Sailing against the Wind

A real sail is far more complex. This Demonstration is a good simplified model to understand the role of reaction forces, which are forces that will grow just enough to make the system obey the constraints. The shape of a real boat makes it very difficult to move sideways in the water. The constraint in this oversimplified model is that sideways motion is totally impossible; therefore, the reaction force of the water is always just enough to have the total force (the vector sum of the wind and water forces) in the direction of the boat axis.