Centripetal force in loop motion

Question

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

Answer 1

Diavolo does exert a force on the loop by Newton's 3rd law, just as you said. The reason this force isn't shown on the free body diagram is that only forces on the object of interest are shown, not forces by the object. Diavolo is the object of interest, so we don't include forces that he exerts on other things.

This is all justified because only forces acting on an object cause that object's motion to change. Forces exerted by objects don't directly affect their motion (except by 3rd law).

As for the side of the loop, I'm sure Diavolo did gain speed.