Final answer:
The discrepancy between an airplane's intended direction and actual movement is due to the influence of wind as an external force. The airplane's total velocity is affected by both its northward direction and the westward force of the wind. This results in the airplane moving in a direction that is different from where it points.
Step-by-step explanation:
The question refers to how external forces, like wind, can alter the path of a moving object, in this case, an airplane. When an airplane is heading straight north but is carried to the west and slowed down by wind, this is due to the wind acting as an external force that changes the airplane's intended direction of travel. The airplane's movement relative to the ground is the result of its total velocity, which is a combination of its velocity towards the north and the velocity of the wind acting upon it from the side.
In a similar manner, when discussing how the orientation of a compass needle might change as the airplane continues to fly due west along the equator, the Earth's magnetic field influences the compass. As the plane flies west, if the Earth's magnetic and geographic north poles coincide, the compass needle should maintain its perpendicular orientation to the direction of motion without change, because the airplane remains equidistant from the magnetic north pole.