The wind pushes on wave crests. If the wind is relatively weak, it may add or subtract energy from the wave by doing work or negative work on it. But if the wind is too strong, it can blow the top off a crest. Choppy seas occur when the wind is so strong that it blows the surface water right out of wave crests and turns them white with foam.
They appear in rigid systems, such as beams or bridges. The Tacoma Narrows bridge failed because of a torsional (twisting) motion of its deck, driven by the wind. Before it failed, it was carrying torsional waves back and forth along its length. Torsional waves also appear in less spectacular engineering situations. When you lean on a loose tabletop, you actually send a torsional wave through it. However, it’s so rigid that the wave is tiny and travels too quickly for you to see.
According to the dictionaries, it’s just a form of fast moving current, a rip current. Water returning either from the shore (after a wave breaks) or from a channel (as the result of the tide) can create a strong current that’s difficult to stand still against.
The moon’s gravity affects both the earth’s path through space and the earth’s shape. If the moon were to disappear, the earth’s path would change but probably not enough to cause a noticeable difference. The earth and the moon normally orbit one another but the moon, which has much less mass than the earth, does most of the moving. Without the moon, the earth would just orbit smoothly around the sun. As for the earth’s shape, the only part of the earth that responds noticeably to the moon’s gravity is the water on its surface. The tides are caused mostly by the moon’s gravity. Without the moon, the tides would be much smaller and caused only by the sun’s gravity. Thus, in the long run, you would probably have trouble telling that the moon was gone without looking overhead—the earth’s path wouldn’t change much and you would have to look carefully to see the effect on the earth’s oceans.
However, in the moments following the moon’s disappearance, there might be some dramatic waves and a few stress-related earthquakes. The oceans and the earth’s crust do experience substantial stresses due to the unevenness of the moon’s gravity (it’s stronger on the side of the earth nearest the moon than it is on the side of the earth farthest from the moon). But I doubt that the sudden change in stress caused by having the moon disappear would do more than temporarily flood a few coastal cities. One last effect worth noting is that the precession of the equinoxes, a 26,000 year process that shifts the earth’s rotational axis in space and causes the stars that are overhead at night during a particular season to change gradually, is driven by the moon’s gravity and would disappear if the moon were to disappear.
When a wave breaks and then rushes up the beach, it leaves the water on the beach with excess gravitational potential energy. That’s what’s left of the wave’s energy. The water accelerates back down the beach and returns to the sea. This returning flow of water tends to go under the sea’s surface, probably because of the water’s circular motion in waves. Remember that the water in a wave travels in a circle, always moving forward (in the direction of the wave’s motion) when it’s at its highest point and backward (away from the direction of the wave’s motion) when it’s at its lowest point. I suspect that the returning flow of water from the beach joins this backward moving low water. When this low-lying returning water flows past you, it tends to sweep you along with it, hence the name undertow.
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.
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.
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.
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.
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.