You saw that the drag force depends upon the dynamic pressure. There is another force that also depends on the dynamic pressure: it's called lift. More precisely, it is the difference in dynamic pressure on opposite sides of an object that is responsible for these forces. Whereas the drag force on an object acts opposite to the velocity of the object, the lift force acts perpendicular to the velocity (see Figure 7-14). Now, because there is always a difference in the dynamic pressure between the front and back of a moving object, there is always a drag force on a moving object. However, if an object is perfectly symmetrical along an axis in the direction of motion, the flow (and therefore the dynamic pressure) on either side of the object ...

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