1. The problem statement, all variables and given/known data What's the MAXIMUM velocity the roller coaster should have at pt A in order for the passengers to stay in contact with their seats ? calculate the velocity for r=7 It kinda looks like this [img=http://img94.imageshack.us/img94/6994/image39.gif] 2. Relevant equations 3. The attempt at a solution The friction is not mentioned anywhere, so let's forget about it, i thought that it has something to do with the normal force, so it should be like this: Nmax + mg = mv^2/r does it need some advanced stuff about the normal force to solve this? does it have something to do with friction between the passenger and the seat ? Thanks for clearing this up for me, and sorry for my poor English.