Final answer:
To mount a bike, it is advisable to do so from the left side to avoid the chain and gears and for safety reasons. The physics of bicycle riding involves balancing the center of gravity with the force exerted by the ground, accounting for friction and centripetal force during turns. From a bicyclist's viewpoint, standing observers appear to move in the opposite direction to their travel.
Step-by-step explanation:
When mounting a bike, the standard recommended practice is to mount from the left side. This is because the chain and gears of the bike are typically on the right side, and mounting from the left helps avoid getting your clothes caught or soiled. Furthermore, in many countries, this also means you would be mounting from the side away from traffic, which is safer.
Regarding the physics involved in riding a bicycle, when you lean into a turn, you are actually balancing forces. The force exerted by the ground must align with the center of gravity for stability. Forces on the bicycle wheel can be resolved into a frictional component that must provide the centripetal force to turn and a vertical normal force equal to the weight of the system.
In the context of seeing a bicyclist while standing on a sidewalk, from the bicyclist's reference frame, you would appear to be moving in the opposite direction to their travel. So if they are passing to your right, you would appear to be moving leftward from their perspective.