Makes me wonder... Is there a measurement for potential energy stored in a segment of rubber? I'm sure its air perturbations that keep it unbalanced, but surely the rubber wears out eventually, so it could be given a number for how much work it can do.
Discussion
Its going to be a measurement based on how much force was used in pulling it. Looking at those bands, the pulling of one band has to use up its force pulling the next band. Energy is lost in this system from friction of the rubber against the rod (minimized with lubricant) and the movement of the air as a result of the fan spinning. As well, some heat is lost heating up the rubber (the side effect of the tension of the rubber changing). Some is also lost in friction of the fan shaft.
I posit that the fan (if there is no inbound air force) would likely spin longer without the rubber bands given the same amount of input rotational force.
I'm not sure. I'll take the other side, and posit that the air hitting the fan is making it spin longer. The lost energy could be a loss of elasticity in the rubber.