|MadSci Network: Physics|
Greetings, Sarah: Water swirls down a drain because Planet Earth is rotating. Possibly, one could go so far as to say it is proof that the world turns. But to derive such an uncommon statement from such a common event is going to take some explanation.... Get yourself an ordinary ball and set it on a table. Place one finger on top of the ball to hold it in place. Now take a marker and put some dots on the ball, in a nice vertical straight row, from near your finger, down toward the table. Finally, keep your finger in place while you turn the ball slowly, so that the vertical row of dots stays vertical. After one complete turn of the ball, take a moment to think about how far each dot moved. Each moved in a circle, but the ones near your finger, or near the tabletop, moved in small circles, while the ones near the middle of the ball moved in larger circles. Now, since each dot took the same amount of time to move in its circle, how fast did each dot move? It should be obvious that the dots that moved in the larger circles had to move faster. Now think about the Planet Earth: It is like a ball, only bigger. The "middle" of this ball is usually called the Equator; it is a circle more than 24000 miles in circumfrence. Next, since there are 24 hours in a day, it is plain that for a "dot" on the Equator to move more than 24000 miles in 24 hours, it must move at over 1000 miles per hour. Meanwhile, a "dot" located near either the North Pole, or the South Pole, will move very little in 24 hours. --------------- Time to switch subjects for a bit. Take a look at a music disc, and consider the small circles near the hole, and the larger circles near the rim. You know that the whole disc turns together, so just like the ball, a dot on the rim has to move faster than a dot near the middle. Next, think about some astronauts orbiting the Earth. It is easy to imagine them going around the world like a dot on the rim of a disk. BUT: suppose we look very closely inside the astronauts' spaceship; some parts of the spaceship are closer to the Earth than others, right? If you were to place two dots inside that spaceship, the one closer to the Earth would go around the Earth just a little bit slower than the dot located farther from the Earth, wouldn't it? Whenever any object, such as the spaceship, contains spots that have to try to move at two different speeds at the same time, there will be some stress in that object. If you try to spin that music disc too fast, it will fly to pieces. Fortunately for the spaceship, it only experiences some very small stress. We call it "tidal stress". It is sufficient to cause loose objects in a spaceship to gradually move. Some people may erroneously refer to it as "microgravity", because of that motion. On the other hand, there is ALSO some miniscule amount of real gravitation present, between those loose objects. Anyway, it is important to know that tidal stress is a FORCE -- if something can cause an object to begin to move, it is always because that something is a force. ---------------- Meanwhile, back on Earth, we have all this water, which most certainly is a very loose collection of molecule-sized objects. In whatever manner you wish to describe water moving, it does so because some kind of force acts upon it. Note a drain, for example. At most places on the surface of the Earth, you can place two dots near the opening, such that one dot is closer to the Equator, and the other dot is closer to a Pole. As the world rotates, one dot will be moving very slightly faster than the other. Therefore, we have a condition similar to that inside the spaceship, where a tiny force is present. (In this particular case, the correct name is "Coriolis Force".) One of the basic facts about forces is that not only do they cause objects to move, the longer a force is applied, the faster an object will move. So...when we have a tub full of water, and pull the plug to let it drain out, there will be a tiny force causing some of the water molecules to begin to move in a swirl. This initial motion is far to small for you to see, however. BUT: The force exists steadily for as long as there is water in the tub to drain; gradually the swirl goes faster and faster. After a few seconds, you will become able to see it; after a few more seconds, the swirl is unmistakable. The more water available, the longer it will take to drain, and the longer that tiny force will have, to build up a larger and larger swirl. One final point is relevant: The location of the drain, on the surface of the Earth, affects the swirl significantly. In the Northern Hemisphere, draining water naturally swirls counterclockwise; in the Southern Hemisphere, it naturally swirls clockwise. Also near either Pole, the Coriolis Force has maximum effect upon water going down a drain. At the Equator -- EXACTLY at the Equator; imagine it as a mathematical line that perfectly bisects the drain -- the Coriolis Force is zero. (More precisely; two Coriolis Forces are present, one for each half of the bisected drain, but they cancel each other out.) In this special case water SHOULD be able to go down a drain simply straight, just as you wondered about in your Question.
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