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Inspired by this Phys.SE question, Coriolis force in free fall, I have to ask:

Why does the Coriolis force act in the east-direction?

I would say, if I jump from a high distance and look at the earth at the beginning and at the end, in the end the earth has rotated under me and I am more west, thus the Coriolis-effect deviated me in the west-direction.

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  • $\begingroup$ It doesn't. For things falling towards the surface of the Earth, it acts in the west direction. $\endgroup$
    – Brionius
    Feb 8, 2015 at 18:43
  • $\begingroup$ Why is it dependent whether the object falls towards the surface of the Earth or is fleeing from Earth ? $\endgroup$
    – Christian
    Feb 8, 2015 at 19:26
  • $\begingroup$ Oh, yeah, it's not, sorry - it's also westward for people going upwards. $\endgroup$
    – Brionius
    Feb 8, 2015 at 20:10
  • $\begingroup$ When you say "jump from a high [place]" do you mean you're leaving a static platform such as the top of a tall building? 'cause if so consider your linear E-W velocity up there compared with E-W velocity at the ground. $\endgroup$
    – Carl Witthoft
    Feb 8, 2015 at 22:08
  • $\begingroup$ Related: physics.stackexchange.com/q/43124/2451 $\endgroup$
    – Qmechanic ♦
    Feb 8, 2015 at 23:13

3 Answers 3

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Because while you fall the earth is still rotating and thus the ground moving from west to east relative to you.

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  • $\begingroup$ Looking from space on the Earth, I would be deviated in west-direction? $\endgroup$
    – Christian
    Feb 9, 2015 at 15:40
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    $\begingroup$ But you had W-->E speed too, based on whatever platform you dropped from. $\endgroup$
    – Carl Witthoft
    Feb 9, 2015 at 15:54
  • $\begingroup$ But the earth is moving in a circle and your tangential speed starts to mismatch the ground as soon as you leave it. $\endgroup$
    – John Alexiou
    Feb 9, 2015 at 19:54
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When the Earth pulls one of its inhabitants closer to its center of mass, that causes the Earth to spin a little bit faster.

When the inhabitant hits the ground he is moving into same direction as the ground (to the east), but faster than the ground.

Let us consider the angular momentum of two masses at the opposite sides of the Earth, at the equator, on top of two trees. As the masses are falling from the trees, their angular momentum is conserved. As the masses get closer to each other, and as there is no torque, the masses gain more speed (to the east).

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Imagine you are on a very high pole. Kilometers high. But you are still "attached" to the ground. So you rotate with the earth around its axis.

You move to the east. But your tangential speed is greater than if you were on the ground because you are further away from the axis. Its the same like when you are on a merry go around, its different when you're sitting on the edge than when you are sitting on the center.

So you "move to the east" faster than the ground. When you hit it, you have covered a slightly greater distance to the east.

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