Physics: Why does a flag flap in the wind instead of remaining erect when the wind blows?

Answer 1

Because the wind is not at a constant speed from a constant direction, but varies both almost constantly.

Answer 2

The wind blowing around the flag pole creates eddys. As eddys move across the material of the flag, the material is pulled into voids behind the eddys. This in turn changes the flow pattern of the wind thus changing the eddy pattern.

Answer 3

First the wind is not constant in velocity or direction, second the flag itself has very bad aerodynamic structure thus creates unstable turbulence around its trailing edge and given the flags lack of torsional rigidity it flaps around in the turbulence

Answer 4

the material of the flag is light enough to move with the wind
and the wind is not constant in speed and direction. even in hurricane winds the flag is never ramrod straight

Answer 5

a flag is limp cloth and, when the wind blows it is meeting resistance from both sides.

Answer 6

Turbulence. There is a thing called Reynolds' number which you can calculate to see whether a flow will be laminar or turbulent. Flags generally qualify for turbulent.