# Thread: Why does flag flutter when wind blow?

1. Why does flag flutter when wind blow?
thanks

2.

3. i think this is due to centrifugal force act on the edges of the flag when the when the wind is blowing

4. Originally Posted by jssb
i think this is due to centrifugal force act on the edges of the flag when the when the wind is blowing
I am sorry. I don't understand. Can you expain more? pls

5. A flag takes the shape of least resistence to the wind, but having achieved this, there is nothing then to hold it up so it falls, losing its shape, again gaining resistence. It can never find a perfect balance; one reason being as it moves to find the shape of least resistence it cannot instantly stop so goes out of shape automatically. There is turbulance at the edges, as the wind slips off the edge suddenly releasing its pressure. Crows have lose feathers on the end of their wings to reduce this turbulance for smoother flight. There are many other reasons why a flag will never stay still in the wind.

6. Originally Posted by jssb
i think this is due to centrifugal force act on the edges of the flag when the when the wind is blowing
Don't centrifugal forces not exist... :S aren't they just an illusion of sorts and centripital forces are the ones that are real?

7. Originally Posted by Mick
A flag takes the shape of least resistence to the wind, but having achieved this, there is nothing then to hold it up so it falls, losing its shape, again gaining resistence. It can never find a perfect balance; one reason being as it moves to find the shape of least resistence it cannot instantly stop so goes out of shape automatically. There is turbulance at the edges, as the wind slips off the edge suddenly releasing its pressure. Crows have lose feathers on the end of their wings to reduce this turbulance for smoother flight. There are many other reasons why a flag will never stay still in the wind.
Is it this effect is also present on the wings of planes?

8. Just to let everyone know, centrifugal and centripetal force both exist. The basic definition of each are:

Centrifugal (meaning "center fleeing" in Latin) means directed outwards from the center. Centrifugal force is a force that acts upon a body spinning on an axis. This force is directed away from the body. Centrifugal force is equal to, but opposite the centripeta force.

Centripetal (meaning "center seeking" in Latin) means directed towards the center. Centripetal force is a force that acts upon a body moving in a curved path. This force is directed towards the center of the curvature of the path. It is equal to, but opposite the centrifugal force.

9. But there is no force acting outwards on a spinning body, the only force is towards the center keeping the object in 'orbit' around the midpoint. Example: you have a rock tied to a string and you're spinning it, it has a velocity perpendicular to the string and the force inwards (centripital force) is changing the velocity so that it keeps going in circles. Where would a centrifugal force come from?? Is there a magic, invisible man running around the outside of the orbit with another rope?? I sure hope not..

10. The wings of a plane are completely different to how a flag flies. They are designed not to change shape. The top of the wing is curved and the air passes over this at a faster rate than under the wing. When air or water moves fast past something it exerts less pressure on the object. The less pressure above the wing allows the pressure under the wing to push it upwards. Obviously the plane must keep moving through the air for this effect to work. If you blow over a sheet of paper hanging off the edge of a table you will see the paper lifting because the moving air creates less pressure above the paper making it move upwards.

11. it's easier to imagine it in water. Fast moving water passing an obstacle that won't move. Water will even change to gas under great pressure. You can see it on the bubbles that will come. Even when you have a hose with only water in it, if you increase the pressure, you'll hear air coming trought.

it's fun

12. There 2 forces that with constant reaction on flag , "force of wind" and force of gravity, flag will achive it stillnes if there no gravity, and allso it will achive it stillnes if wind would be constant wind blow, but due to turbulence that form on edges of flag, force of gravity that allso variate, and variable direction of wind simply it canot stay still cause wind itself dont have equalized pressure by surface, even watter dont have equalised pressure due to mechanik of fluids. that is explanation.

13. flutter on the flag is due to fact that being blown by the wind is an unstable state, thus it oscillates and cannot find a steady state until the wind dies and the flag is laid to rest by the much more stable and constant force of gravity. If you could assure a constant wind speed and completely laminar flow(smooth and parallel), the flag would not flutter, but this is unattainable in real life.

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