Right now in my physics class I am teaching about conservation of momentum. My students struggle with these problems because their are so many variables to identify and solve for. To solve those problems you have to be able to clearly separate the before from the after. A year ago is the before. This is the after. A lot of people have worse afters, but I have never looked at my life so distinctly in those two halves as I do right now. It has been nearly a year since I have gone for a long run. I was cleared to run months ago, but e ach time the pain comes back. Like coffee, running is just something I have had to have an ugly break up with. Unlike Taylor Swift, I am still holding out hope for us getting back together. But for now, my running shoes are retired. The cold and the wind are also an issue, and turns out this thing called winter happens every year. For months at a time. So I have had to take my physical activity indoors. Until recently, I assumed this meant N...