sciences Lecture Demonstrations is excellent.
Precession of a small metal top leads to surprising results. Shot in 1200 fps with a Casio EX-F1.





More about them here:






Serious physics in this pdf.

[Tiny]

The spin direction doesn't change.
The top still spins the same direction relative to the floor.
In terms of the of the stem of the top it appears to spin the reverse direction.
It's just conservation of momentum.

Having watched the slow motion video above I now think I know what you were saying here & of course you are correct.

The energy/momentum that is put into the top initially does not change direction, only the orientation of the top to the energy/momentum.

This happens with planets & stars as well. The word for it is precession. Trash; you will surely correct me if I'm wrong.

[trash]

You're within the bounds of science to say "baffling with bullshit".
What I say may be correct, but if so and I can't explain it so that you can understand it, then it is my fault for not being able to achieve that.

I haven't been able to find a video of the QI episode showing it.
I can only guess that watchdog is misinterpreting what he saw, in the same respect that I consider the top to be spinning the same direction inverted, even though the top is spinning reversed in it's frame of reference. From the metal top in the video you can see that when the top lands back right side up, it's spinning the same direction as the arrow.

When inverted it spins the opposite direction from the arrow drawn on it.
Is this the situation you saw on QI.

I suspect if it isn't, then it may be a stroboscopic effect of the camera frame rate.
The same reason car wheels on TV ads or waggon wheels in old westerns appear to spin backwards.

[watchdog]

I can only guess that watchdog is misinterpreting what he saw, in the same respect that I consider the top to be spinning the same direction inverted, even though the top is spinning reversed in it's frame of reference. From the metal top in the video you can see that when the top lands back right side up, it's spinning the same direction as the arrow.

When inverted it spins the opposite direction from the arrow drawn on it.
Is this the situation you saw on QI.

I suspect if it isn't, then it may be a stroboscopic effect of the camera frame rate.
The same reason car wheels on TV ads or waggon wheels in old westerns appear to spin backwards.

I'm fully coversant with the stroboscopic effect and this is usually identifiable from the speed of the interference pattern. What was shown on QI was a top being spun in a clockwise direction (to the observer) and then after inverting it was still spinning in a clocvkwise direction (to the observer). This is clearly shown in the 2nd clip in post #10. I have done a bit more reading /research , and while I wouldn't dare try to explain it here, it dose seem that the initial rotation about the pin axis does actually stop & reverse in direction through an "axial transfer of momentum" (my terminology)

[Uncle Fester]

It will always keep rotating in the direction of the force that accelerated it no matter in what position it is.
Think of a spinning ball. I you spin a ball and then roll it, it will just continue to spin in the same direction as it rolls and nobody will question that or find it strange.
… but now imagine yourself inside the ball !
It's spinning axis is constantly changing as it rolls and you would be experiencing the rotation tumbling in all directions including the effect of reversed rotation.

[trash]

Yep. simple conservation of momentum.
Nothing changes, we all see it as rotating the same direction with respect to the tops inversion.

Tiny's comment on precession is correct.
You can see the precession gaining energy from the rotation.

The top stops rotating in the Z axis, and all the energy of rotation is transferred into the X axis.