TITLE: Is there a difference between forces due to pressure differentials and collision?
QUESTION [1 upvotes]: Imagine moving at a constant speed a flat object straight through a fluid: it could be a paddle in the water, the wing of a plane in the air or a sail.
Ignoring whatever pushes the object at its constant speed, with my shaky, at best, understanding of physics I think there are two forces that are applied to the object: one from the collision with the fluid (equal and opposite reaction and all that) and one arising from the pressure differential in front and behind of the object.
Are the two just the same thing (from conservation of momentum?) or do they differ? And if they differ, which is usually bigger?

REPLY [1 votes]: They are the same.
There is always at least a static pressure acting on the wings.  This is the case for any object in a fluid.  This is what leads to buoyancy for example.
When you begin to move in the fluid; things change as the relative movement between the fluid and the surface you examine now becomes a factor.  The object that you send through the fluid now has different pressures acting on it; because the air is now not just static; but able to "hit" the surface (and these will vary based on the geometry of the object).  These interactions will also cause a change in the momentum of the air; and of the object itself.
Basically, we design wings to push air the opposite direction we want our objects momentum; and the force of that generated momentum manifests itself as pressure gradients across the surface.
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