Domain: Natural Science — Physics
Target concept: Verification of the Independence of Free Fall Motion from Mass

Video generation prompt: Take a wooden ball with a mass of 100 grams and an iron ball with a mass of 1000 grams, which have the same shape and volume. Release them simultaneously from the same height and observe the order in which they hit the ground.

Evaluation guide:
(i) Target concept: Free-fall acceleration is independent of mass under uniform gravitational field and negligible drag.
Minimum mechanism: Identical shape and volume ensure near-identical air resistance; simultaneous release and same height ensure identical initial conditions; visual simultaneity of impact implies identical acceleration profiles.
(ii)Storyline: Frame 1: Two balls held side-by-side at identical height, stationary, clearly distinguishable by material appearance. Frame 2–N−1: Both balls descend vertically in parallel, maintaining lateral alignment, accelerating smoothly with no deviation. Final frames: Both balls contact ground surface in the same frame or consecutive frames with no perceptible delay: no bounce, no stagger, no differential deceleration. Viewpoint is fixed frontal/slightly elevated, full-body view of fall path and ground plane; no occlusion of balls or impact zone at any point.

Manual:
Experimental Procedure
1. Place wooden ball (100 g) and iron ball (1000 g) side-by-side at same height.
2. Release both balls simultaneously from rest.
3. Observe impact with ground.
Observation of Experimental Phenomena
Both balls strike the ground at the same instant — no observable time lag between impacts.
Summary of Experimental Principles
In vacuum or negligible air resistance, all objects fall with identical acceleration (g) regardless of mass — confirming independence of free-fall motion from mass.

Prompt Grounding rubric:
level_1: No wooden ball and no iron ball appear; or only one ball appears; or the balls differ in shape or volume; or the balls are not released simultaneously; or the release height is not the same; or no ground impact is shown.
level_2: Both balls appear but differ in shape or volume; or the balls are released at visibly different times (e.g., one ball starts falling while the other remains stationary); or the release heights are clearly unequal (e.g., one ball originates noticeably higher or lower in frame); or the ground impact is missing for either ball.
level_3: The wooden ball and iron ball both appear, matching in shape and volume, released from the same height and at the same moment, and both eventually hit the ground; but the moment of impact for one or both balls cannot be clearly seen (e.g., motion blur obscures contact, or the camera angle hides the ground-contact point).
level_4: All prompt elements are present and correctly set up: the wooden ball and iron ball match in shape and volume, are released from the same height at the same moment, and both ground-impact moments are clearly visible on screen; but the shot has a minor framing flaw, such as the camera briefly clipping part of one ball's fall path, or one ball drifting slightly out of lateral alignment with the other during the fall.
level_5: All prompt elements are present with clean, unobstructed framing: the wooden ball and iron ball are identical in shape and volume, are released simultaneously from the same height, and the entire fall path and both ground-impact moments for both balls are fully visible throughout, with no cropping, occlusion, or camera disruption.

Scientific and Causal Correctness rubric:
level_1: Phenomenon contradicts physics: e.g., iron ball hits ground first *and* is visibly heavier (e.g., labeled or color-coded as iron), or wooden ball hits first, or one ball floats/falls slowly while other accelerates abnormally, or both balls show inconsistent acceleration (e.g., one moves linearly, other curves).
level_2: Balls are correctly labeled or distinguishable by color/material (e.g., brown wood, gray metal), but impact order is unambiguously sequential (iron hits first *or* wood hits first) with clear temporal gap, no attempt to convey equivalence; or balls differ in size/shape implying mass difference unrelated to material.
level_3: Balls are distinguishable by material-appropriate appearance (e.g., brownish wooden surface, metallic-gray iron surface), same shape/volume, released together, both hit ground; but impact timing is ambiguous due to occlusion, motion blur, or low frame fidelity, yet no explicit violation of free-fall equivalence is shown.
level_4: Balls are visually distinguishable by material (wood vs. iron), same shape/volume, released simultaneously from same height, both impact ground, and impacts occur at indistinguishable moments; but one ball exhibits non-physical motion artifact (e.g., jitter, teleportation between frames, unnatural bounce, or sudden stop before impact).
level_5: Balls are visually distinguishable by material (e.g., matte brown wood texture, reflective gray iron texture), identical shape and volume, released simultaneously from same height, accelerate identically throughout fall (smooth, continuous downward motion without deviation), and impact ground in perfect visual synchrony: no motion artifacts, no acceleration disparity, no post-impact anomaly.

Spatiotemporal Consistency rubric:
level_1: Severe spatiotemporal break: one or both balls vanish mid-fall or appear discontinuously in a new position; the ground plane visibly shifts, tilts, or warps between frames; the lighting on the balls and ground flips abruptly between bright and dark with no visible cause; the camera viewpoint jumps or rotates abruptly mid-fall; or the background scenery changes entirely partway through the sequence.
level_2: Moderate break: one ball's position stutters or jumps briefly during the fall while the other continues smoothly; the ground plane shows a visible seam, doubling, or scale change; the shadows cast by the balls point in a different direction than the unchanged light source would allow; or the hand/support releasing the balls disappears inconsistently after release.
level_3: Minor but visible flaw: a brief, noticeable stutter or hitch in one ball's fall path, or a momentary shadow/lighting flicker, or a slight camera wobble; but the ground plane, viewpoint, and both balls' overall trajectories remain stable and clearly readable throughout.
level_4: Both balls' side-by-side spacing, the ground plane, the background, and the lighting/shadow direction stay visibly constant throughout the fall, with only one small, easily overlooked imperfection (e.g., a faint, brief flicker in one ball's outline, or a slightly soft shadow edge) that does not disrupt the viewer's read of the fall or impact.
level_5: No visible spatiotemporal flaws: the two balls keep the same side-by-side spacing from release to impact, the ground plane and background stay perfectly fixed, shadow direction and light intensity stay constant throughout, and both falls read as smooth, continuous, physically natural motion with no jumps, warps, or flicker anywhere in the sequence.