Domain: Engineering — Mechanical Engineering
Target concept: Large and Small Spur Gear Pairs, Gear Transmission

Video generation prompt: A large gear and a small gear are meshed on parallel shafts; the large gear has three times as many teeth as the small gear (a 3:1 tooth ratio between the large and small gear); the initial image is stationary, with a red dot marked at the highest point of the large gear and a green dot marked at the highest point of the small gear; the large gear is slowly rotated clockwise, and there is no slipping at the meshing point.

Evaluation guide:
(i) Target concept: Kinematic transmission in external spur gear pairs under no-slip rolling contact.
Minimum mechanism: Teeth must be rigid, non-deforming bodies in pure rolling contact; rotation must obey gear ratio constraint; shafts must be fixed and parallel; marker dots must be kinematically bound to gear bodies.
(ii)Storyline: [Frame 0] Static scene: two spur gears meshed on parallel shafts, red dot centered on apex of large gear tooth, green dot centered on apex of small gear tooth; both dots at topmost vertical position. [Frame 1–N] Large gear begins smooth clockwise rotation; meshing forces induce immediate counterclockwise rotation in small gear; dots trace circular arcs around respective shafts; relative angular displacement accumulates such that small gear completes three full cycles per one cycle of large gear. Perspective is fixed frontal orthographic view: no camera motion, no occlusion, no depth ambiguity; all motion is planar and constrained to XY plane of gear faces.

Manual:
Experimental Procedure
1. Place large and small spur gears on fixed parallel shafts so they mesh fully.
2. Mark red dot at highest point of large gear tooth; mark green dot at highest point of small gear tooth.
3. Hold gears stationary, then rotate large gear slowly clockwise.
Observation of Experimental Phenomena
- Red dot moves clockwise with large gear; green dot moves counterclockwise with small gear.
- Small gear rotates faster than large gear.
- Dots remain fixed on tooth tips; no slipping occurs at mesh point.
Summary of Experimental Principles
- Gear transmission requires external meshing of teeth on parallel shafts.
- Rotation direction reverses between driver and driven gear.
- Angular velocity ratio equals the inverse of the tooth-number ratio: since the large gear has three times as many teeth as the small gear (tooth ratio large:small = 3:1), the small gear turns three times for every one turn of the large gear (ω_small : ω_large = 3:1).

Prompt Grounding rubric:
level_1: No two gears are shown, or they are not meshed; or no red dot appears on the large gear; or no green dot appears on the small gear; or neither dot sits at the highest point of its gear at the very start; or the gears are not mounted on parallel shafts; or the scene is already in motion in the opening frame instead of stationary.
level_2: Both gears are present, meshed, and mounted on parallel shafts, and both dots are present, but at least one dot is not sitting at the highest point of its gear in the first frame; or the opening camera view is not a fixed frontal shot (e.g., it is angled, tilted, or already moving).
level_3: A large spur gear and a smaller spur gear are meshed on visibly parallel shafts; a red dot sits exactly at the top of the large gear and a green dot sits exactly at the top of the small gear in the first frame; both gears are motionless before any rotation begins; the camera holds one fixed, frontal view of the whole gear pair with no occlusion of either gear.
level_4: All conditions for score 3 are met, and the large gear is noticeably bigger in diameter than the small gear, consistent with it carrying more teeth; the teeth of both gears are clearly interlocked with no visible gap or overlap at the mesh point in the first frame; no hand, tool, or label covers either gear before motion starts.
level_5: All conditions for score 4 are met, and both dots remain crisp and clearly colored (red versus green), with no blur or distortion, in the opening stationary frames; the two shafts are visibly straight and parallel with no tilt between them; the frame composition keeps both full gear faces in view with no cropping at the picture's edge.

Scientific and Causal Correctness rubric:
level_1: The small gear turns the same direction as the large gear instead of the opposite direction; or the dots move independently of their gears' rotation; or the small gear turns slower than or at the same rate as the large gear; or the red and green dots swap which gear they're on; or the teeth visibly slide past each other without gripping (slipping).
level_2: The small gear correctly turns counterclockwise while the large gear turns clockwise, but their speeds look about the same, with no visible difference; or a dot sits on the side or base of a tooth rather than its tip; or one of the two dots disappears partway through the clip.
level_3: The large gear turns clockwise and the small gear turns counterclockwise; the red dot stays on the large gear and moves with it from its starting position at the top; the green dot stays on the small gear and moves with it from its starting position at the top; the teeth stay meshed with no sideways slipping at the contact point; the small gear is visibly turning faster than the large gear.
level_4: Everything required for score 3 holds, and the speed difference matches the 3:1 tooth ratio in an obvious way: by the time the large gear completes less than one full turn, the small gear has completed more than two full turns; both dots keep tracking the tips of their teeth, never the base or side; the small gear's rotation never pauses, slows, or speeds up in a way that would suggest anything other than a constant drive from the large gear.
level_5: Everything required for score 4 holds, and the small gear completes exactly three full turns for every one full turn the large gear completes, with the dot positions confirming this by returning to the top of their own gear the matching number of times; the meshing point between the two gears never shows a visible gap or overlap between teeth at any point in the clip.

Spatiotemporal Consistency rubric:
level_1: The large or small gear jumps to a new position between frames instead of rotating smoothly; the shafts suddenly tilt or reorient; a dot vanishes and reappears without visibly traveling along the gear's edge; a gear's outline stretches, shrinks, or warps; the lighting or shadows flicker violently or jump between frames.
level_2: A dot floats briefly off the gear surface before settling back on; a gear's center shifts sideways by more than a dot's width between two consecutive frames; the shaft alignment visibly wobbles across the clip; background elements jitter or change scale inconsistently frame to frame.
level_3: Both gears stay rigidly mounted on their shafts throughout the clip; the dots stay attached to the gear surfaces they started on; neither gear ever jumps position, warps, or changes size; the rotation is smooth and continuous with no stutters or sudden repositioning; the shadows cast by the gears keep a consistent direction and darkness across the clip.
level_4: All conditions for score 3 are met, and the two shafts stay parallel to one another with no visible convergence or divergence anywhere in the clip; the distance between the two gear centers stays fixed; each dot traces a smooth circular path around its own gear's shaft; neither gear is ever shown appearing in front of or behind the other in a way that contradicts their earlier depth relationship.
level_5: All conditions for score 4 are met, and the dots move along their circular paths with no visible skipping, flickering, or shimmer along the gear edges; the rotating teeth show no strobing or reversal-illusion effect; every part of the scene stays consistent with a single fixed camera viewpoint, with no element appearing at the wrong depth relative to the others.