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gravitation2.gif (445 bytes)

Legend

ac

 = centripetal acceleration

v

 = velocity

r

 = radius
Fc = centripetal force

m

 = mass
Fg = force of gravity
G = universal constant

網上實驗
　
You do not have Shockwave Flash support. Please go to this page to view this page. http://www.macromedia.com/shockwave/download/download.cgi?P1_Prod_Version=ShockwaveFlash In reality, an apple and a feather do not fall at the same speed.  They don't reach the ground at the same time because the feather is so light, Fg (mg) can be easily effected by its surroundings.  Small wind is enough horizontal force to move it side ways.   Plus the air drag and friction, forces of the environment can easily overcome Fg.
However, in a vacuum, a feather and an apple fall at the same rate.  Although the apple has a greater Fg, they fall at the same rate because acceleration due to gravity is independent of the mass of the falling object.  And it can be shown by the below formula.

g = Gmearth/r2
mearth = mass of earth
r = distance from center of earth to object (m)
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