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About 1600, Galileo performed his famous inclined plane experiments and discovered that the acceleration of a freelyfalling object is constant  that is, the object's acceleration does not change while the object is in free fall. The acceleration of a freelyfalling object  any freelyfalling object  near the surface of the Earth is about 9.8 m/s^{2} (which is conveniently close to 10 m/s^{2} for rough calculations). In the "customary" system of units, g = 32 ft/s^{2} or about 22 mi/hr/s. This acceleration value is commonly called "g". The direction of this acceleration is downward (toward the center of the Earth).
Free fall acceleration is different on other planets  it depends on the planet's size and mass. The table below shows some approximate values of "g" for selected objects in our solar system.
Acceleration of gravity, "g" in the Solar System 


At the surface of 
g (m/s^{2}) is 
Mass (kg)^{*} 
Radius (m)^{*} 
Sun^{1 } 



Mercury 



Venus 



Earth 



Moon (of Earth) 



Mars 



Phobos (moon of Mars) 



Deimos (moon of Mars) 



Jupiter^{1} 



Ganymede (moon of Jupiter) 



Europa (moon of Jupiter) 



Saturn^{1} 



Uranus^{1} 



Neptune^{1} 



Pluto 

1.27 x 10^{22} 

^{*}Mass and radius data taken from The Nine Planets
To say that the acceleration of a freelyfalling object is constant means that the velocity of a freelyfalling object changes at a constant rate.
If we say that g = 10 m/s^{2}, therefore, we mean that the velocity of the falling object changes by 10 m/s each second that it falls. If we drop it from rest, it will be going 10 m/s downward in 1 second, 20 m/s downward in 2 seconds, 30 m/s in 3 seconds, and so on. As the object falls, its velocity increases by about 10 m/s each second. In tabular form:
Velocity vs. Time for Free Fall From Rest 


time (s) 
velocity (m/s) 
















Since free fall is a motion with constant acceleration, the kinematics equations for constant acceleration apply. Particularly, the equations:
and
can be used to calculate an object's velocity in free fall.
BHS > Staff > Mr. Stanbrough > Physics > Mechanics > Kinematics > this page