5) Suppose a student finds that the total y-component of the outgoing momentum is consistently non-zero. Besides not correctly choosing the x-axis to be parallel to the direction of motion of the incoming puck, what else could be causing this discrepancy

The geological theory that states that pieces of earth's lithosphere are in constant slow motion is the theory of plate tectonics.

They like garbage and they hunt during the night.

Sounds propagates through some kind of medium. usually it's air, but it goes just as well through solid objects like walls, windows etc. a soundwave will compress the (air)molecules in front of it, and if this increased density wave hits your ears, you register it as sound. solid objects like walls are a lot denser than air, this means that they will damp a wave, but also carry it easier.