Why do flags flutter in the wind?

Ripples of wind pass through the Indian tricolour flutters near Wagah village, Pakistan, July 24, 2018.
| Photo Credit: Naveed Ahmed

A: The fluttering of flags is a dynamic phenomenon involving three forces.

The first one is a constant pulling force due to the rope used to tie the flag to a pole.

The second force on the flag is due to gravity, which acts downwards.

The third force is responsible for fluttering is the wind. This force is not constant, both in magnitude and direction. It can be greater or lesser than the other forces.

The interesting wavelike motion of a flag (made of cloth) results when the wind speed goes above and comes below (in other words, oscillates about) a critical value of the force equal to the flag’s weight.

As the force due to the wind speed exceeds the weight, the flag tends to fly, and when it is lower, the flag sags.

Thus the rapid changes in wind speed around the flag results in fluttering.