Hi there. On a midterm we just had on Monday, one question asked about a magic mountain ride called the rainbow. You get in this circular apparatus and it starts to spin, you're against the wall. Then it goes from horizontal to vertical and you stick to the walls. Anyways, one question asked that I pick a coordinate system and then set up Newton's 2nd Law for the person at the top. I chose x to the right and y going down. Then I said F=ma=mg+Fn (normal force from wall down on person to keep them in the centripetal motion). Now I didn't get any points marked down but I'm still unsure of the direction of forces. I said Fn=mv^2/r-mg. Is that right? Is it minus mg, or + mg? What signs do I use? I ride stays at constant speed. I figured the normal without gravity would just be mv^2/r. When I add gravity I figure it'll allow the person not to push on the wall as much so I said mv^2/r-mg. Is that right? Part two. I just did this for my own sake. At the bottom of the ride, I wrote in ma=mg-Fn where Fn=mg+mv^2/r. The professor wrote in a minus sign in front of mg. What happened here? I figured I'd add the weight this time instead of subtract it. Maybe I did it wrong in the first part? Thanks, Josh Additional question. I'm holding a pencil in my hand and turning it in a circle. I feel it weighs more on the way up, but once I reach the top it almost flies out of my hand. Likewise it weighs less on the way down. Now in my head I can't figure if it weighs more at the top or bottom. At the top, is it flying out of my hand, or is it just about to stop feeling heavy and fly out of my hand? The bottom? These forces are confusing me.