Free fall

Free fall

From Wikipedia, the free encyclopedia

For other uses, see Free-fall (disambiguation).

File:Apollo 15 feather and hammer drop.ogg

Commander David Scott conducting an experiment during the Apollo 15 moon landing.

In Newtonian physics, free fall is any motion of a body where gravity is the only force 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 and it moves along a geodesic. The present article concerns itself with free fall in the Newtonian domain.

An object in the technical sense of 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 thus is in free fall.

In a uniform gravitational field, in the absence of any other forces, gravitation acts on each part of the body equally and this is akin to weightlessness, a condition which also obtains when the gravitational field is zero such as when far away from any gravitating body. A body in free fall experiences "0-g".

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.

Contents [hide]

1 History

2 Examples

3 Free fall in Newtonian mechanics

3.1 Uniform gravitational field without air resistance

3.2 Uniform gravitational field with air resistance

3.3 Inverse-square law gravitational field

4 Free fall in General Relativity

5 Record free fall parachute jumps

6 Surviving falls

7 See also

8 References

9 External links

History[edit]

In the Western world prior to the Sixteenth Century, it was generally assumed that the acceleration of a falling body would be proportional to its mass — that is, a 10 kg object was expected to accelerate ten times faster than a 1 kg object. The ancient Greek philosopher Aristotle (384-322 BCE), included this rule in what was perhaps the first book on mechanics. It was an immensely popular work among academicians and over the centuries it had acquired a certain devotion verging on the religious.

It wasn't until the Italian scientist Galileo Galilei (1564-1642) came along that anyone put Aristotle's theories to the test. Unlike everyone else up to that point, Galileo actually tried to verify his own theories through experimentation and careful observation. He then combined the results of these experiments with mathematical analysis in a method that was totally new at the time, but is now generally recognized as the scientific method. For the invention of this method, Galileo is generally regarded as the world's first scientist.

In a tale that may be apocryphal, Galileo (or an assistant, more likely) dropped two objects of unequal mass from the Leaning Tower of Pisa. Quite contrary to the teachings of Aristotle, the two objects struck the ground simultaneously (or very nearly so). 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 and photogates 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."

Examples[edit]

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)

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