Falling
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Falling is movement due to gravity, but also has other uses not directly related to gravity.
Sensation
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A sensation of falling occurs when the labyrinth or vestibular apparatus, a system of fluid-filled passages in the inner ear, detects motion. The same system also detects rotary motion. A similar sensation of falling can be induced when the eyes detect rapid apparent motion with respect to the environment. This system enables us to keep our balance by signalling when a physical correction is necessary.
When a human is in free fall in an orbiting spacecraft, or in an aircraft in a steep dive, the sensation of falling is constant, and the sensation of there being an "up" and a "down" is missing or much attenuated. Some medical conditions, known as balance disorders, also induce the sensation of falling.
Accidents
Falling is a major cause of personal injury, especially for the elderly whose vision, nerve conduction and muscles are weaker, whose vestibular sense is diminished, whose neurological responses are extended, and whose bones have grown brittle. Builders and miners represent worker categories representing high rates of fall injuries. The WHO estimate (2002) that 392,000 people die in falls every year.
Falls from buildings
Falls from buildings are often accidental but can also be caused intentionally, such as by defenestration. Injuries resulting in falls from buildings vary depending on the building's height, and also depend on the type of person (infant, child, adult, elderly adult, etc.) Falls from the second floor usually result in injuries, but are not fatal.
Record Fall
In 1972, Vesna Vulović survived a fall from 33,000ft without a parachute.
Falls in the Elderly
Stephen Lord at the University of New South Wales studied 80,000 elderly persons in Australia and found that the risk of falling increases for any who are taking multiple prescription medications and for all who are taking psychoactive drugs. This increased risk was demonstrated through the use of a variety of balance and reaction time tests. Inexplicably, the older men when matched with women of identical height, weight, and age, on average, performed measurably better in all of the balance and reaction time tests.
Classical physics
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Falling is descent under gravity. All objects have mass and in the presence of sufficiently massive objects such as planets or moons they experience a strong attraction due to gravity. This is known as weight. If the force of gravity is not equalized by an opposite force directed away from the planet, the object will start to fall towards the center of mass of the system--in effect, towards the center of the planet. The acceleration of gravity is directly proportional to the mass of the planet. The planet will also fall towards the center of the system but, if the object is much less massive than the planet, this motion is imperceptible.
The way in which an object moves under gravity (not necessarily a descent), in the absence of other forces, is known as free fall, and is described by a conic section whose parameters are dependent on the object's initial velocity. If the speed is above the escape velocity, and the object has no downward vertical component, the force of gravity is not enough to reverse the motion away from the planet and it will continue indefinitely on its path away from the planet. Otherwise it will fall back towards the planet and may go into orbit around it or collide with it.
In the presence of an atmosphere, the conditions for free fall are broken and the object will experience atmospheric drag, and the speed at which it falls towards the planet is subject to a terminal velocity when the force due to drag equalizes the force of gravity. Note that in common usage the term free fall does not take account of atmospheric drag.
Mathematics
In mathematics, the word falling describes a scalar value that decreases with respect to time or another variable.