# Centripetal force in loop motion

I saw it from the book which talk about centripetal force in circular motion  The main idea here is the balance in the term of relation between velocity and centripetal acceleration - centripetal force to maintain the uniform circular motion. However what I don't understand here is the normal force component which have the same direction with the gravitational force. The normal force exert on Diavolo from the top but according to Newton's 3rd law there must be a force exert on the loop to make the action-reaction pair. As I consider there is nothing from Diavolo exert on the loop so what make the normal force here

In another case, not on top of the loop but in the place of the red dot In this case, there is nothing to nullify the effect of gravitational force and combining it with the centripetal force that will make the net force vector not perpendicular with the velocity vector so it will not the uniform circular motion. So how Diavolo can keep the velocity constant but not slowing down. Assumed that he reached the required velocity to no falling down from the loop at the first place so he don't need to use any force to accelerate his speed

Sorry for the ridiculous title, pls change it to the more suitable one

• Your title is fine. – BMS Oct 1 '14 at 16:10
• Are you following Flying Circus of physics by Jearl Walker or Resnick, Halliday's Principles of Physics?? – user36790 Dec 10 '14 at 8:05