As the air is drawn in over long distances some of the Earth's rotation is coupled to the air mass by something known as the Coriolis effect. This rather small rotation of the air mass is amplified by the conservation of angular momentum as the air is drawn toward the center of the storm where the pressure is lowest. The net effect of this is that instead of blowing straight in toward the center, the winds circulate about the center with increasing speed as the center is approached.
At some point the circular motion of the air is so fast that the centripetal force required to maintain its circular path is equal to the force from the pressure difference so near the center of a hurricane there is a balance of forces. In the absence of any force, according to Newton's second law, the air near the center of the storm is not accelerated to high velocity and a calm area exists.
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