Falling (physics)

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.