Centripital Acceleration

[amcavoy]

Could someone tell me how this equation was derived or where it came from:

$$a_c=\frac{{v_t}^2}{r}$$

This isn't a homework question, but it seemed better to put it in this section (I apologize if it's in the wrong section).

 

[FredGarvin]

http://hyperphysics.phy-astr.gsu.edu/hbase/cf.html

Look down at the bottom for a quick and dirty explaination. Or...

http://www.knowledgerush.com/kr/encyclopedia/Centripetal