Centripetal acceleration

When stuff goes in circles there must be a force causing this to happen. This force towards the centre is a centripetal.
Key ideas: http://hyperphysics.phy-astr.gsu.edu/hbase/cf.html
In a circus: http://www.pbs.org/opb/circus/classroom/circus-physics/centripetal-acceleration/
Here is the derivation we looked at: http://dev.physicslab.org/Document.aspx?doctype=3&filename=CircularMotion_CentripetalAcceleration.xml