What you aren't taking into account is that object on your
bridge IS accelerating - it could not possibly be applying a
force to your bridge if it weren't. The pressure you are
describing is a force, and the Statics equation for a force is
F=ma. If your object's acceleration = 0, the force being applied
to your bridge would also = 0. In this case, the object's
acceleration = gravity = -9.83 m/s^2. So yes, the speed of the
wind in itself is nothing - it's the acceleration of the air
molecules (and whatever else is being carried by the wind) as a
result of the wind times the mass of the air molecules (and
whatever else is being carried by the wind) that applies the
force to the scoreboard - not the wind. agrabes is saying the
wind is being decelerated from 90mph to 0mph in such a small
amount of time (when it hits that board) that this acceleration
component of a 90mph wind will be so much greater than any other
factor, assuming the board's shape is constant, that you don't
have to consider whether the wind is