Definition and conditions of free fall

What is free fall?

An object is in free fall when the only force acting on it is its weight. No other forces (such as air resistance or Archimedes’ principle) must be involved. This is an idealised model, but one that is very useful in physics for understanding the motion of falling objects.

Conditions for real free fall

In practice, an object is considered to be ‘in free fall’ if:

• it is released with no initial velocity or is thrown vertically,
• air resistance is negligible (dense object, low speed, short distance),
• Archimedes’ buoyancy due to air is neglected.

Example: a steel ball dropped from a 3-metre-high balcony falls in near-free fall. By contrast, a sheet of paper fluttering in the air is not in free fall: the air slows its descent to a non-negligible extent.

The role of gravitational acceleration g

In the gravitational field, all objects in free fall experience the same acceleration, denoted by g, directed downwards.

Location	Value of g (m/s^2)
Earth’s surface	g ~ 9.8
Moon	g ~ 1.6
Mars	g ~ 3.7

A common misconception is to believe that a heavy object falls faster than a light one. In the absence of friction, mass has no effect on the rate of fall: a feather and a hammer fall at the same speed in a vacuum (an experiment carried out on the Moon in 1971).