For an index file listing questions by topic, click here.
Why did the Moon appear Red?
(1) Why don't its particles separate by weight?
(2) What accelerates the solar wind?
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114. Why not use a heat shield going up?
ReplyThe reason you suggested yourself is pretty much what happens.
Going up, it is the rocket engine which provides acceleration and energy. Because air resistance robs energy and is undesirable, the rocket deliberately rises vertically, to go though the denser atmosphere as quickly as possible. The vehicle gets most of its velocity, and almost all of the kinetic energy, at high altitudes where air density is too low to make a great difference. With the space shuttle "Columbia," even that might not have been enough: by the time it reached twice the velocity of sound, the atmosphere around it was still dense enough to rip a piece of foam insulation off its fuel tank, and it hit the orbiter with great force, breaking the heat shield.
On coming down (with spacecraft which we want to come down undamaged), the atmosphere is the brake absorbing the energy. We need that air resistance, and the heating is a result of absorbed energy! The big concern that energy should not be absorbed too fast, otherwise the heat gets too intense and may melt the heat shield and everything else. That is why the space shuttle comes down at a low angle, trying to stay as long as possible in a layer with the right density: If the shuttle comes in too high, not enough energy is lost, if too low, too much. The density is also important in supporting the shuttle, which--like a kite--needs part of the resistance to help it from coming down too fast.
A final note: heat shields get very hot, but most of the energy, almost all of it, is given to the shock which forms ahead of the heat shield. It thus heats the air, not the re-entering vehicle.
115. When and where can rainbows be seen?
Believe it or not this all has to do with rainbows. I had an argument with someone whom I told that I have never observed a rainbow except in the morning or evening. The Sun has to be low in the east or west, and the bow then appears on the opposite side of the sky. Never observed one in the northern sky.
Could you help me out by answering these questions?
On the winter solstice, the noontime Sun will be 42 + 23.5 = 65.5 degrees from the zenith or 24.5 degrees from the horizon
The center of the rainbow is always in the direction opposite from the Sun, and the primary rainbow has a radius of 42 degrees (secondary, 54). Thus it is required that the Sun be appreciably closer to the horizon that 42 degrees. This does happen in the morning or evening, but in principle could also occur near noon in mid-winter, in which case the rainbow would be centered on north. Having an end of the rainbow pass north of you (its part near the horizon) is much more likely.
116. The unusual rotation of the planet Venus
"Venus is the only planet that rotates clockwise."
Is the above statement true? And if so, why? Why would Venus rotate in the opposite direction of all the other planets?
"Why" is harder--I am not sure that anyone knows for sure. The sense in which all planets in our solar system (and the Sun) rotate is presumably the same as that of the cloud from which the solar system condensed. When that happened, presumably many fragments collided to create each planet, and some average behavior of those collisions produced the rotation. One might even speculate that Mercury and Venus, the ones closest to the Sun once had large outer gas envelopes, and the way these evaporated in the Sun's heat may have contributed to the loss of rotation. That, though, is just a guess.
By the way, "clockwise" only has a meaning when you give a point of observation. You might write "clockwise when viewed from north." Viewed from south, the same rotation is counterclockwise. In the same manner, if you have a transparent clock and stand behind it, the handles will seem to move counterclockwise.
117. Why not use nuclear power for spaceflight?
ReplyNice idea. However, to fly in space takes rocket thrust, not just energy.
By Newton's laws, the forward momentum given to any rocket is always equal to the backward momentum given to the jet fired backwards. That momentum, in its turn, depends on two factors--how much mass is expelled by the jet, how many tons per second, and the speed with which it is expelled. Nuclear energy can supply the speed, but something must provide the expelled mass.
You might think next that given some source of mass (say, a tank filled with water), plentiful nuclear energy would make it possible to eject it much faster. But how? Rocket engines work by converting heat into directed motion, in a very efficient way, but they already run about as hot as available materials can stand. Nuclear energy could provide more heat, but no rocket engine could stand it.
Early in the space age a serious effort existed to build a nuclear rocket, getting its thrust by heating hydrogen with nuclear fission. A jet of hydrogen, coming from a rocket engine at a certain temperature, is much faster than a jet of burned rocket fuel, coming from a rocket engine at the same temperature. The reason is linked to the fact that hydrogen molecules are much lighter than those of any burned fuel.
However, the rate at which rocket engines used in spaceflight supply energy is enormous--e.g. the shuttle's engines burn a ton of fuel or more each second. The stresses are enormous, and the risk of nuclear material and waste products of fission getting into the atmosphere was too great, and so the project ended.
A visionary proposal of the 1950s proposed a "rocket" cabin with a strong flat plate on the bottom (oil would be sprayed on it for protection), and a trapdoor through which small nuclear bombs could be dropped, detonating some distance away and pushing the craft forward. On paper, it seemed feasible, but an actual nuclear test was deemed hazardous, sure to release contamination. The nuclear test-ban treaty of 1963 ended all efforts in this direction.
I should add here that a book has recently appeared about "Project Orion", by (what I take as) the son of Freeman Dyson, prime mover in that project: Project Orion: The True Story of the Atomic Spaceship George B. Dyson, George Dyson, David Sobel (Editor)
118. "Doesn't heat rise?"
I was really caught off guard with a question from the "peanut gallery." It
was - "Doesn't heat rise?" I said that that is correct, and conversely cold
would tend to remain near the ground. He further questioned - "If heat rises then
why wouldn't it get hotter as you increase your altitude? I had no
explanation. Can you help?
That heat arrives when sunlight hits the ground and warms it up. Think of what would happen if no way existed for removing it! The ground would get hotter and hotter--oceans, lakes and rivers would boil away, life would become impossible. Actually, we see rather little temperature change near the ground--just day-to-night fluctuations, and slow changes with weather and seasons. Such observations suggest that on the average, heat is removed just as fast as it is received.
Where can it go? Only one place--outer space, the sky above! We know that anything that is warm shines in some sort of light ("radiates"). A lightbulb filament is hot enough to shine in visible light, but a hot teapot (say) also shines (radiates). Our eye cannot see such "infra-red" light, produced (at a much lower rate) by moderately warm objects, but a hand held close to the pot will sense the radiation, as heat streaming out. (Rattlesnakes have special sensors to detect infra-red (IR) light, helping them find warm blooded prey.)
So at first sight, this appears to be a simple situation. The Sun shines on the ground and gives it heat, and the ground returns that heat to space as invisible infra-red radiation.
But this simple process is made complicated by the so-called "greenhouse effect." Air is relatively transparent, but some gases in it absorb and re-emit infra red very efficiently--water vapor, methane and increasingly, carbon dioxide, so called "greenhouse gases." Put a few drops of India ink in a glass of water, and it darkens appreciably; with these gases, similarly, a little bit of those gases ABSORBS A LOT. By absorbing and re-emitting IR, they make the IR light bounce around, rather than letting it head straight to space.
That makes it difficult for heat to escape, and keep the ground warm; something similar happens in a gardener's greenhouse, enclosed by glass panes, which let sunlight in but absorb IR. Such random bouncing-around continues until the IR (some of it, anyway) reaches a layer so high that not enough air and water vapor are left to send it back, and then the radiation escapes to space. That layer is known as the "tropopause" and it is typically 8 miles up.
This "greenhouse effect" would keep the air warm near the ground, even if our cars and power plants did not emit carbon dioxide (although those emissions make the effect more pronounced). The air then does a second thing to help getting rid of its heat: IT RISES.
Air cools as it rises, because air pressure around it is lower, and an expanding gas cools (that is how air-conditioners work). But because this process is happening everywhere and all the time, the SURROUNDING air is already cooler than the air near the ground. As long at air is warmer than its surroundings, it keeps rising. If a chunk of air started out extra-warm, it may well STILL BE warmer than the air around it, even higher up, so it keeps rising. Ideally, it rises until it arrives near the tropopause, where it can get rid of its heat. After that, being cooler, it sinks down again and is replaced by more heated air from below.
(Why does air pressure get lower as one rises? Because air near the ground is compressed by the weight of all the other air above it. If you rise about 5 kilometers--3 miles and a little bit more--half the air is below you, only half of it is above and contributes to the compression, and so, the pressure there is only half of what it was near the ground. Go up 5 more kilometers and the pressure is about half as much again--a quarter of what it is near the ground. That is where jetliners fly, and the low pressure is the reason their cabins are sealed and pressurized--also why climbers on Mt. Everest carry oxygen bottles).
This process of cooling, rising and finally radiating heat away is very important. THAT IS THE REASON WE HAVE WEATHER! The whole weather process is driven by the heat of the Sun, and by the collection of processes by which the Earth returns heat to space.
The above is very, very simplified, especially since it ignores humidity. Actual air also contains water vapor--water which was evaporated by the Sun and dissolved in the atmosphere, just as sugar dissolves in a cup of coffee. Since the Sun has provided heat energy for the evaporation, water vapor acts a bit like extra heat given to the air; when the water is removed as rain, air gets that heat back and is warmed, which is what drives thunderstorms. More about all these in
I am not sure, however, whether the role of water can be explained at the level of 3rd grade. Please let me know how all above explanations were received!
119. Have any changes been observed on the Moon?
I once had an office on the same floor as a lady scientist, Winnifred Cameron, who very much wanted to find such changes. She used a special viewing device looking at two pictures of a region on the Moon, taken under similar conditions but at different times, flipping from one to the other and looking to see if anything changed. I don't think anything ever did. She was particularly interested in observations of a Russian named Kozyrev, who claimed to see glows.
As for impacts, it is only possible to see pretty big ones. If meteorite impacts are your interest, read the chapter "The Shoemaker Comets" in "First Light" by Richard Preston. It's a great book. Gene Shoemaker is unfortunately gone from us, killed in a head-on collision while rounding a blind curve in Australia's outback. Those roads are usually completely empty, but you never know fate.
120. Why isn't our atmosphere flung off by the Earth's rotation?
I have wondered for years how the earth keeps our atmosphere. The equator moves at almost 1000 MPH and the atmosphere is fluid. The fact that there isn't any wind (to speak of, at least resulting from the earth's rotation) says that the attraction of gravity is stronger than the centrifugal force trying to throw it off. Do we know if the amount of atmosphere is increasing, decreasing or remaining the same? It just seems to me that there should be a lot of turbulence in the atmosphere/space boundary region, although the 'emptiness' of space probably can't provide any drag on the atmosphere.
If you went around Earth at orbital velocity--one circuit in 90 minutes--the centrifugal force would just balance gravity. Our rotation speed (one circuit in about 24 hours) is nowhere near that.
121. Can kinetic energy be reconverted to work?I read your article about energy from the following web site:
and have a question. Is kinetic energy available to do work later?
It depends. Kinetic energy is all too easily converted to heat by friction, and if this is allowed to happen, that energy is rarely recoverable. However, if you can convert it to another form, you can extract at least some useful work from it (there is always some friction loss). Examples:
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Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: audavstern("at" symbol)erols.com .
Last updated 9-17-2004