Domain: Engineering — Mechanical Engineering
Target concept: Connecting Rod Mechanism

Video generation prompt: The figure shows two planar four-bar linkages built from the same rigid material. The two linkages have different link-length proportions, and each mechanism has one link fixed to the ground. A motor turns the driving link of each mechanism at a steady, continuous rate; observe how the other links in each mechanism respond.

Evaluation guide:
(i) Target concept: Grashof criterion determining whether continuous input rotation is possible in a planar four-bar linkage.
Minimum mechanism: Four rigid links connected by four revolute joints in a closed loop; one link is fixed as ground; a driving actuation is applied to one moving link; whether any link in the loop can complete a full revolution is determined solely by the Grashof condition on the four link lengths (comparing the sum of the shortest and longest link lengths against the sum of the other two).
(ii)Storyline: Frame opens to a static side-by-side view of the two four-bar mechanisms with their ground pivots visible. Actuation begins on both mechanisms at the same moment: the left mechanism's driving link turns through complete, continuous revolutions, and its follower link responds with a bounded back-and-forth arc. On the right mechanism, the driving link is turned at the same steady rate, but because its link lengths do not satisfy the Grashof condition, no link in that loop (including the one being driven) can complete a full revolution; every moving link instead swings back and forth within a limited range, with the two non-driven links showing amplitudes and phases visibly different from each other and from the left follower. Viewpoint remains fixed; lighting, scale, and rigidity are preserved throughout; motion stops cleanly after one representative cycle.

Manual:
Experimental Procedure
1. Place two planar four-bar linkages side-by-side, both built from the same rigid material; the two mechanisms have different link-length proportions and different ground-link choices.
2. Turn the driving link of the left mechanism continuously.
3. Attempt to turn the driving link of the right mechanism at the same steady rate; because this mechanism's link lengths do not satisfy the Grashof condition (the sum of its shortest and longest link lengths exceeds the sum of the other two), no link in this loop — including the driving link itself — can complete a continuous revolution, so it too only oscillates back and forth within a limited angular range.
Observation of Experimental Phenomena
- Left mechanism: the driving link completes full, continuous revolutions, and the follower link oscillates between two fixed angular positions (no full rotation).
- Right mechanism: neither the driving link nor the other two moving links can complete a full revolution — all three oscillate back and forth, with different amplitudes and phases from each other and from the left follower.
Summary of Experimental Principles
Whether a four-bar linkage's links can rotate fully depends on the Grashof condition (comparing the sum of the shortest and longest link lengths to the sum of the other two). When the condition is satisfied and the shortest link is grounded (or made adjacent to the ground) appropriately, that shortest link can act as a full-rotation crank while the opposite link only rocks. When the condition is violated, as in the right-hand mechanism here, no link can complete a full revolution — every moving link, including the one being driven, can only oscillate.

Prompt Grounding rubric:
level_1: No two linkage mechanisms visible; or only one mechanism shown; or mechanisms not side-by-side in same frame; or driving link absent in either mechanism; or follower component absent in either mechanism.
level_2: Two mechanisms appear side-by-side, but at least one lacks a clearly identifiable input link or a clearly identifiable output link; or the two mechanisms visibly differ in material appearance (different colors, textures, or shading); or, within a single mechanism, the four link lengths look randomly mismatched rather than forming a coherent rigid linkage (e.g., one rod looks wildly out of scale with the other three in the same mechanism).
level_3: Two mechanisms appear side-by-side in the same frame, each with one clearly identifiable input link and one clearly identifiable output link; both mechanisms share the same material appearance; the four link lengths within each mechanism appear internally consistent and rigid, i.e., each mechanism's own rods maintain a coherent proportional relationship to one another (the two mechanisms need not match each other in these proportions).
level_4: The setup from the 3-point level holds, and both mechanisms are simultaneously animated from the same starting moment: motion begins on both at once, with no mechanism sitting static while the other moves, and no link freezing, skipping a position, or snapping backward unnaturally during the sequence.
level_5: The setup from the 4-point level holds exactly, and the animation is fully continuous and legible for the entire sequence: every joint in both mechanisms stays connected and in view, the ground pivots never shift position, and each mechanism completes at least one clear, unambiguous back-and-forth or rotational cycle of its input link before motion stops.

Scientific and Causal Correctness rubric:
level_1: The observed motion contradicts basic four-bar kinematics: e.g., the left mechanism's follower link completes full 360-degree rotations (impossible for a rocker), or any link of the right mechanism (including the one being driven) completes a full 360-degree rotation (impossible when no link in that linkage can fully revolve), or the two mechanisms show the exact same type of motion for their outputs (both fully rotating or both oscillating identically), erasing the distinction the setup is meant to reveal.
level_2: At least one mechanism's motion is inconsistent with its own link-length configuration: e.g., the left follower fails to rotate fully when it should, or any link of the right mechanism completes a full revolution when none should be able to; or the output motion lacks any oscillatory back-and-forth character at all (e.g., pure translation, jerky teleporting between positions, or no visible displacement).
level_3: The left mechanism's follower oscillates back and forth without ever completing a full 360-degree turn, and every moving link of the right mechanism (including the one being driven) oscillates back and forth without completing a full 360-degree turn; motion in both mechanisms visibly originates from the applied drive; no link in either mechanism rotates fully when the geometry forbids it.
level_4: Everything in the 3-point description holds, and in addition: the left follower's oscillation is visibly bounded by two consistent angular extremes with no overshoot past them, and the right mechanism's two non-grounded moving links show distinguishably different oscillation envelopes from each other (different amplitude and/or timing), rather than swinging identically in lockstep.
level_5: Everything in the 4-point description holds, and the joint layout itself visually explains the difference: on the left, the shortest link sits directly next to the ground pivot in a way consistent with it being the fully-rotating input, while the opposite link only rocks; on the right, the arrangement of ground pivots and link lengths is visibly consistent with no link being able to fully revolve, so that every moving link's restriction to oscillation follows directly from the visible geometry rather than looking arbitrary.

Spatiotemporal Consistency rubric:
level_1: A mechanism or one of its links disappears and reappears during the sequence; a joint visibly detaches and floats free of its link; a rod stretches, shrinks, or bends at a point that should stay rigid; a follower link visibly moves before its own driving link begins turning; the camera view or the scene's lighting changes abruptly partway through.
level_2: A structural break appears without being a full discontinuity: for example a joint disconnects and reconnects momentarily, a rod visibly kinks during motion that should keep it straight, or the motion freezes for a noticeable moment before continuing; or the camera viewpoint rotates, zooms, or pans during the sequence.
level_3: All rods in both mechanisms stay straight and rigid throughout; every joint stays fixed relative to its own ground pivot with no slippage or gaps; motion in both mechanisms is continuous with no freezing, jumping, or sudden reversal; the lighting on the two linkages stays steady in brightness and direction throughout the shot, with each mechanism's cast shadow anchored consistently beneath it.
level_4: Everything in the 3-point description holds, and in addition both mechanisms sit on the same visible ground plane with no parallax shift between them, and neither mechanism appears to grow or shrink in scale relative to the other at any point in the sequence.
level_5: Everything in the 4-point description holds, and the motion is fully physically plausible throughout: each follower link traces a smooth, continuous arc rather than a straight line or a jagged path; no two rods in either mechanism pass through one another; every joint center stays fixed in its own local frame with no drifting; each link's angular speed changes smoothly through the turning points of its oscillation rather than snapping instantly from one direction to the other.