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The two primary forces in aerodynamics are lift and drag. Lift refers to (usually upward) forces perpendicular to the direction of motion of an object traveling through the air. For example, airplane wings are designed so that their movement through the air creates an area of low pressure above the wing and an area of high pressure beneath it; the pressure difference produces the lift needed for flight. This effect is typical of airfoil design. Drag forces are parallel and opposite to the object's direction of motion and are caused largely by friction. Large wings can create a significant amount of lift, but they do so with the expense of generating a great deal of drag. Spoilers that are extended on airplane wings upon the vehicle's landing exploit this tradeoff by making the wings capable of high lift even at low speeds; low landing speeds then still provide enough lift for a gentle touchdown. Aeronautical engineers need to take into account such factors as the speed and altitude at which their designs will fly (lower air pressures at high altitudes reduce both lift and drag) in order to optimally balance lift and drag in varying conditions.