#### Galileo and the acceleration due to gravity

The value  9.8 N/kg or 9.8 m/swas first calculated rather accurately by Galileo Galilei using a series of extremely simple but ingenious experiments.

The acceleration of an object down an inclined plane (a) is equal to the value of the acceleration due to gravity (gdiluted by a factor corresponding to the sine of the angle of the incline with respect to the horizontal (sinq). If you want to read more about Galileo's life and his classic experiments go to www.pbs.org/wgbh/nova/galileo

Galileo noticed that objects (pendulums, and cannon balls) rolled down inclined planes at the same rate regardless of their mass but would speed up if the angle of the incline increased (that is of course if we ignore air resistance and keep all other things controlled -- like the frictional force).  The steeper the slope of the incline, the higher the acceleration of the object down the ramp.

The acceleration of an object down an inclined plane (a) is equal to the value of the acceleration due to gravity (gdiluted by a factor corresponding to the sine of the angle of the incline with respect to the horizontal (sinq).

 a = g x sinq

#### Facts and Misconceptions about falling objects

##### FACTS
• All objects fall at the same rate of change of downward speed or acceleration.
• The mass of an object does not influence the acceleration of an object in free fall.
• The surface area of an object will influence the acceleration of an object in free fall because of the added air resistance factor
• In the absence of air, close to the Earth's surface (in a vacuum) the vertical descend of an object will be constantly accelerated at an average rate of 9.8 m/s2
• The value 9.8 m/s2 is referred to as the acceleration due to gravity or the gravitational field strength of the Earth.
• The size of a planet will determine the value of g around its surface.  On the Moon gmoon  is 1/6 of the value of  gEarth.
• g is a vector and as such it has magnitude and direction.   Therefore g =  -9.8 m/s2or
g =  9.8 m/s2 [down]  because its vector direction is down (pointing towards the center of the Earth.).
• An object thrown up it the air will have positive but decreasing initial velocity.  And negative but increasing final velocity.
•  An object in free fall (in air) will not continue to accelerate indefinitely.  Depending on its size and cross sectional area an object will reach a point where it will stop accelerating due to air resistance.  After this point it will continue to fall at a constant rate called TERMINAL VELOCITY.
• The terminal velocity of an averaged size human is about 55 m/s.