Drag! Drag is a factor of any moving body through a fluid. Drag works in the opposite direction of motion. Drag is a product of colliding with air molecules. Due to the law of conservation of momentum, the energy between the air molecules and the front of the plane is conserved resulting in a net force acting against the motion of the plane! Drag is a function of air density, velocity, cross sectional area, and wing shape.   http://www.pilotfriend.com/training/flight_training/aero/drag.htm A key component of drag is the fluid density. A dense fluid means that the air molecules are more compact which results in more collisions and more drag on the object moving through the fluid. This results in slowing down of the object meaning more fuel and power is needed in the case of an aircraft. The velocity of an object also plays a large role in the creation of drag. An increase in velocity results in an increase in the amount of molecular collisions thus increasing the drag of the object. Plane wings are engineered precisely to combat the inevitable force due to drag. Reducing the overall are of the wing reduces the amount of collision between it and the air, this is why a streamline design is desired for the wing. How do we overcome this drag force?? THRUST Home Page