Apologia Physical Science test question help needed - Coriolis effect

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Module #7
Question#8: Suppose you are at the equator and want to fire a missile at a target due north of your location. Would you aim the missile north, northeast, or northwest to ensure it hits the intended target?

Answer: You would fire it northwest to counteract the Coriolis effect. See figure 7.8.


We estimate that if you are standing on the equator and aiming north, you would fire the missile ahead of your target, thus fire the missile northeast, to actually hit the target.

Please explain why you would fire northwest....

[ccm]

The On Your Own 7.7 is similar to your question. The explanation is on page 178 of the text. I've read it a couple of times and I still don't get it! :confused:

[GVA]

Only freely moving objects are influenced by the Coriolis effect, and it's the earth's rotation that causes the influence of Coriolis forces to be a factor here.

If you look at wind patterns and water patterns, both the wind and the ocean tend to move to the east in the Northern Hemisphere because the molecules of these large "masses" are pulled that way by the Coriolis forces. I live on the East Coast, and the storms almost always come from the west.

By shooting the missle northwest you are countering the Coriolis force to some extent and probably compensating for common wind direction. Shooting it true north will mean that it doesn't go true north, but that it will be pulled somewhat east by the Coriolis force (like storms going from west to east across the U.S.) and prevailing wind direction. Shooting it northeast will mean that it's movement in that direction will be boosted by the Coriolis effect.

Shooting a missle northeast with the idea that the target will "catch up" assumes information that is not in the problem. Maybe the missile is only a simple Scud that is in the air for minutes total. In that period the immensity of the earth means that it will not "catch up" in a measurable way and that the rotation of the earth is really not a factor in calculating it's target (a tiny effect there, but just like when you throw a ball in the air -- it really doesn't make a big difference that the earth is rotating). The rotation of the earth is indeed an factor (but not the only one) for inter-continental missiles because they are in the air awhile, but the problem doesn't state what kind of missile it is. They're giving you a more simple, general problem focused on a simple shot and they way it would go.

HTH!

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I will check out some online sources this weekend to try to understand it better. Your great description makes me think we aren't understanding it correctly. Thanks for your detailed answer