How does the force of wind come from?

[KFC]

Hi there,
There is a question of text about a sailing boat and the force of wind. Suppose the boat is directed downwind as shown in the image.

I think for fluid, the interaction force on a smooth surface is along the direction perpendicular to surface. Hence, force F is result from the wind, right? If that's true, then it is the component F2 of F to drive the boat forward, is that correct?

 

[K^2]

It's not generally perpendicular to the surface, but yes, there is going to be an F1 and F2 component.

Though, it's still a little more complicated than that. F1 component doesn't just go away. There are more forces due to the keel in the water. The boat is going to be turned slightly away from F1, so the keel will generate a hydrodynamic lift force in direction opposite to F1, canceling it, and a drag force opposite F2, but significantly smaller than it. So the net force is going to be in direction of F2, and that's where the boat will travel, but the actual orientation of the boat will be at a slight angle to it.

That's if you want to be complete in the description. It's also the only way you are going to get proper description if the sailboat is doing anything other than running downwind.

 

[HallsofIvy]

The force of the wind comes from the change of momentum of air molecules striking the sail. That's the answer to the question posed in your title.

The question in the body of your post is a bit more complicated. When the wind strikes the sail, it imparts a force equal to the component of "momentum change" that is perpendicular to the sail at that point. Since the sail is curved, that is very complcated to calculuate but the sail can be approximated by a plane.

So the first force, your "F", is the projection of the wind's momentum vector on the vector perpendicular to the sail. But in order that the boat not simply go in that direction, rather that the direction we want to go, we have a keel that prevents motion in the direction perpendicular to the boat. The actual force moving the boat is the projection of vector F on a vector in the direction of the boat itself. That is your "F2".

 

[Naty1]

More here:
http://en.wikipedia.org/wiki/Sail#Sail_aerodynamics

 

[PJC01]

The force of wind comes from a combination of factors, including differences in air pressure and the Earth's rotation. When air moves from an area of high pressure to an area of low pressure, it creates wind. This movement of air is influenced by the Coriolis effect, which is caused by the rotation of the Earth. This effect causes winds to curve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

In terms of a sailing boat, the force of wind is harnessed by the sails, which are designed to create lift and propel the boat forward. As the wind flows over the curved surface of the sail, it creates a difference in air pressure, with higher pressure on the front side and lower pressure on the back side. This pressure difference creates a force that pushes the sail and the boat in the direction of the lower pressure.

In the case of a boat sailing downwind, the wind is coming from behind the boat, and the sails are angled to capture the force of the wind and propel the boat forward. The component of the force of wind that drives the boat forward is indeed F2, as it is the perpendicular component of the force that acts in the direction of motion.

Overall, the force of wind is a complex phenomenon that is influenced by various factors and can be harnessed for various purposes, such as sailing. As scientists, we continue to study and understand the mechanics of wind in order to make use of it in the most efficient and effective ways.