1. The problem statement, all variables and given/known data A 1500-kg roller coaster car starts from rest at a height H=23.0m above the bottom of a 15.0-m-diameter loop. If friction is negligible, determine the downward force of the rails on the car when the upside-down car is at the top of the loop. 2. Relevant equations Conservation of energy: U+K=Esys U=mgh K=0.5mv2 F=ma acentripetal=v2/r 3. The attempt at a solution Uinitial=Esys because starts from rest. Uinitial=mghinitial at the top of the loop: Esys=U+K=mghtop+0.5mv2 meaning mghinitial=mghtop+0.5mv2 simplifying: v2=2(ghinitial-ghtop). so: F=macentripetal=mv2/r= 1500kg*2(9.81m/s2*23m-9.81m/s2*15)/7.5m So that gives an answer of 31,392N. I know the answer is 16.7 kN (back of book), and I know you get there by subtracting mg. So why am I subtracting mg? I would think that would be the TOTAL force and not solely the force of the tracks on the car. Any and all help is much appreciated! Edit: You know, I think I get it. I solved for centripetal force, which is the total inward force toward the center of the circle. Because mg is in that direction, I subtract it and get the force from the tracks. Is this right?