At any rate, momentum is certainly an unquantifiable element in a sporting competition. You often hear color commentators report about it based on highly unscientific factors, like "which team needs the win more" "which team scored more in a row" or "which team overcame an enormous psychological obstacle by accomplishing something they've been struggling at" (i.e. Brandon Jacobs' monster catch and run for a TD on Sunday). They factor in excitement and morale. They factor in winning streaks. In short, they factor in things that are completely irrelevant to a team's likelihood to win.
Momentum is not real; not in the physical world and not in sports. But what is real and what isn't real anyway? Is evil real? Augustine famously made the argument that evil has no real substance; it is the absence of something. I always hated that argument. Anyone who wants to argue that cold is not real because it is the absence of heat, or darkness is not real because it is the absence of light can first spend the night naked and blindfolded on my front porch before we engage in meaningful discourse.
(I believe the fulcrum of this debate is your metaphysical foundation. Augustine and Aquinas presuppose a universe constructed out of matter. Varying degrees of being have varying degrees of form and less unintelligible matter, until you get to the highest being, which is actus purus, pure act. According to this metaphysical worldview, things that lack form, such as darkness or evil, lack existence. But I embrace a good deal of Humean skepticism in this matter. For me, perception doesn't point to a metaphysical structure. All I can be sure of is the information of my senses. For me, that's where existence starts. Sorry, Aquinas, but the problem of theodicy remains)
My point is, though there are no scientific ways of quantifying momentum, in sports or in physics (I would argue that the current "scientific way" of quantifying momentum isn't scientific; it's mathematical), it exists. We can't actually grasp it, and we argue about who has more of it (or at least Al Michaels and Cris Collinsworth do), but it's real. Ask any football player what it's like to be down two TDs in the first quarter of a four-quarter game. Ask him if playing the other 45 minutes of football is easier if you're down two TDs or up two TDs. Ask him which one is more like stopping a freight train and which one is more like accelerating a freight train going downhill. When your opponent has all the momentum, it is very difficult to regain it by sheer force of will.
Normally in sports, when you're sent reeling you need to stop the game in order to regroup. You can call a timeout or just hang on and try not to lose any more yardage before halftime. During the break, you need to re-center yourself, get hyped up, adjust to what your opponent is giving you, and hopefully turn it around. I guess all this is to say that, when you're in the middle of the game, it's hard to stop something that's happening.
In no particular order, the list of amazing subjects I touched upon that I did immense injustice to: theodicy, metaphysics and its death after Modernity, physics and the nature of the world, being and existence, the arbitrariness of time and position, thrownness, continental philosophy and the connectedness of the universe, and of course, going for it on 4th and 2 at midfield.