This is my first attempt to find help online and my first time at this forum, and there are a lot of sections here. I hope I located this topic correctly. This is from my physics book. I'm not currently taking any classes, so this isn't a homework question per se. OK, -3m/s to -2m/s seems simple enough. Some negative work is done because the velocity decreases, lowering the kinetic energy. My problem is with the change of direction in the second case. The book says the kinetic energy remains the same and the work done is zero. I get this, but according to W = Fd, for the W to equal 0 the displacement must be 0 or the F must be 0. Clearly the F isn't 0 since the particle changed direction. So the displacement must be 0. Since the displacement is 0, does this mean the particle instantly changed direction, or that it went some to the left and then that same amount to the right? Or could it be either?