Given that I owe my first class degree in mathematics to a big bet on being able to master "dynamics", I shouldn't admit that I couldn't answer this question immediately. However, I thought I'd throw it open.
I was in a bar on Saturday afternoon watching the England vs Portugal match.
I say "watching". I was, for the most part, bored out of my mind. I find it very, very hard to get excited by football. I watched the penalties; they were fun. I grasped, correctly, that it was not appropriate to demonstrate my enjoyment of the penalties; everybody else there was taking them extraordinarily seriously.
I digress. My point is this: I found myself interested by the fan that was being used to circulate air in the warm bar.
It was one of those fans that moves about a vertical axis, hence blowing air across an arc.
I was sitting about three metres away.
I observed that it took four seconds for me to feel the rush of the cool air from the time the fan was directly pointing at me.
From this, I think it is safe to surmise that the air was being pushed across the room at 0.75 ms-1.
My question is this: do we have enough information to calculate how fast the blades were spinning? And if so, what was that speed?
(Like I say, I was bored...)