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Page 21 of 160 (1595 Questions and Answers)

201. Is water, at say 35° F, more dense than water at 80° F?
Yes, water at 35° F is more dense than water at 80° F, so that the hotter water will float on the colder water. But water is special (and almost unique) in that it does expand slightly as it cools to its freezing temperature. Water's density reaches a maximum at 3.98° C (about 39° F) and then actually becomes less dense as you cool it toward 0° C (32° F). That means that 33° F water will float on 39° F water! This bizarre behavior allows ice and very cold water to float above slightly warmer water and keeps ponds from freezing solid. Without it, many animals would perish during the winter.

202. What is the function/purpose of water in your body? What happens if your body doesn't get sufficient water or gets too much water? What are the best sources of water?
Water plays so many roles in your body that I'll have to choose which one to describe. The role that strikes me as most important is water's capacity to dissolve chemicals and carry them about. Our bodies deal with thousands or perhaps millions of different molecules, each of which must migrate from place to place to perform its required tasks. For example, the molecules that are responsible for energy—carbohydrates, fats, oxygen, and carbon dioxide—must all move through us to keep us alive. Water acts as the carrier for all of these molecules. Many of the molecules dissolve easily in water—meaning that water molecules surround these molecules and carry them about one molecule at a time. Carbohydrates, carbon dioxide, salts, and many other organic compounds dissolve easily in water and are carried about by it. Other chemicals, particularly fats and oxygen, don't dissolve easily in water and need help. Hemoglobin in red blood cells is the molecular structure that carries oxygen through our blood. So my answer to the first question is that water is the principal solvent for all the chemicals in our body.

If your body gets too much or too little water, the dissolved chemicals become either too dilute or too concentrated. When you have too much water in your body, the water tends to flow out of you through any membranes that are permeable to water. Among other things, this process tends to make you excrete extra water as urine. When you drink too much water, it goes right through you. When you have too little water in your body, water tends to flow into you through any permeable membranes, or at least tends not to flow out of you. When you eat lots of salt and are overconcentrated, you don't excrete much water and feel extremely thirsty—your body is looking for more water.

Finally, the best sources of water are those that simply don't have many dissolved chemicals; or at least none that cause trouble for your body. That means that your water shouldn't have much lead or arsenic dissolved in it or any of a number of noxious organic chemicals. The purest waters are distilled water, rain water (assuming minimal air pollution), and water that has been chemically filtered (via ion exchange, reverse osmosis, and/or activated carbon). Spring and well waters tend to contain substantial amounts of dissolved calcium and magnesium salts, which make the water less pure but probably don't affect its healthfulness. One special case to look out for is water that was very hard but that has been passed through a water softener. The dissolved minerals that made the water hard will have been replaced by sodium compounds during the softening process and excessive sodium consumption may be a problem for some people.

203. Why does water boil quicker when you add salt?
Salt should actually delay boiling in water by raising the water's boiling temperature. The water simply won't boil until its temperature reaches a certain value and that requires heat. There isn't any trick that will shorten the heating time, except to add chemicals that boil at lower temperatures than water (e.g. alcohol). But sprinkling salt into very hot water may trigger boiling by providing nucleation sites on which the bubbles can begin to form.

204. How does a "water" clock work? What would be the simplest way to make a water clock that would maintain accuracy to say three minutes per hour?
The most common water clock, the clepsydra, has a reservoir that drips water into a cylindrical vessel. The height of the water in that vessel indicates the amount of time that has passed since the clock was started. The simplest version of the clepsydra just has a small hole in the bottom of the reservoir and doesn't take into account the decreasing drip rate that comes with the dropping water level in the reservoir and the decreasing water pressure at the hole. To make a more accurate clock, you should maintain a constant water level in the dripping reservoir. The simplest way to do this is to place an inverted bottle of water in the reservoir so that as the water level drops past the lip of the inverted bottle, air bubbles can enter the bottle and allow more water to flow out of the bottle and into the reservoir. If you keep the water level in the reservoir constant in this manner, you ought to be able to calibrate the clock to better than 3 minutes per hour. Use a carefully made graduated cylinder as the time-measuring cylinder and watch the water level gradually rise.

205. My grandmother used to have this watch she wound by shaking it. How is it possible?
Her watch was called an automatic watch and contained a heavy piece of metal at the end of a tipping arm. As her wrist moved during daily activity, the metal piece would swing back and forth, twisting the arm first in one direction and then in the other. The arm's twisting motion would wind the watch's mainspring, just as you can wind the mainspring of a normal wristwatch by rolling the knob back and forth in your fingers. On some automatic watches, you can feel and hear the weight swinging back and forth as you shake the watch.

206. Suspension bridges today can't oscillate like the Tacoma Bridge, can they?
Apparently not, because we've never seen or heard of one doing it. To prevent that sort of thing from happening, the bridge builders probably do two things. First, they damp all of the resonance in the bridge. By this, I mean that they introduce energy loss mechanisms that sap the energy out of all the resonant motions. For example, they could add plates that slide against one another as the bridge bends so that sliding friction will waste energy and spoil the resonant motion. Second, they make sure that there are no mechanisms for resonant energy transfer. The wind blowing on the Tacoma bridge gave it tiny pushes at just the right frequency. It oscillated the way a reed does in a musical instrument. These days, bridges are probably tested with computer modeling before they're built to make sure that they don't begin to oscillate when wind blows across them.

207. What are period, amplitude, and frequency?
Period is the time it takes for a resonant system to complete one cycle of its motion. For example, if a pendulum takes two seconds to swing over and back, then its period is two seconds. Amplitude is the maximum amount of motion a resonant system undergoes as it oscillates or vibrates (same thing). For example, if the pendulum swings one meter to the left of center and then one meter to the right of center, its amplitude of motion is one meter. Frequency is the number of cycles a resonant system completes in a certain amount of time. For example, if a pendulum swings over and back twice each second, then its frequency is two cycles-per-second or 2 hertz or 2 Hz.

208. What does the length of the string (in a pendulum) have to do with resonance?
When you lengthen the string or rod of a pendulum, you weaken the restoring force on the pendulum's weight. That weight must then drift farther from its equilibrium position to experience strong restoring forces. The result is that the pendulum swings more slowly through its cycles (its period increases and its frequency decreases). But no matter what the string's length, the pendulum will exhibit a resonance. The frequency at which this resonance occurs is all that changes.

209. Why are Rolex watches able to spin smoothly or what do jewelry inspectors look at to tell the difference between a fake and a real Rolex watch?
A real Rolex watch has a sweep second hand that appears to move steadily around the watch face. A fake, like most other watches, has a second hand that moves with a jerky motion, advancing a little bit each time the balance ring completes one half of a cycle. However, a reader has informed me that even a real Rolex moves its second hand in tiny steps—they're just very small. If you look closely, he writes, you'll see that the second hand makes 5 tiny steps each second. Evidently, the hand-advancing mechanism steps at a higher frequency (5 Hz) than in most other watches (1 Hz). These tiny steps are hard to see so the hand appears to move smoothly. I was relieved to hear this news because the balance ring mechanism is inherently jerky and it's hard to imagine a balance ring-based watch that avoid the jerkiness.

210. How can you make noise by running your finger (if it's wet) along the rim of a glass?
If you run your damp finger lightly along the rim of a crystal glass, it should begin vibrating. This trick involves a resonant transfer of energy in which your finger rhythmically pushes on the glass to make it vibrate more and more strongly. It takes a delicate touch. If you press to hard, you will prevent the glass from vibrating. If you press too lightly, you won't give it any energy. Your finger must stick and slip alternately, just as a bow does while sliding across a violin string.
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