Final answer:
At the top of a loop, a roller coaster's velocity is at its minimum due to the gravitational force opposing its motion.
Step-by-step explanation:
The question is asking about the velocity of a roller coaster at the top of a loop. When a roller coaster moves through a loop, it experiences a change in speed and direction due to the forces acting on it. At the top of the loop, the roller coaster's velocity is at its minimum. This is because the gravitational force acting on the roller coaster is directed downwards, opposing the roller coaster's motion. As a result, the roller coaster's velocity decreases, but the speed may remain constant if there are no external forces.