During the Thai high-school physics teacher training programme, we used an aluminum loop-the-loop system built by the Institute for the Promotion of Teaching Science and Technology (IPST) to demonstrate a circular motion and investigate the concept of the conservation of mechanical energy. There were 27 high-school teachers from three provinces, Bangkok, Ra-yong and Phuket. A steel ball was released at a certain height and then moved into the circular loop and underwent a projectile motion upon leaving the track. We asked the teachers to predict the landing position of the ball if we changed the height of the whole loop-the-loop system. We analysed the methods they used to determine their answers. It turned out that most teachers could not get the correct landing position of the ball. Some did not realize the effect of the friction between the ball and the track. Some did not realize the similar condition of the loop-the-loop system before and after it was moved up by a certain distance. Only a few teachers did realize this and got the correct answers. (Contains 1 table and 2 figures.)