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The Acceleration due to Gravity (Acceleration of Free Fall)
Experiments show that, when air resistance can be ignored, all bodies fall with the same acceleration.  
This acceleration is given the symbol g.  
The value of g on (or near) the surface of the earth is about 9.81ms-2.  
   
The acceleration due to gravity is not exactly the same at all points on the earth’s surface.  
Small variations in g are due to  
1. altitude as you go higher you are moving away from the centre of mass of the earth
2. latitude the earth is not a perfect sphere so at different latitudes you are at different distances from the centre of mass of the earth
3. the rotation of the earth The value of g is less than it would be if the earth did not rotate. The value of g is reduced most at places where the speed of circular motion is greatest, that is, on the equator.
 
   
A value for g at any point on the earth's surface can be calculated using the following formula (from "Physical Constants", W.H.C. Childs)  
 
if you have a few minutes to spare!  
A quick look at the numbers in this formula tells us that, for moderate altitudes, g does not change very much at different places.  
   
Velocity against Time Graphs for a Falling Body  
   
This graph shows what we would expect to happen if a body could be allowed to fall freely, near the earth's surface, in a vacuum.  
   
With no air resistance, there is only one force acting on the body, gravity, so the slope of the graph would be constant at about 9.81ms-2.  
   
   
In practice, there is air resistance acting on falling bodies.  
   
The slope of this graph starts out at 9.81ms-2 but gradually decreases as the body moves faster.  
   
After a certain time (which depends on the shape of the body and the density of the materials of which it is made) the body reaches a maximum speed called the terminal speed (or terminal velocity), vt  
   
At this speed, the magnitude of the force due to air resistance, is equal to the force of gravity.  
   
The fact that bodies reach a terminal speed when falling through air reminds us that the force of air resistance increases with the speed of the body.  
This is true for other fluids.  
For example, a body falling through water gives a similar graph though with much smaller terminal speed (for a given body).  
   
A person "sky-diving" typically reaches speeds of 50 to 60ms-1, however it can be much faster.  
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