Final answer:
The Coriolis force only affects horizontal motion because it acts perpendicular to an object's trajectory, and gravity effectively cancels the Coriolis force in the vertical direction. The Coriolis effect becomes significant in large-scale motions, such as in the formation and rotation of weather systems.
Step-by-step explanation:
The Coriolis force is an apparent force described within the context of rotating systems. Specifically, it is the force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. When it comes to the Coriolis force affecting motion on Earth, only the horizontal motion of an object is considered because the Coriolis force acts perpendicular to the object's motion. This is due to Earth's rotation, which imparts a velocity to moving objects that is perpendicular to their direction of travel. Vertical motion is largely excluded because gravity acts directly upon objects moving vertically, effectively canceling out the Coriolis force in that direction. Therefore, the correct answer to the student's question is b) Coriolis force acts perpendicular to the motion, indicating that the Coriolis effect is primarily concerned with horizontal, not vertical, motion.
On a larger scale, such as with weather systems, the Coriolis force has substantial effects. For example, it is responsible for the counterclockwise rotation of hurricanes in the Northern Hemisphere and the clockwise rotation in the Southern Hemisphere.
In summary, while the Coriolis force can be negligible in everyday circumstances due to Earth's small angular velocity, it becomes significant in large-scale motions like those of wind patterns, where it causes deflection to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.