The track of a roller coaster is shown below.
at which point will the riders experience centripetal acceleration?
a)w
b)x
c)y
d)z

The correct answer is

C)Y

In fact, centripetal acceleration is the acceleration exerted on an object moving on a circular trajectory. Looking at the picture, we see that the riders are moving of circular motion only at point Y, while in all other points the riders are moving in a straight line.

At point Y, therefore, there is a force (called centripetal force, and that corresponds to the normal reaction exerted by the track on the riders) that is keeping the riders in circular motion, giving them a centripetal acceleration.