Lift              (UNSW) The concept of how a sail uses the wind to propel the boat forward into the wind (at an angle) is very similar to that of an airplane wing. The shape of a sail redirects the direction of the air flow from the wind to create lift (Kampf). In order for the sail to change the direction of the wind’s velocity, it must exert a force on the air hitting it. As Newton’s 3rd Law of Motion states, “Every force, acts as part of an action/re-action pair” (Knight). Since the sail exerted a force on the air, the air is exerting a force onto the sail in the opposite direction, which becomes the direction of motion. Besides redirecting the airflow on the inside of the sail, the sail uses the convex shape of the backside of the sail to its advantage as well. The outwards shape of the sail causes a change in the airflow on the backside. The change of direction of the air as it flows around the sail causes the air particles to speed up (Wolfe). Based off of Bernoulli's equation, if the air (the fluid) flowing along speeds up, there is a decrease in pressure as a result. With less pressure on the exterior of the sail than there is on the interior, so the net force on the sail is in the forward direction.(Knight)