This presentation connects familiar playground physics to inertial motion on rotating systems and explains how these ideas lead to circulation patterns in planetary atmospheres.
The animation opens on a playground. At its center sits a rotating merry-go-round: a five-meter-diameter circular platform divided into wedge-shaped sections by handrails extending from the center to the rim.
We observe the platform from the ground, looking slightly downward. From this viewpoint, we are in an inertial reference frame. A rider on the platform experiences a non-inertial, rotating reference frame.
We examine trajectories from launchers attached to the rotating platform. Each particle’s initial velocity combines the kick from the launcher with the tangential motion of the launcher as the platform rotates.
In the inertial frame, particle motion is shown by X-shaped markers tracing straight-line paths. Circular dots mark the particle’s path relative to the rotating platform.
A launcher near the rim is aimed toward the center. In the inertial frame, the particle follows a straight path that is offset from the launcher’s aim due to the tangential speed of the rotating platform.
When the same launch is viewed relative to the platform, the particle’s path begins in the launcher’s direction but then curves to the right. This right-hand turn is measured relative to the particle’s own direction of motion.
Launches from different locations and directions—spinward, anti-spinward, inward, and outward— all show straight motion in the inertial frame and rightward curvature in the rotating frame.
We now consider multiple launchers firing simultaneously. Four launchers are arranged so that their initial directions point toward a common center.
Although each particle moves inertially in a straight line, the right-hand turns of their paths relative to the rotating platform combine to form a net counterclockwise circulation.
Repeating the experiment with four launchers aimed outward from a common center produces a net clockwise circulation in the rotating frame. This circulation is opposite the direction of the platform’s rotation.
From the perspective of someone riding on the platform, these circulation patterns are manifestations of the Coriolis effect. Inward flow produces circulation in the same direction as the platform’s rotation, while outward flow produces circulation in the opposite direction.
The scene transitions to space, where a sphere floats freely. Objects launched on the sphere are constrained to move along its surface, which we take to be frictionless.
Although this motion is not strictly inertial, in an inertial frame objects on the sphere follow great-circle paths. These great circles are the spherical analog of straight-line motion.
A grid and axis marker reveal that the sphere is rotating. Viewed from above mid-latitudes, the sphere rotates counterclockwise.
Four inward-aimed launchers fire simultaneously on the rotating sphere. Inertially, their paths follow great circles, while their paths relative to the rotating surface curve to the right.
Together, these rightward deflections produce a counterclockwise circulation pattern. When Earth’s surface is revealed, this circulation corresponds to large low-pressure systems, such as hurricanes in the northern hemisphere.
Outward-flowing launches produce circulation opposite the planet’s rotation. This behavior corresponds to large high-pressure systems, which are not prominent on Earth but are observed on other planets, such as Jupiter’s Great Red Spot.
The viewpoint shifts below the planet, focusing on the southern hemisphere. From this perspective, Earth’s rotation appears clockwise.
Inward flow in the southern hemisphere again produces circulation in the same direction as the observed planetary rotation, which is now clockwise. This explains why hurricanes rotate in opposite directions in the two hemispheres.
Outward flow in the southern hemisphere produces circulation opposite the planet’s rotation, appearing counterclockwise from this viewpoint.