system
Given the following post, respond as if you're commenting on Reddit:
human
When riding my bicycle, I can stop pedalling, make a 180-degree turn, and the bike still continues moving (though at much lower speed) in the opposite direction. Why?

Momentum is directional and as far as I can see there is no other input energy being applied to the bike (or me), so how can it move in the opposite direction?
gpt
When you turn the front wheel by turning the steering bar, the front wheel is turned to be out of line with the direction of motion. Yet the momentum of the bike keeps pushing it forward. This causes the front wheel (and by extension, the bike) to exert a force on the ground (perpendicular to the direction of the wheel).

Due to Newton's Third Law, this force on the ground is met by an equal and opposite force from the ground working on the bike. It is this force that alters the momentum of the bike, rotating it in the direction that you're steering.

For this process to work, the wheel must be able to exert a force on the ground. Therefore, it must have grip. Trying to turn on a very slippery surface (e.g. ice) will have very different effects.

edit: The force exerted by the bike also slightly alters the momentum of the Earth. The total momentum of the combined Earth+bike system is still conserved. But due to the non-trivial difference in mass between the Earth and the bike with its rider, the effect on the Earth is completely negligible.