Domain: Natural Science — Physics
Target concept: Motion of Objects in a Vacuum

Video generation prompt: Two identical vacuum tubes are placed side by side on a table. A metal coin is released in the left vacuum tube, while a paper of the same size as the coin is released from the same height in the right vacuum tube.

Evaluation guide:
(i) Target concept: Identical gravitational acceleration of dissimilar masses in absence of drag.
Minimum mechanism: Tubes must be evacuated (no air), sealed, rigid, and vertically oriented; objects must be released from rest at same height with no initial velocity or torque; gravity must act uniformly downward in scene.
(ii)Storyline: [Frame 0] Two upright vacuum tubes side-by-side on table, coin centered in left tube opening, paper centered in right tube opening; [Frame T1] Both objects begin downward motion simultaneously with zero lateral displacement; [Frame T2] Both objects remain fully inside tubes, maintaining original orientation and shape, descending in strict vertical alignment; [Frame T3] Both objects contact tube bottoms at identical instant, with no rebound or tilt; [Perspective constraint] Fixed frontal view showing full height of both tubes, unobstructed line of sight to both falling paths; [Visibility constraint] No occlusion between tubes or objects at any frame; coin and paper must be distinguishable by material appearance (metallic vs. matte) but not by motion behavior.

Manual:
Experimental Procedure
1. Place two identical sealed vacuum tubes side-by-side on a flat table.
2. Insert a metal coin into the left tube and a paper sheet of identical size/shape into the right tube.
3. Release both objects simultaneously from the same height at the tube openings.
Observation of Experimental Phenomena
Both objects fall at identical speed and land simultaneously; neither flutters, rotates, nor deviates laterally; both remain rigid and vertically aligned inside their tubes.
Summary of Experimental Principles
In vacuum, all objects fall with identical acceleration regardless of mass or composition, as air resistance is eliminated — demonstrating Galileo’s principle of equivalence in free fall.

Prompt Grounding rubric:
level_1: Fewer than two vacuum tubes appear, or the tubes are not both visible; no metal coin and no paper sheet appear together; the coin and paper are released from noticeably different heights; or the two tubes are not positioned beside each other on a tabletop.
level_2: Two tubes are visible side by side on a table, but the coin and paper are not dropped at the same moment; or the coin and paper are clearly different in size or shape; or the release heights inside the two tubes are clearly different; or one item is thrown, slid, or pushed into the tube rather than simply let go.
level_3: Two matching vacuum tubes stand side by side on a table; a metal coin sits inside the left tube and a paper cutout of the same size and shape sits inside the right tube; both are let go from the same height at the same moment; no hand, jet of air, or other force pushes either object during release.
level_4: Everything required for a 3 also holds, and in addition: both tubes appear fully closed with no visible opening or vent; the coin and paper are already resting inside their tubes before release begins; and nothing (fingers, tools, tube labels) blocks the view of either object's fall.
level_5: Everything required for a 4 also holds, and in addition: the coin and paper start at matching heights measured from their own tube's rim; both tubes stay in the same place and orientation from start to finish; and a viewer can always tell which object belongs to which tube.

Scientific and Causal Correctness rubric:
level_1: The coin and the paper visibly fall at different speeds; one of them floats, stalls, or hangs in place while the other drops; their downward acceleration is clearly unequal; or the paper flutters, bends, or spins as it falls.
level_2: The coin and paper fall at roughly the same speed, but one clearly reaches the bottom noticeably before the other; or the paper keeps curling, folding, or fluttering as it descends; or the coin wobbles, spins, or drifts sideways inside its tube.
level_3: The coin and paper descend together from release to landing and touch down at the same moment; both stay rigid and flat the whole way down, with the paper showing no fluttering, sagging, or rotation that would indicate air resistance.
level_4: Everything required for a 3 also holds, and in addition: neither object drifts sideways or brushes against its tube wall during the fall; and both keep their original shape and orientation (coin flat-on, paper flat and level) without bending or warping.
level_5: Everything required for a 4 also holds, and in addition: both objects fall in a straight vertical line; there is no perceptible lead or lag between the coin and the paper at any point during the fall; and both touch the tube bottom at the same instant with no bounce or tilt.

Spatiotemporal Consistency rubric:
level_1: The coin, the paper, or either tube jumps to a new position or angle between frames with no motion shown in between; the lighting flashes, flickers, or otherwise changes with no visible cause; either object vanishes and reappears mid-fall; or a tube visibly cracks, splits, or bends into an impossible shape.
level_2: The coin or paper shifts sideways between frames for no visible reason; the base of a tube wobbles or lifts off the table; the tabletop or background warps or jitters; or the fall looks jerky, with three or more noticeable skips or stutters in the motion.
level_3: The coin and paper move smoothly and continuously throughout the fall; both tubes stay planted in the same spot and orientation on the table; the direction and brightness of the light on the tubes and objects stay the same from the first frame to the last; and neither object disappears, reappears, or suddenly changes size.
level_4: Everything required for a 3 also holds, and in addition: the coin and paper show no sideways drift relative to the tube walls or the tabletop as the camera view holds steady; the edges of the tubes and the edge of the table line up consistently from frame to frame; and the fall does not visibly speed up or slow down in an unnatural way.
level_5: Everything required for a 4 also holds, and in addition: the motion of the coin and paper is smooth with no visible stutter, flicker, or ghosting; their shadows stay attached to the objects and do not detach or lag behind; and when both objects are visible together, the coin and paper never appear to occupy the same depth or pass through each other's tube.