. If you lived on the moon, would it be easier to adapt to living with the moon's gravity, or to create an artificial environment with the gravity of earth? — MK, Orlando, FL
Building an environment that made you feel what appeared to be the earth's gravity would be a substantial undertaking. The only way to simulate gravity is through acceleration and the only way to make a person experience acceleration continuously is to swing them around in a circle. So this environment will have to swing its occupants around in a circle. However, we are extremely sensitive to changes in orientation, so that we can tell the difference between true gravity and the experience of being swung around in a small circle. To avoid the dizzying feeling of having our orientations changed rapidly, the turning environment would have to be extremely large. It would have to be a huge rotating wheel, looking like a heavily banked circular racetrack that spun at a steady pace and completed something like one full turn per minute. The occupants would have to live on the long, thin surface of this turning racetrack. Building such a device on earth wouldn't be easy. Building it on the moon would be much harder. I wouldn't plan on trying to simulate the earth's gravity on the moon. So I vote for just putting up with the moon's weaker gravity.
. How do thermals affect the atmosphere and air currents? — RM, Praire Farm Schools, Wisconsin
Thermals are air currents in the atmosphere. When sunlight and exposure to warm ground raises the temperature of surface air, that air expands—its molecules travel faster and bounce against one another more vigorously, so they push themselves farther apart. This expanded air weighs less per cubic foot or meter than cooler air, so the cooler air around it lifts it upward in a rising current of warm air—a "thermal." The air can't simply accumulate way up overhead forever, so cooler air descends to take its place. The overall result is rising warm air and descending cool air. These air currents are part of giant circulation loops or "convection cells" that also include surface winds and high altitude winds.
. Would it be possible for a spacecraft to use electrically powered propulsion? Could it gather atoms and molecules from space and then use an electromagnetic field to push them through a nozzle? — JC, Burnaby, British Columbia
Not only is it possible to use electrically powered propulsion, such systems are already in use on several spacecraft. While they don't scavenge atoms and molecules from space, these ion propulsion engines uses electric forces to accelerate ionized atoms to enormous speeds. As the engine pushes on the ions it accelerates, those ions push back on the engine. The ions rush out into space in one direction and the engine experiences a modest thrust in the opposite direction. While the overall thrust from an ion engine is small, it uses its stored-atom "fuel" very efficiently and can be sustained for a very long time in a solar- or nuclear-powered satellite. Ion engines are used in spacecraft that need small but steady thrust for a long time. Scavenging atoms from space would allow these engines to run for an even longer time, but it's probably not realistic. The atoms in space are typically so rare and so fast-moving that they would be more trouble than they're worth.
. Can you explain gyroscopic precession? — BW, Newport, RI
When a gyroscope is spinning rapidly, it has a large amount of a conserved physical quantity called angular momentum. Angular momentum is a special measure of rotational motion that can't be created or destroyed—it can only be transferred between objects. As long as nothing tries to transfer angular momentum to or from the spinning gyroscope, it will continue to spin at a steady pace about a fixed axis in space. But when an external torque (a twist) is exerted on the gyroscope, a transfer of angular momentum takes place. The gyroscope's rate of rotation or its axis of rotation begins to change so that its angular momentum changes. If you apply a twist to the gyroscope around its axis of rotation, it will either spin faster or slower, depending on which way you twist it. But if you twist the gyroscope about a different axis, its axis of rotation will shift—the gyroscope will undergo precession. The direction of this precession depends on how you apply the twist and tends to be very non-intuitive.
. How fast can glaciers move? — SF, Burton, OH
Glaciers move at a variety of rates, ranging from inches per year to many feet per day. Currently there are very few "galloping" glaciers (those that move many feet per day) and most are either stagnant or retreating.
. What happens when salt is added to water? If I mix 1 cup of salt with 1 cup of water, will I end up with 2 cups of solution? - RT
As a crystalline solid, salt consists of a beautiful cubic lattice of sodium atoms that have lost one electron to become sodium positive ions and chlorine atoms that have gained one electron to become chlorine negative ions. The crystal is held together by the attractive forces between these oppositely charged atomic ions. When a salt crystal dissolves in water, it decomposes into individual sodium positive ions and chlorine negative ions that are then carried about by shells of water molecules. Water molecules are electrically polar, meaning that they have positively charged ends and negatively charged ends. The water molecules line up around a positively charged sodium ion with their negatively charged ends inward and carry that ion about. Similarly, water molecules line up around a negatively charged chlorine ion with their positively charged ends inward and carry that ion about. Whether you will end up with 2 cups of solution after mixing 1 cup of salt and 1 cup of water depends on how tightly the atoms and molecules pack together in each case. Remember that your 1-cup of salt contains a fair amount of air between the salt grains. You'll have to try it to find out the answer—I'm not sure what the answer will be.
. Why does popcorn pop? - AB
Inside the hard, dry hull of a popcorn kernel is a portion of moist starch. When you heat the kernel well above the boiling temperature of water, the water in the starch converts to hot, high-pressure steam. The hotter this steam gets, the higher its pressure rises and the stronger the outward forces it exerts on the hull. Eventually, the hull rips open under the stress and exposes the starch to the low-pressure air around it. The pressurized steam then pushes the starch outward, expanding it to many times its original size. The kernel "pops."
. Are there any levitated trains in the world? - BP
At present, I believe that the only magnetically levitated trains are those undergoing development and testing.
. In the simplest terms, how does a basic electrical circuit work? — CC, Port St. Joe, FL
An electric circuit is racetrack for electric charges. It must be a complete loop—a "circuit"—so that the charges don't pile up somewhere along the track. The simplest circuit has a source of energy for the electric charges (e.g., a battery) and a device that takes energy away from the electric charges (e.g., a light bulb). When the charges are in motion through the circuit, they are an electric current. By convention, current points in the direction of positive charge flow, so you can imagine a stream of positive charges circling this circuit over and over again, with current pointing always in the direction that those positive charges are moving. As the current passes through the battery, entering it at the battery's negative terminal and leaving it at its positive terminal, the charges pick up energy. The battery is converting some of its stored chemical potential energy into electric energy and giving that energy steadily to the current flowing through it. The battery is "pumping" the charges from its negative terminal to its positive terminal. The current continues around the circuit and then passes through the light bulb. In the light bulb, the charges give up most of their energies to the filament and the filament becomes white hot. The current continues out of the bulb and returns to the negative terminal of the battery to pick up more energy. This simple circuit is present in a flashlight. The same charges complete this circuit millions of times each second, shuttling energy from the battery to the bulb.
. I'm doing a science fair project on electricity and I need to know how to make a homemade hot dog cooker. - BE
Although I have never done it myself, I understand that it is possible to run electric power directly from the power line through a hot dog and to use the resistive heating that occurs as electric current struggles to pass through the hot dog to cook that hot dog. While I can't recommend doing this and caution anyone trying it to be extremely careful with the electricity (i.e. seek adult supervision from someone who is experienced with the safe handling of electricity), I believe that it can be done. My understanding is that you should carefully connect each wire of an electric power cord (unplugged!) to its own nail (choose an uncoated steel nail to avoid toxic materials). You should then insert one nails into each end of the hot dog and place that hot dog on a safe, nonconducting surface where no one and nothing can touch it. Finally, you should plug the electric cord into an electric socket that is properly connected to a working circuit breaker. I would recommend using a socket protected by a ground-fault interrupter (GFI) such as are used in modern bathrooms (the ones with a "test" and "reset" button). (As you can see, I don't want anyone hurt!) I'm not sure how quickly the hot dog will cook, but I'd expect it to be quite fast. Be sure to unplug the cord before getting anywhere near the hot dog.