In Newtonian physics, free fall is any motion of a body where its weight 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 only 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 is thus 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 weightlessness, a condition that also occurs 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.
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...
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