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Sailboat

In a situation where a boat is using a sail to move across the water, the boat is being pushed by the wind. Our diagram represents a time when the boat is moving at constant speed with zero acceleration and there are no waves moving across the water.

Free Body Diagram

As described in other scenarios, the gravitational force and the buoyant force acting on the boat cancel out because there is no acceleration in that direction. The push force causing the boat to move comes from the wind that is pushing in the direction of motion. In the situation shown above, the sail is perpendicular to the direction of motion and only the force from the wind in the direction of motion is considered. When the sail or the wind is at a different angle, the force vectors have different components. For simplicity, only the ideal situation with the push force from the wind perpendicular to the motion is depicted. The last force on the diagram is the F_drag. As discussed earlier, the amount of drag from the air is negligible compared to the drag from the water, so air resistance is left out of this diagram. The F_drag is opposite the F_push and they are equal in magnitude because the boat is not accelerating. The net force in this diagram is zero, showing that the boat is moving at a constant velocity.

Energy Cubes

The video above represents how energy is transferred when a boat uses a sail to move across the water. As the snapshots show, a lot of the kinetic energy from the wind is transferred into kinetic energy of the boat. Not a lot is opposing the boat, but some thermal energy is transferred into the water. It was depicted this way because the water is causing the drag force that opposes the motion. Some energy is lost to the water then, and the most likely way the energy would be transferred to the water is through thermal energy. The rest of the energy, though, is used to move the boat in the form of kinetic energy.

Other Scenarios - Rowboat (  please click me!)