Anti-Gravity Double Cone - It Rolls Uphill! (physics illusion, 4 colours)

The GIF shows the real motion, computed by the physics check and rendered frame by frame: a striped double cone that rolls UP a ramp by itself. Put it at the low end of the track and it climbs 17 mm to the finish posts, where it stops. The red ball on the axis and the grey bar show the trick: while the rails go up, the cone's centre of mass actually goes down.

How the illusion works

The two rails open into a V (12 mm apart at the bottom, 92 mm at the top, 16° each side) while they climb at 7.3°. A double cone is thick in the middle and thin at the tips, so as the gap widens it rests on thinner and thinner parts of the cone and its fat middle sinks between the rails. Net result: rails +16.6 mm, centre of mass −6.4 mm. Gravity is happy, your eyes are not. Picture 2 shows the cross-section at the start and at the finish with the real numbers.

Details

• Track: 166 × 128 mm base, 54 mm tall at the finish posts. Rails with a 3 mm rounded top, built as a "ladder" of coral / yellow / white bands every 4 mm, so you can count how much they climb.

• Chequered start and finish lines, yellow arrows between the rails, the words "It rolls uphill" and a small HANMA inlaid flush in the base.

• Double cone: Ø70 × 109 mm, printed as two halves standing on their flat base and joined with a Ø9.8 mm pin (0.25 mm clearance; a drop of glue is optional). The colour rings have zigzag edges shifted from ring to ring, so you can see it spin as it rolls (about 1.1 turns on the way up).

Printing

• Bambu Studio, A1 mini + AMS lite, 0.2 mm layers, 15% infill, no supports. Two plates:

• Plate 1, track: 2 h 36 min, 93 g (navy 61 g, white 10 g, coral 12 g, yellow 10 g).

• Plate 2, two cone halves + pin: 3 h 52 min, 88 g (navy 15 g, white 13 g, coral 38 g, yellow 22 g).

• AMS colours: 1 navy, 2 white, 3 coral red, 4 yellow. All colour changes on the track are horizontal (bands and top-layer inlays), so there are few swaps.

• Use the same settings for both cone halves so the cone stays balanced. Put the track on a level table; if the rail tops feel rough, a light pass with fine sandpaper helps it roll smoothly.

How it was checked

• Physics: for 400 positions along the track, the exact height of the cone's axis was found where the solid cone touches the round rail tops. The centre of mass goes down at every step (52.8 → 46.4 mm, a steady 2.9° downhill) while the rail under the contact point goes up at every step (17.8 → 34.4 mm).

• Controls: with parallel rails the centre of mass would rise 16.5 mm, and with a 10° opening (below the critical 11.6°) it would still rise 2.4 mm, so in both cases the cone would not move. At 13.1° it already goes down: the critical angle is real.

• With the real meshes, at 24 positions (cone turned by how much it has rolled): it never cuts into the track (worst 0.001 mm³), 0.3 mm lower it would overlap both rails (so it really rests on them), it stays at least 5.4 mm above the base, the contact stays on the straight part of the rails and the cone tips stick out more than 5 mm past them.

• Stop: it stops at x = 128 mm against the finish posts (0.5 mm further would overlap them), and the posts are taller than the axis so it cannot jump them.

• No colour overlaps another (0.0000 mm³) and the four colours fill each part exactly; one track part and three cone-plate parts, all on the bed; no overhangs above 45° (only the 10 mm bridge over the pin holes). Pin fits, a 0.2 mm thicker pin does not.

About this design

Designed with AI assistance and checked in code before release (see above). Pictures are 3D renders and the slicer preview, not photos. Not printed yet: makes and feedback are very welcome.

Español

El clásico truco del doble cono que "sube" la cuesta: los raíles se abren en V mientras suben 17 mm, el cono se hunde entre ellos y su centro de masas baja 6.4 mm. Pista con raíles a franjas de colores, cono a anillos en zigzag, dos placas sin soportes en el A1 mini con AMS (4 colores, 181 g en total).

العربية

الخدعة الفيزيائية الشهيرة: مخروط مزدوج يبدو أنه يتدحرج صعودا. تنفرج القضبان على شكل V وهي ترتفع 17 مم، فيغوص المخروط بينها وينخفض مركز ثقله 6.4 مم. قاعدة بقضبان ملونة ومخروط بحلقات متعرجة، لوحان بدون دعامات على A1 mini مع AMS بأربعة ألوان.