Proof In Motion
Region: US
Thursday 01 October 2026 01:01:11 GMT
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Marcus Colwell :
this was on the SAT in 1982 and everyone got it wrong, the correct answer didnt appear on the test. it is now called the "coin rotation paradox."
2026-10-01 15:07:37
152
ETHAN ZIMMERMAN :
from the coins pov it only turned once, arrow facing away from the coin and then rolling until it’s facing away again
2026-10-01 01:13:53
564
Dean P :
You’re incorrect, your arrow causes an optical illusion. They’re both the same one rotation.
2026-10-21 19:57:41
2
Fastpaws 👾 :
It’s still only one rotation, it’s just that the contact point rotated relative to our view so it LOOKS like two rotations
2026-10-01 11:31:03
214
jonathan harwood :
Not sure why when you are rotating half way then saying it’s one rotation. In the flat line, halfway is shown by the arrow pointing down towards the white line and a full rotation points away from the white line. When you turn the line into a circle halfway is still pointing towards the white line, and the full rotation is pointing away, but for some reason you’ve got it wrong. I assume deliberately to get engagement, but I’m not sure why misinformation is the way you’re choosing to get interaction. Why not say ‘it appears that it rotates twice, but in reality the perspective change is the reason’?
2026-10-12 07:28:43
8
Aras :
first graphic is wrong, it still only spins halfway around the coin, when you look at it from a stationary standpoint it looks like it spins twice, but the second rotation is because of the curvature of the path itself
2026-10-01 21:56:48
26
Zeus Maine Coon & family :
Totally wrong. 1 turn.
2026-10-13 08:19:20
1
Ants3001 :
Relative to the other coin surfaces, it still makes only one rotation, not two.
2026-10-13 04:28:49
0
Mudskipaaaaa :
This is because the center of the coin travels over a circle twice the size of the coin. Not hard to understand really.
2026-10-12 10:14:47
4
Iamalwaysright :
???? it makes 1 turn in both ??? it just gonna point up 2x because its a circle.
2026-10-12 14:00:47
0
Longshanks :
Just because the turns number shown says 2 does not mean that is true. Imagine it says 0.5 for half and 1.0 for all the way!!!
2026-10-01 12:03:13
44
JGrace :
🙄 it still one turn either way the direction the arrow points is not the deciding factor, it's the point of contact.
2026-10-01 05:00:30
10
GI Drumbot :
From the perspective of the plane of the coin, it only turned once. That’s how it works.
2026-10-01 11:40:35
20
Robert E. Eddy :
I asked ai and it still doesn’t make sense. It is basically like yeah it does this because it does
2026-10-01 17:05:36
1
PapaYankee :
The two clips perfectly explain why the answer is two rotations. In the second clip, the surface is flat and has the length of the circumference, so the coin rotates once. In the first clip, the surface also rotates a full circle, so the coin also rotates once more along with the circular surface for a total of two rotations.
2026-10-01 19:56:33
8
williams.joshuajames :
At the half, the top points INTO the coin/bottom. It is only a half turn, regardless. Visually, it LOOKS LIKE a full turn.
2026-10-01 15:39:54
0
User :
If it was spinning around a point it would make one rotation so by spinning around a circle of its own diameter it makes two rotations - basically the number of rotations adding together
2026-10-01 03:26:10
17
Shawn Nicholson :
is perspective. it's still 1
2026-10-02 01:48:02
1
general quakington III :
um maybe because it didn't rotate it only rotated 180 not 360 perspective matters
2026-10-02 14:15:26
5
Jaike :
nope
2026-10-01 16:09:41
2
Random Person :
They are both one full revolution
2026-10-02 05:08:35
4
user4767791633490 :
Watch the blue lines. The lines are only half done, meaning not a full rotation. So no paradox is there.
2026-10-02 15:34:16
4
626offroad :
This is equal to two times the circumference, so, yes, the coin will do two rotations when turning it around another coin of the same size like in the graphic, and only once when in a straight line
2026-10-01 13:55:58
4
Beautiful Discovery :
The skeptics are correct about the 1D distance. try it with 2 quarters: the outer coin only rolls across one circumference worth of metal. But because that metal track is bent into a circle, the outer coin's center is swung through a wider orbit with a radius of two quarters. You cannot move a wheel's center a distance of two circumferences through 2D space without that wheel physically spinning around its own center axis twice. The second rotation isn't 'magic'—it is the geometric tax required to travel through that wider 2D orbit. Mechanical engineers cannot afford to argue about this in comment sections—if they got this math wrong, real machinery, engines, and watches would physically break or fail to function. In engineering, there is a strict, undeniable mathematical rule used to calculate gear speeds called the Willis Equation. The equation dictates exactly how fast components turn relative to each other. If an engineer builds a machine where the central Sun gear has 20 teeth and the rolling Planet gear has 20 teeth (a 1:1 ratio, just like our identical quarters), the engineer uses the formula: planet rotations = (teeth on sun/teeth on planet) + 1. The math dictates that when the arm swings the planet gear exactly one loop around the identical fixed sun gear, the planet gear must physically spin on its axle exactly 2 times. Used in automatic transmissions, bicycle hub gears (like 3-speed or 7-speed commuter bikes), wind turbine gearboxes. But you can prove it with a simple spirograph gears.
2026-10-03 03:33:50
0
the_Greedy_Mind :
1 turn.
2026-10-02 02:17:03
1
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