Acceleration Due to Gravity

An object in freefall near the surface of the earth has an acceleration of 9.80 meters-per-second-squared downward (toward the center of the earth).

Galileo is credited with demonstrating the fact that objects differing in mass, in freefall near the surface of the earth, experience one and the same acceleration. In his famous Leaning Tower of Pisa demonstration, Galileo is said to have simultaneously dropped two cannon balls, one having twice the mass of the other, from one and the same elevation. The cannon balls hit the ground simultaneously. The prevailing belief was that the more massive ball would hit first because the earth pulled harder on it. The earth does indeed pull harder on the more massive cannon ball, but that ball also has more inertia, more natural resistance to a change in how fast it is going. The two effects (harder pull, more inertia) exactly cancel so two objects of differing masses, released simultaneously from rest over level ground at one and the same elevation, under the influence of the earth's gravitational force alone, do experience one and the same acceleration, namely 9.80 m/s2, and do fall side by side and hit the ground simultaneously.

Pitfall Avoidance Notes:
  1. The magnitude of the acceleration due to gravity is a positive number. It is not okay to arbitrarily put a negative sign in front of the value 9.80 m/s2.
  2. An object is not in freefall near the surface of the earth if there are any non-negligible forces acting on it other than the gravitational force of the earth. Do not assume the acceleration of the object is 9.80 m/s2 downward if there is another force or there are other forces acting on the object besides the gravitational force of the earth. It isn't.