Drag force exists in any motion through a fluid and acts opposite to the direction of motion.  A pilot moving to right experiences a drag force acting to the left.  The cause of drag is the result of numerous collisions with air molecules.  When a Lockheed Martin F-22 Raptor is cruising through the sky, it is colliding with many individual particles.  Although the air molecules collide with the jet in all directions, the net collisions is greater in the direction the jet is moving.  This is because the front of the jet is moving towards the air molecules and the rear is moving away from them, resulting in more collisions on the front of the jet.  Because of the law of conservation of momentum, momentum between the air molecules and the jet is conserved, resulting in a net force acting against the motion of the jet.  Drag is affected by four different variables: air density, velocity, cross section area, and shape.

• FD = $\frac{1}{2}CpAv^2$

    One of the key components of drag is the fluid density.  A dense fluid means that the molecules comprising the fluid are more compact.  This results in more collisions for the object moving through the fluid's molecules, slowing the object down due to the conservation of momentum.  A similar analogy would be a person moving through a crowd.  A dense crowd makes it hard for the person to move, but if the crowd was dispersed the person would have no trouble moving through.  The greater fluid density is the reason why it is harder to throw punches underwater and is also why submarines will never move as fast as fighter jets.
    An object with a higher velocity also creates more drag, which is why a person reaches terminal velocity when they go skydiving  The increase in velocity results in an increase in the bulk flow of fluid colliding with the object in motion.  In the case of a skydiver, the faster he moves the more air collides with him.  F-22 Raptors can move at a top speed of roughly 670 m/s, which generates immense amounts of drag.  However fighter jets have streamlined shapes to minimize drag force.
    A streamlined shape decreases drag in two ways.  It reduces the area and the coefficient of drag of the jet.  Reducing the area also reduces the total amount of air that collides with the jet, which in turn reduces the momentum loss due to collision.  The parachute above is one example of how large surface areas can increase drag.  If parachutes had a smaller surface area, they would not be effective in slowing skydivers or planes down.  The role of streamlining affects drag because it affects the direction in which air molecules collide.  A streamlined object would collide with most of the air molecules at an angle, reducing the momentum lost due to collision.