Cause of the Coriolis Force

Here's a short look at the cause of the Coriolis force.

Imagine something flies overheadTucson. It travels straight from west to east at constant speed. You would, more or less subconciously, plot its path relative to reference points on the ground.The next figure shows the path that the object followed, relative to the ground, as it passed over the city.

It would appear to be moving in a straight line at constant speed. You would conclude there was zero net force acting on the moving object (Newton's first law of motion).

In this second picture the object flies by overhead just as it did in the previous picture. In this picture, however, the ground is moving (don't worry about what might be causing the ground to move).

This is the path that you would see relative to the ground in this case. Even though the object flew from west to east it appears to have been traveling from the NW toward the SE because the ground was moving as the object passed overhead. Because the motion is still in a straight line at constant speed, you would conclude the net force acting on the object was zero.

In this last figure the object flies by again from west to east. In this case however the ground is rotating.

Now the path of the object plotted on the ground appears to be curved. If you ignore the fact that the ground is moving then there must have been a force causing the moving object to follow the curved path. The needed force would be perpendicular to the direction of motion and to the right in this case.

At most locations on the earth the ground IS rotating (we're just not aware of it). This is most easily seen at the poles.

Imagine a piece of paper glued to the top of a globe. As the globe spins the piece of paper will rotate. A piece of paper glued to the globe at the equator won't spin, it will flip over. At points in between the paper would spin and flip, the motion gets complicated.

The easiest thing for us to do is to ignore the fact that the ground on which we are standing is rotating. However, if we do that we need to account for the curved paths that moving objects will take when they move relative to the earth's surface. That is what the Coriolis force does.