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Four Forces of Flight
Every airplane, from a paper plane to a jumbo jet, has four forces acting on it:
LIFT pushes the plane UP. Air flowing over the wings creates lift. The wing shape is the secret: the top is curved and the bottom is flat, so air moves faster over the top, creating lower pressure that sucks the wing upward.
GRAVITY pulls the plane DOWN. This is the force that eventually brings every paper airplane back to the ground.
THRUST pushes the plane FORWARD. For a real airplane, engines provide thrust. For your paper airplane, YOUR ARM provides thrust when you throw it!
DRAG slows the plane DOWN. Air resistance pushes against the plane as it moves. A sleek, narrow plane has less drag than a wide, flat one.