Today's sailboats are very different in design to the Viking ships and other ships of the era. During the time of the Vikings, sails were only used for traveling downwind. If the ship needed to go upwind, then the warriors would get out the oars and start rowing. So, to analyze the physics of Viking sails, it becomes simple. When the sailboat is traveling downwind, so that the sail is perpendicular to the wind, the sail will capture the kinetic energy and propel the boat forward. Alongside this, the boat will experience a drag force from the water causing the net force to be$$

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