opposing side of air track 601 for example photo detector 618 is mounted on a frame 620 a frame 620 a is pivotally mounted to a frame 622 a frame 622 a includes a slot 624 a that allows screws 623 a to secure frame 622 a to frame 604 a at various heights frame 622 a is mounted to air track 601 light source 616 shines a light toward top surface 625 of air track 601 light source 616 and photo detector 618 are aligned such that top surface 625 reflects the light to photo detector 618 when disk 310 is not in the path of the light in this implementation photo detector 618 detects light when disk 310 is not present alternatively light source 616 and photo detector 618 are aligned such that top surface of disk 310 reflects the light to photo detector 618 when disk 310 is in the path of the light in this implementation photo detector 618 detects light when disk 310 is present stopper 1000 can work equally well for an air track 601 with a concave top surface 625 fig7 a 7 c illustrate a stopper assembly 700 in one embodiment of the invention stopper 700.