Final answer:
In the reference frame of a bicyclist, an observer standing on the sidewalk as the bicyclist passes by is perceived to be moving in the opposite direction of the bicyclist's motion.
Step-by-step explanation:
The question relates to reference frames in physics, specifically regarding the relative motion between an observer and a moving object, in this case, a bicyclist. When you are standing on a sidewalk and observing a bicyclist passing by to your right, in the reference frame of the bicyclist, you would appear to be moving in the direction opposite to the motion of the bicyclist. This is because in the bicyclist's reference frame, they consider themselves to be stationary, and everything else moves relative to them.
This understanding is pivotal in understanding concepts of relative motion, where the apparent motion of an object depends on the reference frame of the observer. If the bicyclist sees you while they pass by, they perceive you as moving backward because they are moving forward relative to the ground and therefore relative to you as well.