Gravitational acceleration (symbolized g) is an expression used in physics to indicate the intensity of a gravitational field. It is expressed in meters per second squared (m/s 2 ). At the surface of the earth, 1 g is about 9.8 m/s 2 .
The use of the term acceleration in conjunction with gravity arises from Einstein's principle of equivalence, which was a cornerstone in the development of the general theory of relativity. This principle states that the force produced by a gravitational field is qualitatively the same (in terms of how it affects physical objects, time, and space) as the force produced when a reference frame accelerates.
Suppose a deep-space vessel accelerates at 9.8 m/s 2 , far away from the gravitational field of any planet or star. The occupants of that space ship experience a force equal to the force of gravity at the earth's surface (1 g), and this force has the same physical effect as gravitation. Conversely, a free-falling object near the earth's surface gains downward speed at a rate of 9.8 m/s 2 .
The gravity on the surface of Mars is approximately 0.37 g; the gravity in the atmosphere of Jupiter is about 2.5 g. The astronauts in space flights, and the pilots of some fighter aircraft, are subjected to gravitational acceleration levels of up to 6 or 8 g.
Also see acceleration and Table of Physical Units and Constants .