Paragliders like all gliders are subjected to three forces: lift, drag, and weight/gravity. All these forces have a magnitude and a direction. The weight acts through the center of gravity and is always directed toward the center of the earth. Lift and drag are aerodynamic forces and act through the center of pressure. The lift is directed perpendicular to the flight path, and drag is directed opposite the flight path. The magnitude and direction of the lift and drag forces depend on the size and shape of the the glider with payload, air density, and airspeed.
Lift is generated by “turning” or redirecting the air around a wing. The turning of the airflow results from the air molecules staying in contact with the surface of the wing due to relatively fast moving airflow which generates low pressure regions at the top surface. Think of the wing as “yanking” down on the air molecules above it. We know from Newton that Force = Mass * (Change in velocity / Time). Velocity has a speed and a direction. By changing either the speed or direction of an airflow, you generate a force, in this case, lift. A change in velocity causes a force, and a force will cause a change in velocity. Because the velocity and direction of the air around the wing vary, so do the amounts of lift generated at various points on the wing. The average of those forces is the lift vector. Simply put, the force of a wing pulling air molecules downward into the low pressure at the top surface, generates an opposite force that pulls the wing upward.
The motion of a glider through the air generates drag. In a powered aircraft, thrust from the engine (kinetic energy) opposes drag. A glider must trade altitude for speed, or trade the potential energy of it’s altitude, for kinetic energy or speed. Thus, gliders must always descend relative to the air in which they are flying.
Gliders are able to stay aloft for hours at a time because they are relatively efficient and descend slowly. If the glider is subjected to air that is rising faster than its descent rate through the air, the glider will gain altitude and increase its potential energy.