At times a very thin invisible layer of ice forms on road surfaces. The road sur…

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At times a very thin invisible layer of ice forms on road surfaces. The road surface appears dry and does not have the telltale reflections of ice. Many people refer to this as “black ice.” How is this ice formed? What are the crystal properties that make it invisible? – BK

Black ice is a layer of ice that is almost free of internal defects or air bubbles and that does not have a smooth surface. The absence of internal defects or air bubbles is what makes it transparent rather than white. Snow and crushed ice appear white because they contain countless tiny surfaces. Whenever light changes speed, as it does in going from ice to air or air to ice, some of that light reflects. Since snow and crushed ice contain many ice/air interfaces, they reflect light extensively and appear white. In contrast, black ice contains no internal ice/air interfaces and doesn’t reflect any light from inside. Any light that makes it into the black ice goes all the way to the roadway. If the roadway reflects any of this light, it again passes unscathed through the black ice. The only evidence that the black ice exists at all comes from its surface, but here again the ice offers little that you can see. Since true black ice is microscopically rough, the small amount of light that reflects as it enters the ice from the air is reflected randomly in all directions. So little of that reflected light travels in any one direction that you can barely see it at all. Overall, black ice reflects so little light that you see only the roadway itself. While I am not sure, I think that it forms when moisture in the air condenses to dew on the roadway and then freezes into ice. Whatever process forms it must leave it almost without holes and therefore invisible.

How do fletchings stabilize an arrow in flight after it is shot from a bow?

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How do fletchings stabilize an arrow in flight after it is shot from a bow? — SH, Newton, TX

Like all isolated objects, the arrow naturally pivots about its own center of mass, a point located near its geometric center. If the arrow had no fletchings (or fins) it would tend to rotate wildly in flight. But the fletchings experience substantial aerodynamic forces whenever the arrow isn’t flying point first and these aerodynamic forces twist the arrow back toward its proper orientation. Thus whenever the arrow begins to rotate so that its point isn’t first, the air pushes hard on the fletchings and returns the arrow to its point-first orientation. The same effect keeps airplanes and birds flying nose (or beak) forward.

Piling sandbags in the back of a truck would increase friction between the wheel…

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Piling sandbags in the back of a truck would increase friction between the wheels and the ground, but wouldn’t it also increase the truck’s inertia, making it harder to stop on an icy road?

Adding sandbags to the back of a pickup truck increases the truck’s traction and adds to the truck’s mass. Fortunately, the truck’s traction increases more dramatically than its mass and it becomes easier to start and stop the truck, rather than the reverse. That’s because even a modest amount of sand can double the force pressing the rear wheels against the road and thus double the frictional forces the wheels can experience. That same amount of sand won’t double the total mass of the truck.

In class, you sat motionless on a cart with a ball in your lap. You said that yo…

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In class, you sat motionless on a cart with a ball in your lap. You said that your momentum was zero. You then threw the ball in one direction and you began moving in the other direction. You said that your momentum was still zero. How can your momentum be zero if you are moving?

In both cases, I was referring to the total momentum of the ball and me. The total momentum of the ball and me was zero before I threw the ball and it was still zero after I threw the ball. However, before I threw the ball nothing was moving and after I threw the ball the two of us were moving in opposite directions. It was our total momentum that was zero after the throw, not our individual momenta. While the ball and I each had a nonzero momentum after the throw, our momenta were equal in amount but opposite in direction—the ball’s momentum was exactly opposite mine. If you were to add our momenta together, they would sum to zero. Since momentum is conserved and we couldn’t exchange momentum with anything around us, the ball and I began and ended with the same total amount of momentum: zero.

If there were no friction or air resistance, would the bowling ball pendulum con…

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If there were no friction or air resistance, would the bowling ball pendulum continue in motion forever?

Yes. If the pendulum had no way to convert its energy into thermal energy (e.g., via friction) and no way to transfer that energy elsewhere (e.g., via air resistance), it would continue to swing forever. While its energy would transform from gravitational potential energy (at the ends of each swing) to kinetic energy (at the middle of each swing) and back again, over and over, the total amount of energy it has won’t change.

How fast is the earth moving through space? Does this movement affect our percep…

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How fast is the earth moving through space? Does this movement affect our perception of time? — GR, Grabil, IN

Because there is no preferred reference frame for the universe, we can only talk about the earth’s speed in reference to other objects. For example, the earth is moving at about 5 kilometers per second relative to the sun and about 30,000 kilometers per second relative to the center of the galaxy. These speeds do affect our perceptions of time, so that times passes at a different rate for us than for someone closer to the sun or to the galactic center. However, gravitational wells also affect the perception of time, so that the effects are complicated. The earth is also receding extremely rapidly from objects at the far side of the universe; so fast that time passage is dramatically affected. Those distant objects appear to be aging very slowly and their light is shifted substantially toward the red.

How does an infrared sensor faucet work?

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How does an infrared sensor faucet work? — DD, Sacramento, CA

The sensor has two lenses: one that emits a beam of infrared light and the other that looks for a reflection of that light. As long as there is nothing beneath the faucet, there is very little infrared light reflected back toward the sensor and the sensor prevents any water from flowing out of the faucet. But when you hold your hands under the faucet, the infrared light reflects from your hands and some of it returns to the sensor. The sensor detects this light and opens an electronic valve to permit water to flow out of the faucet. The lenses are aimed so that only objects under the faucet itself will reflect the infrared light back toward the lens. A more distance object may reflect some of the infrared light, but the light won’t pass through the sensor at the proper angle and won’t be detected.

Can you explain how the telephone wiring in my home works for the telephone? My …

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Can you explain how the telephone wiring in my home works for the telephone? My touch-tone phone has 4 wires, but I understand that only 2 wires are used. Does the phone use the other 2 wires for the light on the phone pad, etc.? — DS, Larkspur, CA

Your telephone performs all of its functions using only those 2 wires. The 2 extra wires are virtually never used by a single-line telephone. The only exception that I’m aware of is the old “Princess Telephone,” which had a special light powered by the extra pair of wires. In most telephones, even the power for the lighted keys comes from the 2 main wires. While the telephone is off the hook, the telephone company sends a constant DC current through those two wires. This current powers the telephone’s electronics and its lights. When you talk, the microphone causes the telephone’s electric impedance to fluctuate up and down and this variation causes sound to be reproduced in your friend’s earpiece. Pressing the dialing buttons causes similar fluctuations in impedance and the telephone company uses these tones to make the proper connections. When the telephone company rings your telephone, they send a higher voltage AC current through the two wires and the telephone’s bell rings.

Radioactive elements’ half-lives are fixed and they decay at a constant rate. Th…

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Radioactive elements’ half-lives are fixed and they decay at a constant rate. Their decay rates have been determined thanks in part to our nuclear weapons research. Under what circumstances can a radioactive element have its decay rate changed? Can the element’s radioactivity be destroyed (cancelled) by applying high temperatures? If so, how high would the temperature have to go to achieve this? — RD, Humble, TX

Since radioactivity is a feature of atomic nuclei, the only way to alter radioactivity is to alter atomic nuclei. But there aren’t many ways to change atomic nuclei. Of various atomic and subatomic particles, only a neutron can enter a nucleus easily and cause it to rearrange. However, it’s more common for a neutron to increase radioactivity than to destroy it, so that’s not a good approach. Furthermore, the only practical way to obtain neutrons is with radioactivity.

Heating a collection of nuclei can cause them to collide and rearrange. However, this process is also fraught with problems. The products of the fusion and fission events that occur when nuclei collide will probably be radioactive themselves, so that it’s unlikely that heating radioactive materials will make them less radioactive. Instead, it’s likely that heating radioactive materials will make them more radioactive. Furthermore, the temperatures at which nuclei will begin to collide are extraordinarily high. Even the smallest nuclei repel one another fiercely so that they need temperatures of 100 million degrees C or more to begin colliding effectively. Larger nuclei, such as those common in nuclear wastes, won’t collide until their temperatures exceed 1 billion degrees C. The only way to reach these temperatures is with nuclear weapons and they certainly don’t reduce the radioactivity of nearby materials. In short, the only way to get rid of radioactivity is by waiting patiently.