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Why is it that a flushed toilet will "swirl" clockwise north of the equator and counterclockwise south of the equator? A physics major told us about this phenomenon and we want to know if it is true...and why?!? Thanks

Your teacher is referring to a phenomenon which is true, but, in truth, a given toilet flushed north or south of the border will swirl the same way. This is because details of the toilet us design, such as from what angle the inlet water comes, has a much larger effect on the resulting flow than does the small force generated from the earth's motion. The force he describes is the Corriolis Force. This force is caused by the rotation of the earth. It is responsible for the fact that weather systems swirl in the manner you described. It is also responsible for rivers eroding their right banks (in the north) more than their left ones, and vice versa in the south. Imagine for a moment that someone at the equator fires a gun due north. Lets also imagine that we are looking down at the earth from above the north pole so that the earth looks like a spinning disk with the equator at the edge. From that vantage, the shooter stands at the edge of the disk and fires toward the center. The bullet has angular momentum even before it is fired just by virtue of the fact that its rotating with the earth. That angular momentum is the product of the angular velocity and the distance from the axis (the radius of the "disk" we are looking at. The angular momentum must be conserved. As the bullet move north, the distance to the center of the "disk" decreases. Th keep the same angular momentum, the angular velocity increases, and the bullet deflects to the right (it gets ahead of the earth.) Similar arguments show that the deflection is always to the right in the north and to the left in the south.

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