Free Fall¶

Classify and predict a body's motion by enforcing the gravity-only condition: after release, no dynamically significant support, drag, thrust, lift, tension, or other non-gravitational force acts, whether the body moves downward, upward, ballistically, or in orbit.

Version: v3 · 2026-09-06 · History
Domain-specific #: 1884
Origin domain: physics
Subdomain: classical mechanics and gravitation
Aliases: Gravitational free fall

Core Idea¶

Free fall is the dynamical regime in which gravity is the only force acting on a body, to the accuracy claimed by the model. The definition is a force criterion, not a visual description. A released ball remains in free fall while rising, at the instant its vertical velocity is zero, and while descending. A satellite in an unpowered ideal orbit is continuously falling even when its distance above Earth's surface is constant. Conversely, a skydiver at terminal velocity is moving downward but is not in strict free fall, because aerodynamic drag supports the body's weight. OpenStax accordingly treats ascent and descent with the same free-fall equations and defines its elementary ideal as motion without air resistance or friction.^[1] Its orbital treatment states the decisive point directly: gravity supplies the centripetal acceleration and is the only force acting on an ideal satellite.^[2]

The word only carries the abstraction. In a Newtonian inertial frame, write the force ledger as.

m a = F_gravity + F_drag + F_normal + F_thrust + F_lift + F_tension + ....

The strict free-fall model applies when every non-gravitational term is zero or negligible at the declared tolerance. This is why dropping, throwing, jumping after loss of contact, ballistic flight, and orbit can share one identity, while standing, parachuting, powered flight, and terminal-velocity descent fail it. NASA's aerodynamics account makes the exclusion quantitative: a body falling through air has both weight and drag; at terminal velocity those forces balance and acceleration becomes zero.^[3] Downward motion therefore is neither necessary nor sufficient.

The classical wording requires a declared frame and gravitational model. Near Earth's surface one may approximate the field as uniform and use constant g. Across orbital distances one uses the central inverse-square field. In general relativity the ontology changes: an ideal test body under no non-gravitational force follows a timelike geodesic, locally experiences weightlessness, and has zero proper acceleration even though its coordinates can accelerate. Tidal effects remain across an extended body or finite laboratory because spacetime curvature cannot be transformed away over an arbitrarily large region.^[4]^[5]

The locked identity is:

body or test particle + declared frame and gravity model + force ledger excluding every dynamically significant non-gravitational force + initial state -> ballistic or orbital trajectory with no support-force loading, valid within an explicit approximation regime.

Free Fall earns an autonomous domain node because this identity organizes classification, equation selection, approximation tests, apparatus design, and failure diagnosis throughout mechanics and gravitation. It is not merely the result of substituting a value into a kinematics formula.

Structural Signature¶

Sig role-phrases:

the falling body or test particle — the object whose center-of-mass motion is modeled, with finite-size, rotation, and self-gravity neglected or separately bounded
the declared reference frame — ordinarily an inertial or sufficiently local frame in Newtonian mechanics, or a named coordinate frame plus proper-acceleration criterion in relativity
the gravitational source and field model — constant near-surface g, a central inverse-square field, a many-body field, or curved spacetime
the force ledger — an exhaustive inventory separating gravity from material non-gravitational interactions such as support, drag, thrust, lift, tension, and electromagnetic forces, with frame-induced inertial terms tracked separately as coordinate bookkeeping
the exclusive-force condition — every non-gravitational force is zero or negligible relative to the required accuracy
the initial state — release position and velocity, which may point downward, upward, sideways, or along an orbit
the gravity-only equation of motion — the field-specific dynamical law that maps the force condition and initial state into a trajectory
the ballistic or orbital trajectory — the resulting worldline, not restricted to decreasing height or straight vertical motion
the support-force or proper-acceleration readout — zero ideal contact loading in the body's local frame, which explains weightlessness without implying weak gravity
the validity envelope — spatial scale, duration, drag ratio, tidal gradient, mass ratio, and measurement tolerance within which “only gravity” is defensible

Recognition test. Inventory all dynamically relevant physical forces and declare the frame. If gravity alone remains at the model's tolerance and the trajectory follows from a gravity-only equation plus initial conditions, the case is free fall. A material normal force, drag, thrust, lift, tension, or electromagnetic interaction makes the physical free-fall identity fail. In non-inertial coordinates, retain the required inertial terms or transform frames when predicting motion; those coordinate terms do not themselves change the body's invariant support or proper-force status. The direction or sign of velocity does not decide the case. A strong empirical test compares accelerometer or trajectory residuals against the predicted gravity-only motion and assigns any departure to a named omitted influence.

What It Is Not¶

Not any downward motion. An elevator descending at constant speed is supported by cable tension; a parachutist at terminal velocity is supported aerodynamically. Both move downward without satisfying the force criterion.^[3]
Not downward motion only. A thrown ball is in free fall throughout its upward leg and instantaneous apex after contact with the thrower ends, provided air resistance is negligible.^[1]
Not zero gravity. Orbiting astronauts remain in a strong gravitational field. They appear weightless because spacecraft and occupants accelerate together without a support force.^[6]^[2]
Not weightlessness by itself. Weightlessness is a local support-force experience and can be approximate. The classification additionally requires a defensible force ledger and gravity-only dynamics.
Not terminal velocity. At terminal velocity drag equals weight, net force is zero, and velocity is constant. Gravity is not the only force.^[3]
Not all projectile motion. An ideal projectile after release is a free-fall case; a projectile subject to appreciable drag, lift, thrust, or guidance is not strict free fall.
Not an inertial straight line in Newtonian space. Newtonian free fall has gravitational acceleration. The general-relativistic statement that a free body follows a geodesic uses a different account of gravitation and must not be imported as “no acceleration” in an arbitrary coordinate frame.^[4]
Not the Equivalence Principle. Free Fall is a motion class with an exclusive-force criterion. The Equivalence Principle is the broader local claim connecting freely falling frames, universality of fall, and gravity-free special-relativistic laws.^[5]
Not “unforced” without qualification. In Newtonian mechanics gravity is retained as a force; in general relativity an ideal freely falling body has no non-gravitational proper force. The theory context must be stated.

Scope of Application¶

Near-surface kinematics: dropped and thrown bodies over distances small enough that g is approximately constant and air resistance can be neglected.
Ballistics and suborbital motion: unpowered trajectory segments after launch or engine cutoff, with atmosphere, oblateness, rotation, and other perturbations admitted only when bounded or added as corrections.
Orbital mechanics: ideal satellites, moons, planets, and spacecraft coasting under gravitation. Circular orbit is a special free-fall solution, not an absence of falling.^[2]
Microgravity research: drop towers and parabolic-flight intervals produce laboratories that fall with their experiments, suppressing relative support loads. NASA's Zero Gravity Research Facility uses a 132-meter evacuated drop and provides 5.18 seconds of near-weightlessness; its low chamber pressure reduces aerodynamic-drag acceleration below 0.00001 g.^[7]
Equivalence-principle experiments: comparison of freely falling test bodies tests whether their accelerations are composition-independent, while drag-free control and differential measurement bound non-gravitational contamination.
General relativity: ideal test bodies follow geodesics, locally define freely falling frames, and have zero proper acceleration; finite-region tidal effects expose curvature.^[4]^[5]
Engineering validation: accelerometer data and orbit residuals test whether a segment is sufficiently close to gravity-only for a specified task. “Free fall” may be an approximation with a numerical error budget rather than an exact fact.

The node does not cover colloquial adventure-sport usage unless aerodynamic loading is explicitly treated as negligible for the purpose at hand. It also does not make an extended deformable body exactly weightless: a shared center-of-mass fall can coexist with gravity gradients, rotation, internal stress, and residual gas drag.

Clarity¶

Three separations make the term reliable.

First, distinguish kinematic appearance from dynamical cause. Position and velocity describe what a body is doing at an instant; the force ledger explains why its velocity changes. An upward velocity and a downward acceleration coexist without contradiction. At an apex the velocity is zero but acceleration is not. A satellite can maintain nearly constant altitude while its velocity direction changes continuously under gravity. Asking “which way is it moving?” cannot replace asking “which forces act?”

Second, distinguish exact identity from controlled approximation. Strictly, a low-altitude object in air experiences drag and a spacecraft experiences solar pressure, atmospheric drag, outgassing, and third-body gravity. Mechanics still uses free fall when omitted non-gravitational forces are below a declared tolerance over the interval of interest. A defensible claim therefore reports a scale such as |F_non-grav|/|F_gravity|, a trajectory-residual bound, or an accelerometer threshold. NASA's evacuated drop chamber demonstrates the intervention: lower gas pressure until drag acceleration is below 10^-5 g, then the gravity-only model becomes experimentally useful.^[7]

Third, distinguish coordinate acceleration from felt acceleration. A person standing on Earth has nearly zero coordinate acceleration in an Earth-fixed description but feels the ground's normal force. An orbiting astronaut has substantial centripetal coordinate acceleration yet feels nearly weightless. In general relativity, an ideal free-fall accelerometer reads zero proper acceleration locally; over a larger cabin, differential acceleration between separated parts reveals tidal curvature.^[4] “Acceleration is zero” is therefore incomplete unless it names the acceleration and frame.

Manages Complexity¶

Free Fall supplies a compact model-selection workflow.

Choose the body and interval. Decide whether the target is a point mass, center of mass, extended vehicle, instrument package, or fluid parcel.
Declare the frame and theory. State inertial versus rotating coordinates, Newtonian versus relativistic dynamics, and the gravitational sources retained.
Build the physical force ledger. Include gravity and material non-gravitational interactions such as drag, normal contact, thrust, lift, tension, electromagnetic forces, buoyancy, and radiation pressure. In non-inertial coordinates, separately retain the required inertial terms or transform to an inertial frame; those bookkeeping terms are not physical contaminants.
Set the tolerance. Specify the maximum non-gravitational acceleration or trajectory error acceptable for the prediction.
Apply the exclusion test. Remove only terms demonstrated negligible. If any non-gravitational term exceeds tolerance, route to a perturbed-motion model rather than labeling the motion strict free fall.
Select the field model. Use constant g only for small near-surface regions; use inverse-square or many-body gravity when field variation matters; use geodesic dynamics when relativistic accuracy is required.
Propagate initial conditions. Direction comes from the initial velocity and changing gravitational field, not from the word “fall.”
Validate against residuals. Compare accelerometer readings or measured trajectory with the gravity-only prediction, then attribute discrepancies to named omitted forces or model limits.

The workflow localizes interventions. Excess drag calls for evacuation, a drag shield, a ballistic-coefficient model, or a shorter interval. A normal force calls for release or suspension removal. Thrust calls for engine cutoff or explicit propulsion dynamics. Rotating-frame residuals call for Coriolis and centrifugal terms or transformation to an inertial frame. Tidal error calls for a smaller laboratory, a shorter duration, or a gradient model. Treating all these as generic “falling error” would obscure the repair.

Abstract Reasoning¶

Near Earth's surface, take upward as positive and approximate g as constant. Once a body is released with no appreciable non-gravitational force,

a_y = -g, v_y(t) = v_0 - g t, and y(t) = y_0 + v_0 t - (1/2) g t^2.^[1]

Suppose a ball leaves a hand upward at v_0 = 19.6 m/s with g = 9.8 m/s^2. It reaches its apex when v_y = 0, so t_apex = v_0/g = 2.0 s. Its rise is v_0^2/(2g) = 19.6 m. If it returns to the release height without drag, the total time is 4.0 s. The force ledger is unchanged through all three visually different phases: upward motion, zero instantaneous vertical velocity, and downward motion. This computation is the simplest proof that direction is not the identity.

For a central body with gravitational parameter mu = GM, the point-mass equation is

r_ddot = -mu r / |r|^3.

A circular solution at radius r has v = sqrt(mu/r) and T = 2 pi sqrt(r^3/mu).^[2] For a nominal altitude of 400 km, using r = 6.771 x 10^6 m and mu = 3.986 x 10^14 m^3/s^2, the ideal circular speed is about 7.67 km/s and the period about 92.4 min. The gravitational acceleration is still about 8.69 m/s^2. Constant altitude does not mean zero gravity; sideways velocity makes Earth's surface curve away at the same rate as the satellite falls.

The force-ratio test turns “negligible” into an operational statement. Let epsilon = |a_non-grav|/|g_model|. A laboratory requiring residual acceleration below 10^-5 g cannot treat an atmospheric drop with epsilon = 10^-2 as free fall, but can accept a vacuum drop measured below its threshold. The exact numerical tolerance belongs to the experiment, not to the universal definition.

In general relativity, the analogous ideal worldline satisfies the geodesic equation. Its four-acceleration vanishes, yet neighboring worldlines can converge or separate through curvature. The Newtonian equation and relativistic geodesic are not rival definitions pasted together: they are theory-relative realizations of the same specialist exclusion—no non-gravitational forcing—while differing on whether gravity itself is represented as force or geometry.^[4]

Knowledge Transfer¶

Within mechanics, the force-ledger method transfers literally. The same recognition test handles a dropped laboratory package, a ball after release, a suborbital payload after cutoff, an unpowered spacecraft, and an orbiting satellite. In each case one declares the frame, retains gravity, excludes or bounds other forces, propagates initial conditions, and diagnoses residuals. What changes is field geometry and the scale of perturbations, not the identity.

The Newtonian-to-relativistic transfer is controlled rather than verbal. “No non-gravitational force” survives; the bookkeeping of gravity changes. Newtonian analysis puts gravity on the right side of F = ma. General relativity treats ideal free motion as geodesic and distinguishes coordinate from proper acceleration. The transfer succeeds only when this translation is made explicitly and tidal limitations are retained.

Outside physics, the generic move “remove perturbations and inspect a baseline” belongs to Zero-Force Null Baseline, while “state the observational coordinates” belongs to Frame of Reference. Calling a market price, software process, or social actor “in free fall” transfers only a metaphor of rapid uncontrolled descent. It loses gravitational source, force ledger, inertial-frame conditions, trajectory law, support-force readout, and the orbital/upward counterexamples. Those losses are exactly why Free Fall remains domain-specific rather than a prime.

Examples¶

Canonical: a ball thrown upward¶

A ball leaves a hand upward at 19.6 m/s. Neglect air resistance over the short interval and analyze from an approximately inertial ground frame with uniform g = 9.8 m/s^2. After release the hand's normal force disappears; gravity is the only retained force. The ball rises for 2.0 s, is instantaneously motionless at an apex 19.6 m above release, and returns after another 2.0 s.^[1]

Mapped back: the ball is the body; the local ground frame and constant-g model are declared; the force ledger removes hand support after release and bounds drag; the exclusive-force condition holds; upward initial velocity generates the rising leg; the gravity-only equation generates apex and descent; and the validity envelope is the short, low-speed near-surface interval. Reversing initial velocity changes the trajectory but not the free-fall classification. Adding appreciable drag changes both classification and predicted return symmetry.

Applied / In Practice: NASA's evacuated drop facility¶

NASA Glenn's Zero Gravity Research Facility releases an experiment vehicle through 132 m inside an evacuated chamber, producing 5.18 s of near-weightlessness. The chamber is reduced to 0.05 torr, and NASA reports aerodynamic-drag acceleration below 0.00001 g.^[7] A uniform-field ideal predicts sqrt(2h/g) = sqrt(264/9.81), about 5.19 s, consistent with the operational duration. The payload and vehicle accelerate together until the decelerator supplies a very large non-gravitational stopping force; the experiment interval ends there.

Mapped back: vehicle and payload form the falling body; Earth supplies the local gravitational field; evacuation is the intervention that suppresses drag; release removes structural support; the force tolerance is explicit; the predicted trajectory and duration follow from gravity-only motion; relative support loading becomes very small inside the vehicle; and the decelerator marks a sharp boundary where the free-fall condition fails. The facility does not remove gravity—it uses gravity to remove support-force differences within the experiment.

Structural Tensions¶

T1 — Exact criterion vs. useful approximation. Real bodies nearly always experience residual gas drag, radiation pressure, third-body gravity, magnetic forces, or outgassing. The failure mode is either denying all practical free fall because perfection is impossible or declaring it whenever gravity is merely dominant. Diagnostic: state the non-gravitational acceleration ratio or trajectory-residual tolerance and test it over the relevant interval.

T2 — Velocity language vs. force identity. “Fall” suggests downward velocity, while free fall includes upward and orbital trajectories. The failure mode is admitting supported descent and excluding an ascending released body. Diagnostic: ignore the sign of velocity and inspect the force ledger after release.

T3 — Newtonian force vs. relativistic geometry. Newtonian mechanics says gravity is the only force; general relativity says an ideal free body has no proper force and follows a geodesic. The failure mode is presenting these as a contradiction or mixing equations from both descriptions without a theory translation. Diagnostic: identify whether acceleration is coordinate or proper and whether gravity is represented as force or curvature.

T4 — Point body vs. finite apparatus. A center of mass may closely follow gravity-only motion while an extended body experiences tides, rotation, internal stress, or differential drag. The failure mode is inferring perfect internal weightlessness from center-of-mass free fall. Diagnostic: compare apparatus size and duration with gravitational gradients and measure relative acceleration across the payload.

T5 — Weightlessness vs. weak gravity. Free-falling observers can feel weightless in a strong field, while a distant supported observer could feel a small force without being in free fall. The failure mode is calling orbit “zero gravity.” Diagnostic: separate gravitational field strength from accelerometer-measured support or proper acceleration.^[6]^[5]

T6 — Frame choice vs. physical contamination. A rotating or accelerating coordinate frame introduces pseudo-force terms that can make the Newtonian ledger appear to violate “gravity only.” The failure mode is treating a coordinate artifact as a new interaction or silently dropping it. Diagnostic: declare the frame, transform to an inertial frame when possible, or retain the required inertial-force terms.

T7 — Autonomous domain node vs. reduction to generic baselines. Free Fall resembles a zero-perturbation model and presupposes a frame, but neither generic prime entails its gravity-only criterion, upward/orbital inclusion, proper-acceleration boundary, drag/support exclusions, or mechanics-specific interventions. The opposite failure is to isolate it from those reusable patterns. Diagnostic: remove gravitational and mechanics vocabulary; if the remaining statement is only “suppress perturbations in a frame,” the transferable shell has been extracted and the indispensable Free Fall identity has been lost.

Structural–Framed Character¶

Free Fall is mixed-structural with a strong specialist frame.

Vocabulary portability: low. Gravity, inertial frame, drag, support force, ballistic trajectory, orbit, geodesic, and proper acceleration are constitutive rather than illustrative.
Evaluative loading: low. The abstraction classifies and predicts motion; it does not praise falling, weightlessness, or model simplicity.
Origin: physical and mathematical. Its force laws and trajectory equations arise in mechanics and gravitation, not in an institutional convention.
Human dependence: none. Moons, dust, and test particles instantiate the regime without observers or social practice.
Operation: partly structural. The force-ledger and perturbation-exclusion method is recognizable as a general modeling move, but it remains Free Fall only when gravity and its theory-specific motion law occupy the retained role.

Its character: a sharply testable gravity-only motion class whose clean structural exclusion rule travels across mechanics applications, while its identity remains anchored to gravitational physics.

Structural Core vs. Domain Accent¶

1. Structural core. Choose a frame and dynamical model, enumerate influences, retain one governing interaction, suppress or bound the others, propagate an initial state, and diagnose departures from the resulting baseline.

2. Indispensable domain accent. The retained interaction is gravity; the excluded influences are non-gravitational forces; the valid outputs are ballistic, orbital, or geodesic trajectories; the characteristic readout is absence of support/proper acceleration; and the boundary conditions include drag, thrust, normal force, field gradients, and inertial-frame choice. Generalization erases the counterintuitive fact that rising and orbiting bodies qualify while terminal-velocity descent does not.

3. Placement result. The structural core connects Free Fall to Frame of Reference and, more loosely, Zero-Force Null Baseline. The surviving specialist obligations justify a domain-specific node. No generic conjunction in the current catalog yields the full force criterion, theory translation, validation instruments, and intervention package.

Frame of Reference — strict prerequisite. Force and coordinate acceleration must be interpreted in a declared frame; inertial versus rotating and local versus global choices change the ledger and equations. This is the minimal prospective parent.
Zero-Force Null Baseline — related modeling pattern, not a strict parent. Practical free-fall calculations suppress named perturbations and read residuals diagnostically. But the prime additionally requires a deliberately false retained reference and deviation-analysis programme, while an actual vacuum trajectory may satisfy free fall without serving that epistemic role.
Equivalence Principle — close explanatory neighbor, not the candidate's genus. It explains local weightlessness and universality of fall and motivates freely falling frames in relativity. Free Fall can be defined and calculated classically without asserting the full principle.
Temporal Dynamics — generic consequence, not a covering parent. Initial conditions and time evolution matter, but this is true of almost every dynamical system and does not discriminate Free Fall.
Constraint — generic formal relation, not a covering parent. “No non-gravitational force” restricts admissible models, yet generic admissibility does not supply gravitational motion or its tests.

Relationships to Other Abstractions¶

Current abstraction Free Fall Domain-specific

Parents (1) — more general patterns this builds on

Free Fall presupposes Frame of Reference Prime

Frame of Reference — strict prerequisite. Force and coordinate acceleration must be interpreted in a declared frame; inertial versus rotating and local versus global choices change the ledger and equations.

Hierarchy path (1) — routes to 1 parentless root

Free Fall → Frame of Reference → Viewpoint

Neighborhood in Abstraction Space¶

Free Fall sits in a sparse region of the domain-specific corpus (87^th percentile for distinctiveness): few abstractions share its structure, so a faithful description tends to retrieve it precisely.

Family — Reference Frames & Inertial Motion (7 abstractions)

Nearest neighbors

Computed from structural-signature embeddings · 2026-09-08

Not to Be Confused With¶

Falling. Ordinary falling names downward motion. Free Fall names a gravity-only regime. Tell: Is motion direction being used instead of a force inventory?
Terminal velocity. Drag balances weight and the object descends without acceleration. Tell: Does an aerodynamic force remain equal and opposite to gravity?
Weightlessness / microgravity. These describe a support-force condition or small residual acceleration, often produced by free fall but not identical to its whole dynamical definition. Tell: Are the trajectory law, frame, and force exclusions specified?
Zero gravity. Gravity can be strong in orbit. Tell: Does the explanation confuse low accelerometer reading with low gravitational field strength?
Projectile motion. Ideal projectile motion after release is a case; real projectile motion may include drag, lift, thrust, or guidance. Tell: Are non-gravitational forces negligible over the modeled interval?
Orbit. An ideal unpowered orbit is a free-fall trajectory, not a contrast class. Tell: Is constant altitude being mistaken for absence of falling?
Inertial motion. In Newtonian mechanics an inertial body has zero net force and constant velocity; a freely falling body accelerates gravitationally. Tell: Has gravity been removed rather than retained?
Equivalence Principle. The principle concerns local physical equivalence and universality; Free Fall is the regime it uses. Tell: Is the claim about which forces act, or about equivalence of local frames and laws?
Mass wasting. Gravity drives downslope earth-material transport, but cohesion, friction, pore pressure, and contact interactions are constitutive. Tell: Is the subject a slope-failure mechanism rather than an isolated body's gravity-only trajectory?

References¶

[1] Moebs, William, Samuel J. Ling, and Jeff Sanny. “3.5 Free Fall.” University Physics Volume 1. OpenStax, 2016. Defines the elementary free-fall ideal through negligible air resistance and friction, emphasizes that “falling” need not mean downward motion, and develops the constant-g kinematics used here. registry ↩a ↩b ↩c ↩d

[2] Moebs, William, Samuel J. Ling, and Jeff Sanny. “13.4 Satellite Orbits and Energy.” University Physics Volume 1. OpenStax, 2016. Derives circular speed and period from gravity as the only force and explains orbiting astronauts as genuinely in free fall. registry ↩a ↩b ↩c ↩d

[3] NASA Glenn Research Center. “Falling Object with Air Resistance.” Beginner's Guide to Aeronautics. Accessed 2026-08-26. Separates weight and aerodynamic drag and shows that terminal velocity occurs when drag balances weight. registry ↩a ↩b ↩c

[4] Pössel, Markus. “Gravity: From Weightlessness to Curvature.” Einstein Online 1 (2005), 03-1011. Max Planck Institute for Gravitational Physics. Explains orbital free fall, local weightlessness, tidal effects, and geodesic motion in general relativity. registry ↩a ↩b ↩c ↩d ↩e

[5] Pössel, Markus. “The Elevator, the Rocket, and Gravity: The Equivalence Principle.” Einstein Online. Max Planck Institute for Gravitational Physics. Explains freely falling frames, local special-relativistic behavior, and the tidal limitation of the equivalence principle. registry ↩a ↩b ↩c ↩d

[6] NASA STEM Team. “What Is Microgravity? (Grades 5–8).” NASA, 2012; page updated 2025. Defines free fall as gravity-only acceleration for its audience, explains orbit as continuous falling, and warns that “zero gravity” is misleading. registry ↩a ↩b

[7] NASA Glenn Research Center. “Zero Gravity Research Facility.” NASA, 2025. Gives the 132-meter free-fall distance, 5.18-second microgravity duration, 0.05-torr chamber pressure, and aerodynamic-drag acceleration below 0.00001 g. registry ↩a ↩b ↩c