Free fall

Free fall describes any motion of a body where gravity is the only or dominant force acting upon it, at least initially. Since this definition does not specify velocity, it also applies to objects initially moving upward. Although strictly the definition excludes motion of an object subjected to other forces such as aerodynamic drag, in nontechnical usage falling through an atmosphere without a deployed parachute or lifting device is also referred to as free fall.

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 going up for some minutes, and then down).

An object dropped at the top of a drop tower or drop tube.

An object thrown upwards or a person jumping off the ground at low speed (i.e. as long as air resistance is negligible in comparison to their weight). Technically, the object or person is in free fall even when moving upwards or instantaneously at rest at the top of their motion, since the acceleration is still g downwards. However in common usage "free fall" is understood to mean downwards motion.

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 acceleration 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 they experience a drag force which equals their weight once they have achieved terminal velocity (see below). However, the term "free fall skydiving" is commonly used to describe this case in everyday speech, and in the skydiving community. It is not clear, though, whether the more recent sport of wingsuit flying fits under the definition of free fall skydiving.