When an object is close to a planet and it has no support, it is said to be in "free fall." For instance, if you toss a ball upwards, then it is free fall until it lands, even though it is first rises then falls.

When an object is in free fall, and if we ignore air friction, its acceleration is always the same, regardless of the mass of the object. The acceleration is straight down and its magnitude is denoted by the constant g. Thus, for any object in freefall with neglible air friction, the following equation holds:

a = g, where a is the magnitude of the acceleration of the object.

An object is not "close to a planet" if we are considering an orbital motion problem. For a discussion of what "close to a planet" means see orbital motion.