Weathervanes point in the direction of the wind.
How do they accomplish this? Which Is the best answer?
a) They are shaped like arrows, or
b) They have a large surface area on one end of the weathervane.
If you answered either (a) or (b), you are correct.
The key point, however, is contained in answer b. The large surface
area is located BEHIND the pivoting point of the weathervane.
What is the pivot point of a rocket?
When the wind hits the weathervane, the greatest
force is on the largest surface area. So the big fins in the picture
to the right will be pushed AWAY from the wind. The arrow then pivots
so that the small end points into the wind.
The physics of a weathervane is presented
in this diagram.
The wind force will be greatest
against the largest surface area. The weathervane will swing
about it's pivot until the force on the surfaces is minimized--when
the arrow points directly into the wind.
If you want to understand the physics of a weathervane in terms
of aerodynamics, consider the following: wind hitting the
large surface area creates a high pressure region. The
opposite side of the large surface sees a lower pressure. Therefore,
the large surface gets drawn into the low pressure region (in this
diagram that would be upward). That’s called “lift.”
The small arrow at the front also experiences “lift.”
But because it’s much smaller than the back surface, the largest
surface area dominates.
