Then, on a spinning sphere, the centrifugal force, an outward force that arises from the spinning, pushes the puck toward the equator. After traveling about 50 miles north, the puck reverses its direction and starts to move south.

At the same time, the Coriolis force, another force that emerges in rotating systems, drives the puck to the west. Part of the reason is that as Earth spins, objects closer to the equator are moving faster than objects closer to the poles, since those points have to travel farther to complete an orbit in the same amount of time. (It’s similar to how a track athlete in an outside lane has to run faster to keep up with a runner in an inside lane.) As the puck travels south, the land beneath it is moving faster eastward, so the puck appears to move west.

Furthermore, as the puck gets closer to the equator, it gets farther from Earth’s axis of rotation, and to conserve angular momentum, the speed at which it moves to the east slows. It’s the same reason why a figure skater spins faster with their arms tucked in, and slower with their arms outstretched. This slowing further pushes the puck to the west.

As the puck travels to the southwest, it speeds up until it crosses the equator, at which point it starts to slow, as the centrifugal force pulls it back to the equator again. The puck gets as far south as Argentina, exactly as far below the equator as its highest point above it, before returning to the north. The puck will continue to zigzag between the northern and southern hemispheres while moving to the west.

A hockey player on a spheroidal Earth

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