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In Newtonian physics, **free fall** is any motion of a body where gravity is the only acceleration acting upon it. In the context of general relativity, where gravitation is reduced to a space-time curvature, a body in free fall has no force acting on it.

**Classical mechanics** describes the motion of macroscopic objects, from projectiles to parts of machinery, and astronomical objects, such as spacecraft, planets, stars and galaxies.

In physics, **acceleration** is the rate of change of velocity of an object with respect to time. An object's acceleration is the net result of all forces acting on the object, as described by Newton's Second Law. The SI unit for acceleration is metre per second squared (m⋅s^{−2}). Accelerations are vector quantities and add according to the parallelogram law. The vector of the net force acting on a body has the same direction as the vector of the body's acceleration, and its magnitude is proportional to the magnitude of the acceleration, with the object's mass as proportionality constant.

**General relativity** is the geometric theory of gravitation published by Albert Einstein in 1915 and the current description of gravitation in modern physics. General relativity generalizes special relativity and refines Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time, or spacetime. In particular, the *curvature of spacetime* is directly related to the energy and momentum of whatever matter and radiation are present. The relation is specified by the Einstein field equations, a system of partial differential equations.

- History
- Examples
- Free fall in Newtonian mechanics
- Uniform gravitational field without air resistance
- Uniform gravitational field with air resistance
- Inverse-square law gravitational field
- Free fall in general relativity
- See also
- References
- External links

An object in the technical sense of the term "free fall" may not necessarily be falling down in the usual sense of the term. An object moving upwards would not normally be considered to be falling, but if it is subject to the force of gravity only, it is said to be in free fall. The moon is thus in free fall.

In a roughly uniform gravitational field, in the absence of any other forces, gravitation acts on each part of the body roughly equally, which results in the sensation of weightlessness, a condition that also occurs when the gravitational field is weak (such as when far away from any source of gravity).

**Weightlessness** is the complete or near-complete absence of the sensation of weight. This is also termed **zero-g**, although the more correct term is "zero g-force". It occurs in the absence of any contact forces upon objects including the human body.

The term "free fall" is often used more loosely than in the strict sense defined above. Thus, falling through an atmosphere without a deployed parachute, or lifting device, is also often referred to as *free fall*. The aerodynamic drag forces in such situations prevent them from producing full weightlessness, and thus a skydiver's "free fall" after reaching terminal velocity produces the sensation of the body's weight being supported on a cushion of air.

An **atmosphere** is a layer or a set of layers of gases surrounding a planet or other material body, that is held in place by the gravity of that body. An atmosphere is more likely to be retained if the gravity it is subject to is high and the temperature of the atmosphere is low.

A **parachute** is a device used to slow the motion of an object through an atmosphere by creating drag. Parachutes are usually made out of light, strong fabric, originally silk, now most commonly nylon. They are typically dome-shaped, but vary, with rectangles, inverted domes, and others found. A variety of loads are attached to parachutes, including people, food, equipment, space capsules, and bombs.

**Terminal velocity** is the maximum velocity attainable by an object as it falls through a fluid. It occurs when the sum of the drag force (*F _{d}*) and the buoyancy is equal to the downward force of gravity (

In the Western world prior to the 16th century, it was generally assumed that the speed of a falling body would be proportional to its weight—that is, a 10 kg object was expected to fall ten times faster than an otherwise identical 1 kg object through the same medium. The ancient Greek philosopher Aristotle (384–322 BC) discussed falling objects in * Physics * (Book VII), one of the oldest books on mechanics (see Aristotelian physics).

**Aristotle** was a Greek philosopher during the Classical period in Ancient Greece, the founder of the Lyceum and the Peripatetic school of philosophy and Aristotelian tradition. Along with his teacher Plato, he has been called the "Father of Western Philosophy". His writings cover many subjects – including physics, biology, zoology, metaphysics, logic, ethics, aesthetics, poetry, theatre, music, rhetoric, psychology, linguistics, economics, politics and government. Aristotle provided a complex synthesis of the various philosophies existing prior to him, and it was above all from his teachings that the West inherited its intellectual lexicon, as well as problems and methods of inquiry. As a result, his philosophy has exerted a unique influence on almost every form of knowledge in the West and it continues to be a subject of contemporary philosophical discussion.

The * Physics* is a named text, written in ancient Greek, collated from a collection of surviving manuscripts known as the Corpus Aristotelicum, attributed to the 4th-century BC philosopher Aristotle.

**Mechanics** is the area of science concerned with the behaviour of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment. The scientific discipline has its origins in Ancient Greece with the writings of Aristotle and Archimedes. During the early modern period, scientists such as Galileo, Kepler, and Newton laid the foundation for what is now known as classical mechanics. It is a branch of classical physics that deals with particles that are either at rest or are moving with velocities significantly less than the speed of light. It can also be defined as a branch of science which deals with the motion of and forces on objects. The field is yet less widely understood in terms of quantum theory.

In 12th-century Iraq, Abu'l-Barakāt al-Baghdādī gave an explanation for the gravitational acceleration of falling bodies. He proposed an explanation of the acceleration of falling bodies by the accumulation of successive increments of power with successive increments of velocity.^{ [1] } According to Shlomo Pines, al-Baghdādī's theory of motion was "the oldest negation of Aristotle's fundamental dynamic law [namely, that a constant force produces a uniform motion], [and is thus an] anticipation in a vague fashion of the fundamental law of classical mechanics [namely, that a force applied continuously produces acceleration]."^{ [2] } In the 14th century, Jean Buridan and Albert of Saxony referred to Abu'l-Barakat in explaining that the acceleration of a falling body is a result of its increasing impetus.^{ [3] }

**Abu'l-Barakāt Hibat Allah ibn Malkā al-Baghdādī** was an Islamic philosopher, physician and physicist of Jewish descent from Baghdad, Iraq. Abu'l-Barakāt, an older contemporary of Maimonides, was originally known by his Hebrew birth name **Baruch ben Malka** and was given the name of **Nathanel** by his pupil Isaac ben Ezra before his conversion from Judaism to Islam later in his life.

In physics, **gravitational acceleration** is the acceleration on an object caused by the force of gravitation. Neglecting friction such as air resistance, all small bodies accelerate in a gravitational field at the same rate relative to the center of mass. This equality is true regardless of the masses or compositions of the bodies.

In physics, **power** is the rate of doing work or of transferring heat, i.e. the amount of energy transferred or converted per unit time. Having no direction, it is a scalar quantity. In the International System of Units, the unit of power is the joule per second (J/s), known as the watt in honour of James Watt, the eighteenth-century developer of the condenser steam engine. Another common and traditional measure is horsepower. Being the rate of work, the equation for power can be written:

According to a tale that may be apocryphal, in 1589–92 Galileo dropped two objects of unequal mass from the Leaning Tower of Pisa. Given the speed at which such a fall would occur, it is doubtful that Galileo could have extracted much information from this experiment. Most of his observations of falling bodies were really of bodies rolling down ramps. This slowed things down enough to the point where he was able to measure the time intervals with water clocks and his own pulse (stopwatches having not yet been invented). This he repeated "a full hundred times" until he had achieved "an accuracy such that the deviation between two observations never exceeded one-tenth of a pulse beat." In 1589–92, Galileo wrote * De Motu Antiquiora *, an unpublished manuscript on the motion of falling bodies.

Between 1589–92, the Italian scientist Galileo Galilei is said to have dropped two spheres of different masses from the Leaning Tower of Pisa to demonstrate that their time of descent was independent of their mass, according to a biography by Galileo's pupil Vincenzo Viviani, composed in 1654 and published in 1717.

* De Motu Antiquiora*, or simply

Examples of objects in free fall include:

- A spacecraft (in space) with propulsion off (e.g. in a continuous orbit, or on a suborbital trajectory (ballistics) going up for some minutes, and then down).
- An object dropped at the top of a drop tube.
- An object thrown upward or a person jumping off the ground at low speed (i.e. as long as air resistance is negligible in comparison to weight).

Technically, an object is in free fall even when moving upwards or instantaneously at rest at the top of its motion. If gravity is the only influence acting, then the acceleration is always downward and has the same magnitude for all bodies, commonly denoted .

Since all objects fall at the same rate in the absence of other forces, objects and people will experience weightlessness in these situations.

Examples of objects **not** in free fall:

- Flying in an aircraft: there is also an additional force of lift.
- Standing on the ground: the gravitational force is counteracted by the normal force from the ground.
- Descending to the Earth using a parachute, which balances the force of gravity with an aerodynamic drag force (and with some parachutes, an additional lift force).

The example of a falling skydiver who has not yet deployed a parachute is not considered free fall from a physics perspective, since he experiences a drag force that equals his weight once he has achieved terminal velocity (see below).

Near the surface of the Earth, an object in free fall in a vacuum will accelerate at approximately 9.8 m/s^{2}, independent of its mass. With air resistance acting on an object that has been dropped, the object will eventually reach a terminal velocity, which is around 53 m/s (195 km/h or 122 mph^{ [4] }) for a human skydiver. The terminal velocity depends on many factors including mass, drag coefficient, and relative surface area and will only be achieved if the fall is from sufficient altitude. A typical skydiver in a spread-eagle position will reach terminal velocity after about 12 seconds, during which time he will have fallen around 450 m (1,500 ft).^{ [4] }

Free fall was demonstrated on the moon by astronaut David Scott on August 2, 1971. He simultaneously released a hammer and a feather from the same height above the moon's surface. The hammer and the feather both fell at the same rate and hit the ground at the same time. This demonstrated Galileo's discovery that, in the absence of air resistance, all objects experience the same acceleration due to gravity. (On the Moon, the gravitational acceleration is much less than on Earth, approximately 1.63 m/s^{2}.)

This is the "textbook" case of the vertical motion of an object falling a small distance close to the surface of a planet. It is a good approximation in air as long as the force of gravity on the object is much greater than the force of air resistance, or equivalently the object's velocity is always much less than the terminal velocity (see below).

where

- is the initial velocity (m/s).
- is the vertical velocity with respect to time (m/s).
- is the initial altitude (m).
- is the altitude with respect to time (m).
- is time elapsed (s).
- is the acceleration due to gravity (9.81 m/s
^{2}near the surface of the earth).

This case, which applies to skydivers, parachutists or any body of mass, , and cross-sectional area, , with Reynolds number well above the critical Reynolds number, so that the air resistance is proportional to the square of the fall velocity, , has an equation of motion

where is the air density and is the drag coefficient, assumed to be constant although in general it will depend on the Reynolds number.

Assuming an object falling from rest and no change in air density with altitude, the solution is:

where the terminal speed is given by

The object's speed versus time can be integrated over time to find the vertical position as a function of time:

Using the figure of 56 m/s for the terminal velocity of a human, one finds that after 10 seconds he will have fallen 348 metres and attained 94% of terminal velocity, and after 12 seconds he will have fallen 455 metres and will have attained 97% of terminal velocity. However, when the air density cannot be assumed to be constant, such as for objects falling from high altitude, the equation of motion becomes much more difficult to solve analytically and a numerical simulation of the motion is usually necessary. The figure shows the forces acting on meteoroids falling through the Earth's upper atmosphere. HALO jumps, including Joe Kittinger's and Felix Baumgartner's record jumps also belong in this category.^{ [5] }

It can be said that two objects in space orbiting each other in the absence of other forces are in free fall around each other, e.g. that the Moon or an artificial satellite "falls around" the Earth, or a planet "falls around" the Sun. Assuming spherical objects means that the equation of motion is governed by Newton's Law of Universal Gravitation, with solutions to the gravitational two-body problem being elliptic orbits obeying Kepler's laws of planetary motion. This connection between falling objects close to the Earth and orbiting objects is best illustrated by the thought experiment, Newton's cannonball.

The motion of two objects moving radially towards each other with no angular momentum can be considered a special case of an elliptical orbit of eccentricity *e* = 1 (radial elliptic trajectory). This allows one to compute the free-fall time for two point objects on a radial path. The solution of this equation of motion yields time as a function of separation:

where

*t*is the time after the start of the fall*y*is the distance between the centers of the bodies*y*_{0}is the initial value of*y**μ*=*G*(*m*_{1}+*m*_{2}) is the standard gravitational parameter.

Substituting *y *= 0 we get the free-fall time.

The separation as a function of time is given by the inverse of the equation. The inverse is represented exactly by the analytic power series:

Evaluating this yields:^{ [6] }^{ [7] }

where

In general relativity, an object in free fall is subject to no force and is an inertial body moving along a geodesic. Far away from any sources of space-time curvature, where spacetime is flat, the Newtonian theory of free fall agrees with general relativity. Otherwise the two disagree; e.g., only general relativity can account for the precession of orbits, the orbital decay or inspiral of compact binaries due to gravitational waves, and the relativity of direction (geodetic precession and frame dragging).

The experimental observation that all objects in free fall accelerate at the same rate, as noted by Galileo and then embodied in Newton's theory as the equality of gravitational and inertial masses, and later confirmed to high accuracy by modern forms of the Eötvös experiment, is the basis of the equivalence principle, from which basis Einstein's theory of general relativity initially took off.

A **centripetal force** is a force that makes a body follow a curved path. Its direction is always orthogonal to the motion of the body and towards the fixed point of the instantaneous center of curvature of the path. Isaac Newton described it as "a force by which bodies are drawn or impelled, or in any way tend, towards a point as to a centre". In Newtonian mechanics, gravity provides the centripetal force responsible for astronomical orbits.

In physics, a **force** is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity, i.e., to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity. It is measured in the SI unit of newtons and represented by the symbol **F**.

**Mass** is both a property of a physical body and a measure of its resistance to acceleration when a net force is applied. An object's mass also determines the strength of its gravitational attraction to other bodies.

In physics, **potential energy** is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors.

In mechanics and physics, **simple harmonic motion** is a special type of periodic motion or oscillation where the restoring force is directly proportional to the displacement and acts in the direction opposite to that of displacement.

In physics, **escape velocity** is the minimum speed needed for a free, non-propelled object to escape from the gravitational influence of a massive body. It is slower the farther away from the body an object is, and slower for less massive bodies.

The **Grashof number** (**Gr**) is a dimensionless number in fluid dynamics and heat transfer which approximates the ratio of the buoyancy to viscous force acting on a fluid. It frequently arises in the study of situations involving natural convection and is analogous to the Reynolds number. It's believed to be named after Franz Grashof. Though this grouping of terms had already been in use, it wasn't named until around 1921, 28 years after Franz Grashof's death. It's not very clear why the grouping was named after him.

**Work** is the product of force and displacement. In physics, a force is said to do work if, when acting, there is a movement of the point of application in the direction of the force.

**Orbital mechanics** or **astrodynamics** is the application of ballistics and celestial mechanics to the practical problems concerning the motion of rockets and other spacecraft. The motion of these objects is usually calculated from Newton's laws of motion and law of universal gravitation. Orbital mechanics is a core discipline within space-mission design and control.

A **trajectory** or **flight path** is the path that an object with mass in motion follows through space as a function of time. In classical mechanics, a trajectory is defined by Hamiltonian mechanics via canonical coordinates; hence, a complete trajectory is defined by position and momentum, simultaneously. Trajectory in quantum mechanics is not defined due to Heisenberg uncertainty principle that position and momentum can not be measured simultaneously.

In continuum mechanics, the **Froude number** is a dimensionless number defined as the ratio of the flow inertia to the external field. Named after William Froude (;), the Froude number is based on the **speed–length ratio** which he defined as:

In classical mechanics, the **gravitational potential** at a location is equal to the work per unit mass that would be needed to move the object from a fixed reference location to the location of the object. It is analogous to the electric potential with mass playing the role of charge. The reference location, where the potential is zero, is by convention infinitely far away from any mass, resulting in a negative potential at any finite distance.

In astrophysics, **dynamical friction** or **Chandrasekhar friction**, sometimes called *gravitational drag*, is loss of momentum and kinetic energy of moving bodies through gravitational interactions with surrounding matter in space. It was first discussed in detail by Subrahmanyan Chandrasekhar in 1943.

In the theory of general relativity, the **equivalence principle** is the equivalence of gravitational and inertial mass, and Albert Einstein's observation that the gravitational "force" as experienced locally while standing on a massive body is the same as the *pseudo-force* experienced by an observer in a non-inertial (accelerated) frame of reference.

**Projectile motion** is a form of motion experienced by an object or particle that is thrown near the Earth's surface and moves along a curved path under the action of gravity only. This curved path was shown by Galileo to be a parabola. The study of such motions is called ballistics, and such a trajectory is a ballistic trajectory. The only force of significance that acts on the object is gravity, which acts downward, thus imparting to the object a downward acceleration. Because of the object's inertia, no external horizontal force is needed to maintain the horizontal velocity component of the object. Taking other forces into account, such as friction from aerodynamic drag or internal propulsion such as in a rocket, requires additional analysis. A ballistic missile is a missile only guided during the relatively brief initial powered phase of flight, and whose subsequent course is governed by the laws of classical mechanics.

A **circular orbit** is the orbit with a fixed distance around the barycenter, that is, in the shape of a circle.

A set of **equations** describe the resultant trajectories when objects move owing to a constant gravitational force under normal Earth-bound conditions. For example, Newton's law of universal gravitation simplifies to *F* = *mg*, where m is the mass of the body. This assumption is reasonable for objects falling to earth over the relatively short vertical distances of our everyday experience, but is untrue over larger distances, such as spacecraft trajectories.

In astrodynamics and celestial mechanics a **radial trajectory ** is a Kepler orbit with zero angular momentum. Two objects in a radial trajectory move directly towards or away from each other in a straight line.

- ↑ Crombie, Alistair Cameron,
*Augustine to Galileo 2*, p. 67. - ↑ Pines, Shlomo (1970). "Abu'l-Barakāt al-Baghdādī , Hibat Allah".
*Dictionary of Scientific Biography*.**1**. New York: Charles Scribner's Sons. pp. 26–28. ISBN 0-684-10114-9.

(cf. Abel B. Franco (October 2003). "Avempace, Projectile Motion, and Impetus Theory",*Journal of the History of Ideas***64**(4), p. 521-546 [528].) - ↑ Gutman, Oliver (2003).
*Pseudo-Avicenna, Liber Celi Et Mundi: A Critical Edition*. Brill Publishers. p. 193. ISBN 90-04-13228-7. - 1 2 "Free fall graph" (PDF). Green Harbor Publications. 2010. Retrieved 14 March 2016.
- ↑ An analysis of such jumps is given in Mohazzabi, P.; Shea, J. (1996). "High altitude free fall" (PDF).
*American Journal of Physics*.**64**(10): 1242. Bibcode:1996AmJPh..64.1242M. doi:10.1119/1.18386. - ↑ Foong, S K (2008). "From Moon-fall to motions under inverse square laws".
*European Journal of Physics*.**29**(5): 987. Bibcode:2008EJPh...29..987F. doi:10.1088/0143-0807/29/5/012. - ↑ Mungan, Carl E. (2009). "Radial Motion of Two Mutually Attracting Particles".
*The Physics Teacher*.**47**(8): 502. Bibcode:2009PhTea..47..502M. doi:10.1119/1.3246467.

- Freefall formula calculator
- The Way Things Fall an educational website

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