How does the force of wind come from?

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Discussion Overview

The discussion revolves around the mechanics of how wind generates force on a sailing boat, particularly when the boat is directed downwind. Participants explore the interaction between wind and the sail, the resulting forces, and the complexities involved in the dynamics of sailing.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that the interaction force on a smooth surface is generally perpendicular to the surface, leading to a force component F2 that drives the boat forward.
  • Another participant counters that the force is not strictly perpendicular and introduces the idea of additional forces due to the keel, which affects the boat's orientation and net force direction.
  • A third participant explains that the force of the wind is derived from the change of momentum of air molecules striking the sail, emphasizing the complexity of calculating the forces due to the sail's curvature.
  • This participant also notes that the actual force moving the boat is a projection of the wind's momentum vector onto the direction of the boat, which they refer to as F2.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the forces acting on the boat and the role of the keel, indicating that multiple competing perspectives remain without a clear consensus.

Contextual Notes

The discussion highlights complexities in the interaction between wind and sail, including the effects of sail curvature and the role of the keel, which may not be fully resolved in the participants' explanations.

KFC
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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.

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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?
 
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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.
 
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".
 

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