1. The problem statement, all variables and given/known data Alright, this problem has been driving me into a confused rage. The wave is moving in an upward direction so particle A should be moving up as well, right? But every single website I've used to study claims that it's moving in a downward direction. How? 2. Relevant equations 3. The attempt at a solution This might be way of the mark, but maybe the phrase "next instant of time" is throwing me off? Does that mean the next cycle or the next nanosecond or what?