The “Physics Pass” Prompt: 9 Lines That Make Motion Look Real in Veo3Gen (Runway Gen‑4.5 ‘Physical Accuracy’ Claims, Applied Today)

What “physical accuracy” means for creators (in plain language)

When video model releases talk about “physical accuracy,” they’re usually pointing at motion that reads like the real world: objects feel like they have weight, movement has momentum, contacts don’t slide through surfaces, and timing doesn’t look floaty. Runway’s Gen-4.5 announcement explicitly frames this as realistic weight, momentum, and force in object motion—alongside broader claims about dynamic action and temporal consistency. (https://runwayml.com/research/introducing-runway-gen-4.5)

The practical takeaway for you inside Veo3Gen is simple: don’t just describe what happens (“the phone drops”). Describe why it moves (gravity), what resists it (air drag, friction), where it touches (contact points), and how it deforms (bounces, dents, ripples). Those details give the model more constraints, which often converts “AI glide” into motion that feels grounded.

The Physics Pass: a 9-line add-on you can paste under any Veo3Gen prompt

This is Veo3Gen-native: you keep your normal creative prompt, then append the Physics Pass as a second block. Think of it as a motion “shader”—a consistent checklist of constraints.

Copy/paste template (9 lines)

Paste this under your base prompt and fill the brackets:

PHYSICS PASS
1) Mass/weight: [light/medium/heavy], described as [dense/hollow/top-heavy], scale [real-world size].
2) Forces: gravity present; motion driven by [push/pull/impact/pour/walk], plus [wind/drag] if relevant.
3) Friction/traction: surface is [rubber/wood/metal/cloth/wet], coefficient feel [grippy/slippery]; no unrealistic sliding.
4) Inertia/momentum: movement eases in/out; follow-through; overshoot allowed only if physically plausible.
5) Contact points/collisions: specify first contact at [edge/corner/sole/lip], then secondary contacts; no interpenetration.
6) Deformation/material behavior: [rigid/elastic/viscous/flexible], show [bend/compress/rebound/ripple/splash].
7) Timing/acceleration curve: [slow start → fast fall → sharp stop], include micro-jitter only from [handheld/impact].
8) Camera physics: [locked tripod / handheld], lens [wide/normal], motion consistent with operator movement; stable horizon unless stated.
9) Continuity checks: consistent scale, shadows, and object identity across frames; no sudden morphing.

Where it attaches (example)

Write your scene normally, then add the Physics Pass:

Base prompt: “Macro product shot of a smartwatch landing on a table, cinematic lighting.”

Then append: the 9-line Physics Pass block with smartwatch-specific details.

The 6 realism levers (and prompt phrases that actually help)

Below are the levers you’ll tweak most often. Each one includes concrete language you can drop into line items.

1) Mass/weight

If weight isn’t specified, motion often looks buoyant.

• Prompt phrases: “heavy, dense, top-heavy,” “noticeable sag when held,” “thud on impact.”

2) Friction

Friction is the difference between “skates across the table” and “catches, pivots, then stops.”

• Prompt phrases: “grippy rubberized surface,” “slightly dusty wood with mild resistance,” “wet countertop: slippery.”

3) Inertia

Inertia is the lag between intent and motion—especially on handheld camera or body movement.

• Prompt phrases: “ease-in, ease-out; follow-through in shoulders,” “object continues a few cm after push then settles.”

4) Deformation

Even “rigid” objects show micro-behavior: bounce, vibration, squash in soft materials.

• Prompt phrases: “minor bounce then damped vibrations,” “fabric ripples propagate then fade,” “liquid forms a crown splash then settles.”

5) Contact points

Believability skyrockets when you name where contact happens.

• Prompt phrases: “first contact on the bottom-right corner,” “secondary contact along the long edge,” “shoe sole contacts heel-first then rolls to toe.”

6) Timing/acceleration curves

“Moves quickly” is vague; curves are interpretable.

• Prompt phrases: “slow start → rapid acceleration → sharp deceleration,” “two quick impacts: tap-tap, then stop.”

Five fully written Veo3Gen prompts (with the Physics Pass attached)

Use these as full prompts or as patterns.

1) Product drop + impact (phone case)

Prompt: A clean studio product video: a matte black phone case drops from 30 cm onto a wooden desk, close-up, crisp softbox reflections, shallow depth of field, realistic sound implied by visuals only.

PHYSICS PASS

1. Mass/weight: medium, described as semi-rigid polymer, scale real-world phone case.
2. Forces: gravity present; motion driven by release from rest; slight air drag.
3. Friction/traction: desk is dry wood, moderately grippy; no long sliding.
4. Inertia/momentum: accelerates naturally, brief bounce, then settles.
5. Contact points/collisions: first contact at one bottom corner, then rotates to land flat; no clipping.
6. Deformation/material behavior: semi-rigid with slight flex on impact; brief vibration.
7. Timing/acceleration curve: fast fall → sharp impact → quick rebound → damped stop.
8. Camera physics: locked tripod, normal lens, no handheld wobble.
9. Continuity checks: consistent shape, logo placement, and shadows across frames.

2) Pouring liquid (iced coffee into glass)

Prompt: Kitchen countertop scene, close-up of a clear glass with ice cubes; iced coffee pours from a small pitcher into the glass, warm morning light, macro detail, realistic turbulence.

PHYSICS PASS

1. Mass/weight: liquid behaves heavy enough to feel like coffee, real-world scale glass.
2. Forces: gravity present; pour driven by tilting pitcher.
3. Friction/traction: liquid wets glass; countertop dry.
4. Inertia/momentum: stream responds with slight lag to pitcher tilt; slosh settles.
5. Contact points/collisions: stream hits ice first, deflects around cubes; no passing through ice.
6. Deformation/material behavior: viscous fluid; visible eddies; small splashes; bubbles rise.
7. Timing/acceleration curve: gentle start → steady pour → taper off → drips.
8. Camera physics: locked tripod, macro/normal lens, stable framing.
9. Continuity checks: consistent fill level increase; ice remains coherent.

3) Fabric/coat movement (turning in a trench coat)

Prompt: Street-style fashion shot: a person in a beige trench coat turns 90 degrees, coat belt and hem trailing naturally, overcast daylight, medium shot, cinematic.

PHYSICS PASS

1. Mass/weight: coat is medium-weight fabric, slightly stiff, real-world drape.
2. Forces: gravity present; motion driven by body turn.
3. Friction/traction: shoes on dry pavement, good traction; no foot sliding.
4. Inertia/momentum: coat lags behind torso; belt follows with delayed swing.
5. Contact points/collisions: coat brushes legs on the inside of the turn; no cloth intersecting body.
6. Deformation/material behavior: flexible fabric with ripples traveling from shoulders to hem; settles after turn.
7. Timing/acceleration curve: step → turn → coat follow-through → settle.
8. Camera physics: handheld but controlled, slight natural sway, horizon mostly level.
9. Continuity checks: consistent face, coat details, and lighting across frames.

4) Food steam/sizzle (steak on hot pan)

Prompt: Close-up cooking shot: steak placed onto a hot cast-iron pan, visible sizzle, small bursts of steam, droplets of oil shimmering, cinematic kitchen lighting.

PHYSICS PASS

1. Mass/weight: steak is heavy and damp, real-world thickness.
2. Forces: gravity present; motion driven by hand placing it down.
3. Friction/traction: pan surface high friction; steak doesn’t slide after contact.
4. Inertia/momentum: quick placement then immediate stop; subtle pan vibration.
5. Contact points/collisions: first contact at one edge, then lays flat; no hovering.
6. Deformation/material behavior: slight compress as it lands; steam rises; oil micro-splatter.
7. Timing/acceleration curve: fast downwards motion → sharp contact → small settling.
8. Camera physics: locked tripod, shallow depth of field, no camera shake.
9. Continuity checks: consistent texture; steam direction coherent.

5) Handheld walk-and-talk (street interview)

Prompt: Handheld walk-and-talk on a city sidewalk: subject speaks to camera while walking, natural pacing, subtle background parallax, documentary feel, daylight.

PHYSICS PASS

1. Mass/weight: subject movement grounded; body weight shifts heel-to-toe.
2. Forces: gravity present; motion driven by walking gait.
3. Friction/traction: sidewalk dry, strong traction; no foot skating.
4. Inertia/momentum: head and shoulders have slight bob; arms swing naturally.
5. Contact points/collisions: footfalls are heel strike → mid-stance → toe-off.
6. Deformation/material behavior: clothing shows minor wind flutter and gait-driven folds.
7. Timing/acceleration curve: consistent cadence; small speed changes feel gradual.
8. Camera physics: handheld operator walking backward; mild vertical bob + micro-jitter; no impossible smooth gliding.
9. Continuity checks: consistent identity and outfit; no sudden face morphs.

Common failure patterns (and the one line that usually fixes each)

• Floaty motion / zero weight → Strengthen Line 1: “heavy, dense; thud; brief bounce then settle.”
• Unreal sliding on contact → Strengthen Line 3: “grippy surface; no long sliding; stops within a few cm.”
• Clipping through surfaces → Strengthen Line 5: name the first contact point and the secondary contacts.
• Rubbery objects that should be rigid (or vice versa) → Strengthen Line 6: “rigid with micro-vibration” vs “elastic rebound.”
• Camera motion that contradicts the scene (glide-cam while ‘handheld’) → Strengthen Line 8: pick one camera physics model and stick to it.
• Time feels wrong (teleporty acceleration) → Strengthen Line 7: specify a curve like “slow start → rapid → sharp stop.”

Don’t do this: vague verbs, impossible interactions, conflicting camera physics

A Physics Pass works best when it replaces ambiguity with constraints.

Vague verbs to avoid

• “moves,” “floats,” “glides,” “shifts,” “bounces realistically” (without details)

Upgrade them: “slides 2–3 cm then catches,” “drops 30 cm,” “rotates 15 degrees on first impact.”

Impossible interactions to avoid

• “heavy object falls slowly with no air resistance cues”
• “liquid stream stays perfectly rigid”
• “fabric is both stiff leather and ultra-silky drape in the same shot”

Conflicting camera physics to avoid

• “handheld” + “perfectly stabilized, zero bob, zero micro-jitter”
• “locked tripod” + “large swaying parallax”

Mini prompt library: 12 ready-to-steal Physics Pass snippets

Drop these phrases into the matching line.

1. Weight: “top-heavy; pivots after impact.”
2. Weight: “light but not weightless; responds quickly to small forces.”
3. Friction: “slightly slippery due to condensation.”
4. Friction: “rubber-on-concrete traction; no lateral skid.”
5. Inertia: “follow-through in wrists; object continues briefly.”
6. Inertia: “settles with damped oscillations.”
7. Contact: “first contact on the leading edge; then rolls to flat.”
8. Contact: “finger pad compresses slightly at touch; no penetration.”
9. Material: “viscous, cohesive stream; occasional droplets separate.”
10. Material: “stiff fabric with delayed fold formation.”
11. Timing: “two-stage action: quick hit → slow settle.”
12. Camera: “handheld walking bob, subtle vertical rhythm; horizon mostly level.”

How to A/B test realism without burning credits

Instead of rewriting everything, A/B test only the Physics Pass.

1. Generate Version A with your base prompt.
2. Generate Version B with the same base prompt + Physics Pass.
3. On Version B, do one more iteration where you only change one lever (e.g., friction) to see what it actually controls.

Quick evaluation checklist (one page)

• Weight: Does the object feel heavy/light in a believable way?
• Friction: Does it stop/slide like the surface material you described?
• Inertia: Is there follow-through instead of instant starts/stops?
• Deformation: Do materials bend/splash/ripple in a plausible way?
• Contact: Can you point to the first impact/touch moment clearly?
• Timing: Do accelerations look continuous (no teleporty jumps)?
• Camera: Does camera shake match the declared rig (tripod vs handheld)?
• Continuity: Any sudden morphing, scale changes, or lighting jumps?

Related reading

• Veo 3 API features guide
• Veo 3.1 vs Sora 2 comparison
• Veo 3.1 release date & launch notes

CTA: put the Physics Pass into production

If you’re generating lots of motion variants (product angles, pours, fabric tests), it’s easier when your workflow is programmable.

• Explore the Veo3Gen API for automated A/B testing and batch generations: /api
• See plans and credit options before you scale up: /pricing

This “Physics Pass” is designed to be evergreen: keep your creative direction the same, and tighten the physical constraints until motion reads real.

FAQ

What’s the difference between “physical accuracy” and “photorealism”?

Photorealism is mostly about visuals (lighting, texture). Physical accuracy is about motion behavior—weight, momentum, force, and consistent interactions. Runway’s Gen-4.5 post explicitly calls out weight/momentum/force as part of its accuracy framing. (https://runwayml.com/research/introducing-runway-gen-4.5)

Do I need to use all 9 lines every time?

No. Start with Lines 1, 3, 5, and 7 (weight, friction, contact, timing). Add deformation (Line 6) for liquids, fabric, and food.

Why does specifying contact points help so much?

Because “hits the table” is underspecified. “First contact on the corner, then rotates to land flat” creates a chain of constraints that reduces clipping and float.

How do I keep handheld shots from looking like impossible gimbal footage?

Be explicit in Line 8: “handheld operator walking—mild vertical bob and micro-jitter; horizon mostly level.” Avoid mixing “handheld” with “perfectly stabilized.”

Can this help with consistency across shots?

It can help reduce motion weirdness, but identity/scene consistency is a separate issue. Runway’s Gen-4 post frames Gen-4 as able to generate consistent characters/locations/objects across scenes; your best bet is still to keep prompts consistent and add continuity checks. (https://runwayml.com/research/introducing-runway-gen-4)

Sources

• https://runwayml.com/research/introducing-runway-gen-4.5
• https://www.cnbc.com/2025/12/01/runway-gen-4-5-video-model-google-open-ai.html
• https://www.techbuzz.ai/articles/runway-s-gen-4-5-claims-unprecedented-ai-video-accuracy
• https://runwayml.com/research/introducing-runway-gen-4