[Physics] Why does the Coriolis effect disappear at the equator

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I'm studying from the book "Classical Mechanics" by Goldstein and from a coursebook my Professor provided me.

In the coursebook, it says that "the Coriolis effect disappears at the equator (Where the rotation vector $\vec\omega$ of the Earth is horizontal)"

Now here's my reasoning: Take the Coriolis term:

$-2m(\vec\omega\times\vec v_r)$

Nearby the north pole, this would cause a force pointing towards the equator if you were moving from west to east. The closer you get to the equator, the more this force starts pointing "upwards" if you were to describe the vector from the surface of the Earth.
At the equator, this vector is perpendicular to the tangent of the equator and pointing outwards.
Intuitively, this would mean that if you move from west to east on the equator you would be accelerating away from Earth.

Now if my reasoning is correct, this doesn't mean the Coriolis effect disappears, only it turns into some sort of centrifugal force, but this seems weird because it's described by another term.

Where did I go wrong?

Best Answer

The Coriolis force on the equator indeed does point outwards, if you are moving west to east. This is not the same as the centrifugal force, because the centrifugal force is present always - even if you are not moving. But when you move (west to east), there is an additional force on top of the centrifugal force - the Coriolis force. If you travel east to west on the equator, the Coriolis force is pointing inwards. It has practical effects, as can be seen in this video demonstrating how bullet trajectory is curved upwards or downwards when shooting to the eastern or western direction.

The Coriolis force on the equator disappears if you are traveling in the northern (or southern) direction. In that case your direction is (anti-)parallel to the direction of the $\vec\omega$, so their vector product is zero. Maybe this is what Goldstein had in mind.

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