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Free fall is motion with no acceleration other than that provided by Gravity . This also applies to objects in orbit even though these objects are not falling in the usual sense of the word. Strictly speaking nothing falling through an atmosphere can be in free fall due to the inherent resistance to motion, however in Skydiving , the term is also applied to the period of the jump before the parachute is opened, and in colloquial usage, falling through an atmosphere is normally considered to be free fall.

Examples of objects in free fall include:

Examples of objects not in free fall:
  • Standing on the ground, as the gravitational acceleration is counteracted by the reaction force from the ground.

  • Flying horizontally in a plane, as the wings' lift is also providing an acceleration.

  • Jumping from a plane, as there is a resistance force provided by the atmosphere.


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On Earth

Near sea level, an object in free fall in a vacuum will accelerate at approximately 9.8 m/s^2 regardless of its mass. With air resistance acting upon an object that has been dropped the object will eventually reach a Terminal Velocity (around 120 mph (200 km/h) for a human body). 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.


FREE FALL IN NEWTONIAN MECHANICS


Without air resistance


:v_{yt}=gt+v_{y0}\,

:y_t= rac{1}{2}gt^2+v_{y0}t+y_0

where
:v_{y0}\, is the initial velocity (m/s).
:v_{yt}\,is the velocity with respect to time (m/s).
:y_0\, is the initial altitude (m).
:y_t\, is the altitude with respect to time (m).
:t\, is time elapsed (s).
:g\, is the acceleration due to gravity (-9.8 m/s/s near the surface of the earth).


With air resistance


:ma_{y}=-kv_y+mg\,

:v_y=(v_{y0} + rac{5000}{k}ge^{-kt}) - rac{m}{k}g

:y=- rac{m}{k}\{(v_{yo}+ rac{m}{k}g)(e^{-kt}-1)+gt\}+y_0

:v_{y\infty}=\lim_{t o \infty}v_y = - rac{m}{k}g

where
:m\, is mass of the object
:k\, is the air resistance coefficient
:v_{y\infty}\, is the terminal velocity


SURVIVING FALLS

JAT stewardess , after the plane exploded over Srbská Kamenice in former Yugoslavia (now Serbia ). She broke several bones and was in a coma for 27 days.

In World War II there were several reports of aircrew surviving long falls: Nick Alkemade , Alan Magee , and I.M. Chisov all fell at least 6,000 meters and survived.

It was reported that two of the victims of the Lockerbie Bombing survived for a brief period after hitting the ground, but died from their injuries before help arrived.Cox, Matthew, and Foster, Tom. (1992) ''Their Darkest Day: The Tragedy of Pan Am 103'', ISBN 0-8021-1382-6


RECORD FREE FALL

starting his record-breaking skydive.]]

http://stratocat.com.ar/fichas-e/1960/HMN-19600816.htm According to the Guinness Book Of Records , Eugene Andreev (USSR) holds the official FAI record for the longest free-fall parachute jump after falling for 80,380 ft (24,500 m) from an altitude of 83,523 ft (25,457 m) near the city of Saratov, Russia on November 1 , 1962 . Andreev did not use a drogue chute during his jump.


ACCIDENTAL FREE FALL

In June 6, 1989, a NASA stratospheric balloon launched from Columbia Scientific Balloon Facility (''then known as National Scientific Balloon Facility'') in Palestine, Texas, suffered a uncommanded payload release while flying at 120,000 feet over a thunderstorm near Graham, in Texas. Months after the accident, a post-flight investigation concluded that a lightning bolt traveling upside from the clouds provoked the incident. The payload hit the ground at an estimated speed of 700 miles per hour. No one resulted harmed, but the incident forced the agency to change its policy, forbidding to fly balloons over thunderstorms.1.


FREE-FALLING AIRCRAFT AND MICROGRAVITY

See Also: Microgravity




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