Domain: Natural Science — Physics
Target concept: The Relationship between the Pendulum and the Bob

Video generation prompt: Two pendulums of the same length (20 cm) but different masses (50 g and 100 g) are released from the same angle simultaneously to observe whether there is a difference in their oscillation periods.

Evaluation guide:
Target concept: The oscillation period of an ideal simple pendulum is independent of bob mass, governed solely by length L and gravitational acceleration g (T ∝ √L). Minimum mechanistic assumptions: (i) strings are massless and inextensible; (ii) bobs are point masses undergoing planar, small-angle (≤20°) oscillation; (iii) air resistance and pivot friction are negligible; (iv) gravitational field is uniform. Phased storyline: (1) Setup: Two pendulums mounted side-by-side, both strings measured to exactly 20 cm against calibrated ruler; bobs labeled ‘50 g’ and ‘100 g’. (2) Alignment: Bobs held at identical 20° angle using protractor guide, visual symmetry confirmed. (3) Release: Single coordinated action removes both supports simultaneously, captured in one crisp frame. (4) Observation: First 5 oscillations show perfect synchrony; bobs cross equilibrium, reach extremes, and reverse direction in unison; no phase drift. (5) Verification: On-screen timer confirms identical period (~0.90 s); motion trails or waveform overlays quantitatively match. Visibility constraints: Camera must be front-on, orthogonal to swing plane, with static lighting and ruler/protractor overlays, essential to verify length equality, angle equality, and temporal synchrony. Perspective must exclude parallax to avoid false length/angle perception.

Manual:
I. Basic Demonstration Information
Demonstration Title: Pendulum Period Independence from Bob Mass Demonstration
Subject: Natural Science - Physics - Oscillatory Motion
Demonstration Purpose: To empirically verify that the oscillation period of a simple pendulum depends only on its length and gravitational acceleration, not on the mass of the bob; to visually demonstrate isochronism between two pendulums of identical length (20 cm) but different masses (50 g and 100 g) released simultaneously from the same small angle (~20°).

II. Demonstration Carrier/Equipment
Core Carrier: Two simple pendulums — identical rigid supports, 20.0 cm non-stretchable strings (e.g., nylon monofilament), spherical bobs (50 g steel sphere, 100 g steel sphere; density-matched to ensure size difference reflects mass difference), clearly labeled with waterproof ‘50 g’ and ‘100 g’ tags.
Auxiliary Equipment: Calibrated vertical ruler/grid background (with 20 cm mark), fixed overhead light, protractor overlay (digital or physical), synchronized release mechanism (dual finger-stop or electromagnetic latch), neutral gray background, high-contrast black-and-white bob coloring (e.g., matte black 50 g, glossy silver 100 g) for visual distinction.

III. Demonstration Content Description
Layout: Mount both pendulums side-by-side on rigid frame, aligned vertically, with strings taut and bobs at rest touching same reference line (e.g., laser line or thin thread). Position camera front-on, orthogonal to swing plane, with full view of bobs, strings, pivot, and ruler.
State Control: Ensure bobs hang freely, no air currents; pre-release angle set identically using protractor (20° ± 1° from vertical); release mechanism ensures simultaneous removal of support for both bobs.
Comparison Baseline: Both pendulums share identical L = 20 cm (verified by ruler), identical small release angle, identical environmental conditions (still air, constant g), differing *only* in bob mass — enabling isolation of mass as sole variable.

IV. Observation of Core Phenomena
Synchronization Phenomenon: Both bobs depart from rest at identical instant and return to equilibrium point simultaneously in every oscillation cycle for ≥5 full periods.
Phase Stability: No observable phase drift — bobs reach leftmost, lowest, and rightmost positions at precisely the same moments; motion trails or side-by-side waveform plots confirm identical period (T ≈ 0.90 s).
Mass Irrelevance Evidence: Despite 100% mass difference, no difference in timing, amplitude decay rate, or swing smoothness; visual distinction (size/finish) confirms mass variation is present but causally inert to period.
Visual Verification Features: (1) On-screen 20 cm ruler confirming equal length; (2) Simultaneous release frame showing both supports retracting in same instant; (3) Overlaid protractor showing identical initial angle; (4) Synchronized tick marks or motion trails proving identical period; (5) Side-by-side bob labels ‘50 g’ / ‘100 g’ in consistent font/position.

V. Summary of Key Knowledge Points and Principles
Core Concept Definition: For a simple pendulum under small-angle approximation (θ < 15°), period T = 2π√(L/g) — independent of bob mass m, amplitude (for small θ), and material. This arises because gravitational restoring force F = −mg sinθ ∝ m, while inertia ∝ m, so m cancels in equation of motion.
Scientific Significance: Demonstrates how dimensional analysis and Newton’s second law predict parameter independence — foundational for modeling idealized systems and understanding when approximations hold (e.g., why grandfather clocks use heavy bobs for stability *without* affecting timing).
Common Misconception Alert: Heavier objects do *not* fall faster in gravitational fields — this experiment extends that principle to periodic motion: mass affects force *and* inertia equally, leaving dynamics unchanged.

Prompt Grounding rubric:
level_1: No pendulums are visible, or only one pendulum is shown; the two strings are visibly different lengths; the two bobs cannot be told apart in any way (no labels, no visible size or color difference); the two supports are released one after the other rather than together; or no release moment is shown at all.
level_2: Two pendulums are present and roughly the same length, but the strings look noticeably mismatched (a visible length gap of more than about 15%, not just a subtle one); the '50 g'/'100 g' labels are missing or too small/blurred to read; one bob is clearly let go a moment before the other, with an obvious delay of more than about 0.3 seconds; the two starting angles look uneven (one bob lifted noticeably higher than the other); and there is no ruler, grid, or marked scale anywhere in the shot to check the claimed 20 cm length.
level_3: The two strings look about the same length by eye (within roughly 10% of each other), both bobs carry '50 g' and '100 g' labels (or are otherwise visually distinguishable), and the release looks close enough to simultaneous (delay under about 0.2 seconds) that no delay stands out, but the labels are small, faded, or oddly placed, and there is no ruler, protractor, or alignment guide on screen to actually confirm the length or angle claims.
level_4: A ruler or grid with a marked 20 cm reference sits next to the strings and both appear to match it; both bobs carry large, legible, high-contrast '50 g' and '100 g' labels; the two supports let go with no visible delay between them; both starting angles look symmetric about the vertical, aided by a plumb line or angle guide visible in frame; and the background is plain and doesn't distract from the setup.
level_5: Everything in the 4-point band holds, and in addition: the ruler shows the 20 cm mark lining up with each string's full length from pivot to bob center; the 100 g bob is visibly larger in diameter than the 50 g bob in a way consistent with two same-material spheres of different mass, on top of the text labels; a protractor overlay or mirrored symmetry guide shows both release angles matching each other within about 2°; a single hand or mechanism releases both supports in one visible action; and nothing (no stray object, blur, or lingering hand) blocks the view of either bob during their first few swings.

Scientific and Causal Correctness rubric:
level_1: The two pendulums swing at obviously different rates (for example one completes three full swings while the other has only completed one in the same stretch of time); a bob speeds up or slows down in a way that doesn't look like gravity is driving it; the motion isn't confined to swinging back and forth (drifting sideways, floating, or leaving the vertical plane); or there is no repeating swinging motion at all: the bobs stop almost immediately, move chaotically, or never start.
level_2: The two periods look different within the first couple of swings; one bob's swings shrink away quickly while the other keeps a large swing going much longer (very unequal damping); the plane of swinging drifts or rotates instead of staying fixed; the pivot visibly bends or flexes under the string's pull; or there's no visible cause-and-effect between the release moment and the pendulums starting to swing.
level_3: Both bobs swing smoothly in the same vertical plane with repeatable arcs, and the two periods look the same for the first two or three swings, but a noticeable lag between the two bobs has appeared by the fourth swing; the shrinking of swing size looks roughly similar for both bobs; the pivots stay rigid; release visibly leads to swinging, but nothing in the shot (like a shared timer or aligned reference marks) actively shows the viewer that the two periods match.
level_4: The two bobs stay in step with each other (reaching the bottom and the far ends of their swings at essentially the same moments) for at least four full swings with no visible lag opening up between them; both bobs' swings shrink in size at a similar, gradual rate; the motion stays in one plane and always curves back toward straight-down rather than away from it; the pivots stay still; and if a timer or reference marks are shown, they read the same for both bobs.
level_5: The two bobs stay in step with each other for at least five full swings with no lag ever becoming visible between them; both bobs' swings shrink at a matching, gradual rate; an on-screen timer shows both periods landing at essentially the same value, in the neighborhood of 0.9 seconds for this 20 cm length, with the two readings differing by no more than about 0.03 seconds; the shot gives the viewer a direct way to see the two periods match (for example a shared timer readout or aligned motion trails for both bobs); and each bob's swing always looks governed by gravity pulling it back toward vertical, moving fastest at the bottom of the arc and pausing briefly at each far end.

Spatiotemporal Consistency rubric:
level_1: A pivot or support jumps to a different spot between frames with no motion shown getting there; a bob vanishes mid-swing and reappears somewhere else without visibly swinging there; the room's lighting suddenly switches to a different brightness or color partway through the clip with no visible source causing it; the background scene changes while the pendulums are still swinging; the release mechanism (hand or device) pops into or out of frame instead of visibly acting to release the bobs; or a bob is already swinging before the release mechanism has actually let go of it.
level_2: The pivots creep slightly away from where they were mounted as the swing continues; the two bobs' shadows point in noticeably different directions from each other even though there is only one overhead light; a bob's apparent size changes noticeably as it swings, as if the bob or camera were moving toward or away from the lens; the release mechanism fumbles, sticks, or moves jerkily; the clip stutters or skips frames during the swing; the lighting flickers or gradually brightens or dims; or the camera drifts, zooms, or pans instead of holding a fixed front-on view of the swing plane.
level_3: The camera holds a steady front-on view of the swing plane for the whole clip; both pivots stay put in the frame; the two bobs keep a constant apparent size and shape throughout; both bobs' shadows point the same direction, consistent with a single fixed overhead light; the release mechanism disengages cleanly before either bob starts moving; there are no dropped frames, though the bob paths show a small amount of wobble on close viewing, or the pivots show a slight residual wobble; and the gap between release and the first visible swing is short and physically plausible (roughly a few tenths of a second) rather than instantaneous or noticeably delayed.
level_4: Both strings stay straight and taut throughout the swing with no visible bowing or slack; each bob's shadow swings back and forth directly beneath it and never jumps to a different angle or suddenly changes brightness; the release mechanism disengages in one continuous action, fully clear of both bobs before either begins moving; the swinging motion looks smooth with no jerks or skipped frames; and the ruler or grid in the background stays geometrically fixed, with no drifting or warping as the bobs pass in front of it.
level_5: The two pivots and the ruler/grid markings stay in the same position for the entire clip with no visible drift; both strings stay visibly taut through the whole swing, including at the bottom of the arc where the bobs move fastest, with no slack or sag appearing; each bob's shadow tracks its position continuously, shortening as the bob nears the bottom of its arc and lengthening near the top, with no sudden jumps; the release action looks natural and unhurried for whatever mechanism is used: a hand's fingers opening together with a smoothly rotating wrist, or a mechanical latch disengaging both supports in one smooth, synchronized motion; and both bobs trace smooth circular arcs about their pivots consistent with simple pendulum motion, with the on-screen timer keeping pace with the swinging shown throughout and no visible speeding-up or slowing-down of the footage.