Free fall (physics)
Free fall: motion under gravity alone. Overview of definition, equations, history, examples (projectiles, skydivers, spacecraft), terminal velocity and distinctions like weightlessness and microgravity.
Overview
Free fall is the motion of an object when the only force acting on it is gravity. In ideal free fall there is no thrust from engines and no support or contact forces, and in many discussions air resistance is neglected. For a concise definition see free fall definition. Close to Earth's surface the acceleration associated with gravity is conventionally denoted g and has an average value of about 9.81 m/s2; this standard is often referenced as standard gravity.
Image gallery
3 Images
Key characteristics
In a vacuum, all objects fall with the same constant acceleration regardless of their mass. In the presence of an atmosphere, however, moving objects experience drag; when drag balances the gravitational pull, a falling object reaches a constant terminal speed. A skydiver in a belly-to-earth position reaches a typical terminal velocity, a phenomenon commonly described as terminal velocity. Free fall also includes upward portions of a trajectory: a projectile moving upward after launch remains under gravitational acceleration until other forces act.
Historical context and development
The study of falling bodies was central to the early modern development of physics. Experiments and thought experiments by figures such as Galileo helped overturn Aristotelian ideas that heavier bodies fall faster; later work by Newton placed free fall within a universal theory of gravitation. The modern, quantitative description uses the equations of motion from classical mechanics, which remain accurate for most everyday situations.
Equations and practical description
When air resistance is negligible, motion in one dimension is described by simple kinematic equations with constant acceleration g. For example, distance fallen from rest after time t is (1/2) g t2, and velocity is g t. In more realistic problems the drag force is modeled with terms proportional to speed or speed squared, requiring differential equations to predict changing acceleration and terminal speed. These models are applied in engineering and safety calculations.
Applications, examples and distinctions
Free fall appears in many contexts: a dropped apple in a classroom, an artillery shell in flight, or a skydiver before parachute deployment. An artillery projectile can be considered in free fall for the entire ballistic trajectory once motor or propellant effects cease; see artillery discussions for applied modeling. Spacecraft in orbit are in continuous free fall toward Earth while moving tangentially fast enough to miss the surface; thus astronauts experience weightlessness when engines are off, a situation often described in popular sources on spacecraft. By contrast, a rocket firing its engine produces thrust and is not in free fall during powered flight—refer to material on rocket propulsion for details.
Notable facts and practical notes
- Free fall and weightlessness are related but distinct: weightlessness occurs when support forces vanish, often because object and surroundings share the same gravitational acceleration.
- Measured value of g varies slightly with altitude, latitude, and local geology; the standard 9.81 m/s2 is an average approximation.
- Accurate prediction of fall behavior can require accounting for fluid dynamics, rotation, and shape-dependent drag coefficients in engineering contexts.
For further conceptual or technical reading consult introductory physics texts and authoritative online resources: definitions and summaries are often available via reputable science education pages (definition, terminal velocity, ballistics, orbital mechanics, propulsion, standard gravity).
History
Ancient
In connection with the problem of the motion of bodies, the Greek philosopher Aristotle in the 4th century BC considered bodies in a medium such as water: heavy bodies move downwards because of "their heaviness", light bodies move upwards because of "their lightness" ("heavy" and "light" here mean: greater and lesser specific gravity than water, respectively), and this apparently at a constant speed. So within same medium, heavier bodies sink down faster than less heavy bodies, and within different mediums speed is inversely proportional to resistance of medium. In an empty space with no medium, the sinking speed would then have to be infinite, so such a 'vacuum' could not exist. These views were extended by the late antique, Arabic and scholastic scholars to movements of all kinds, although they do not correspond to the experience of throwing and falling in air and were therefore also doubted as a general property of free fall. Thus, as early as 55 BC, the Roman poet and philosopher Lucretius described in his work De rerum natura ("On the Nature of Things") that falling objects are slowed down only by resistance of the medium, and therefore light bodies must fall more slowly, but in a vacuum all bodies must fall at the same rate.
From Simplikios (approx. 485 - 550) it is handed down that already Straton of Lampsakos (340 B.C. - 268 B.C.) had concluded an accelerated movement due to the drop formation of water when falling from a roof.
Renaissance
In 1554, Giovanni Battista Benedetti showed by means of a thought experiment on the free fall of two single or two connected balls that the speed cannot depend on the quotient of weight and resistance, but on the difference of the specific weights of body and medium. In a vacuum, then, all bodies of equal density would have to fall at the same rate. This was confirmed for the medium air in 1586 by Simon Stevin in one of the first decisive experiments of modern natural science, in which he heard two lead balls of different weights hitting the ground at the same time when falling from a height of about 10 m. Galileo, who is often credited with being the first scientist in the world, was the first person to hear two lead balls of different weights hitting the ground at the same time. Galileo, who was often credited with first performing this experiment a few years later at the Leaning Tower of Pisa, probably never did it.
Galileo's laws of falling
In contrast, Galileo Galilei was still on Aristotle's side in his writing De Motu ("On Motion") of about 1590: "If you drop a ball of lead and one of wood from a high tower, the lead moves far ahead." It was only after his experiments on the inclined plane, with precise measurements and their mathematical analysis, that Galileo was able to describe free fall mathematically correctly in 1609, thus refuting Aristotle's description. He did not yet have an accurate chronometer and therefore slowed down the motion by having a ball roll down a chute. As a timekeeper he used, for example, an accurate scale for the amount of water that had flowed in a thin stream from a bucket into a cup during the passage of a certain distance. He also used his pulse as well as the ability of hearing to judge the accuracy of the rhythm of periodic sounds. In his last work, Galileo puts the following summary in the mouth of Salviati, the personification of his then current views:
"veduto, dico, questo, cascai in opinione che se si levasse totalmente la resistenza del mezzo, tutte le materie descenderebbero con eguali velocità"
"In view of this, I say, I would come to believe that if the resistance of the surrounding medium were entirely removed, all substances would fall with the same velocity."
- Galileo Galilei: Discorsi e dimostrazioni matematiche intorno à due nuove scienze (1638)
This late work of Galileo is also appreciated as the beginning of classical physics, because here the "Galilean laws of falling" are presented: In a vacuum, all bodies fall at the same rate, and their motion is uniformly accelerated. In other words, their falling speed is proportional to the falling time, and the distance they fall is proportional to the square of the falling time. The acceleration is the same for all bodies at the same place.
After the invention of the air pump and the mercury barometer proved the existence of the vacuum, in 1659 Robert Boyle experimentally confirmed that in a vacuum bodies of different mass and composition fall at the same rate.
Newton's law of gravitation
Isaac Newton then formulated - in the Philosophiae Naturalis Principia Mathematica published in 1687 - a uniform law of gravity. With the help of Newton's law of gravitation, named in his honour, the orbits of the moons and planets as well as the free fall of objects on earth can now be explained. Beyond the statement of this mathematical law, Newton refrained from any further explanations as to why the force of gravity imparts the same acceleration to all bodies at the same place, regardless of their material and other properties. A more profound description of gravitation was only found within the framework of the general theory of relativity.
Free fall in homogeneous field
Neglecting buoyancy, air friction, the increase of the gravitational force when approaching the earth and the consequences of the earth's rotation (Coriolis force), a body initially at rest falls vertically with the constant acceleration 
whose value in Germany is about 
(see normal gravity formula). The signs of 
and velocity 
are positive for a downward pointing coordinate axis 
If one chooses the zero points cleverly (start at time 
at 
), then the formulas are also simple:
From this, the fall time and terminal velocity for a given fall height 
become:
A jump from a 5-m-board thus takes about one second and a speed of about 10 m/s (equal to 36 km/h) is achieved. From a height of one meter already 16 km/h are achieved, from three meters already 28 km/h.
In a drop tower of a good 100 m usable height, free-fall times of over 9 seconds with impact speeds of almost 170 km/h can be achieved by using a catapult system.
Related articles
Author
AlegsaOnline.com Free fall (physics) Leandro Alegsa
URL: https://en.alegsaonline.com/art/36457