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114. Why not use a heat shield going up?

Reply

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?

This may be a very unusual request but I need to know the sun's angular elevation at noon , winter solstice, at 42 degrees N latitude. Not necessary but curious same for summer solstice.

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?

Reply

Actually, the problem you ask about is fully discussed on my page

http://www.phy6.org/stargaze/Snavigat.htm

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
On the summer solstice, the noontime Sun will be 42 - 23.5 = 18.5.5 degrees from the zenith or 71.5 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

I am a university student who was asked to research some common legends to determine if they are true. One of the "facts" which I am supposed to verify is:

"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?

Reply

The statement is true. You my look in Bill Arnett's "Nine Planets" at the page on Venus

http://seds.lpl.arizona.edu/nineplanets/nineplanets/venus.html

"Venus' rotation is somewhat unusual in that it is both very slow (243 Earth days per Venus day, slightly longer than Venus' year) and retrograde. In addition, the periods of Venus' rotation and of its orbit are synchronized such that it always presents the same face toward Earth when the two planets are at their closest approach. Whether this is a resonance effect or merely a coincidence is not known."

"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?

Reply

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.
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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 helping my wife in her 3rd grade class yesterday and one of the students came up with an interesting question. We were talking about how it gets cooler as you increase your altitude (specifically in the mountains). I was guessing that the temperature drops about 3 degrees for every 1000 ft that you climb.

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?

Reply

The answer below is tailored (I hope!) to the 3rd grade level, not an easy task.
-----------------------------
It is true--the atmosphere is hot at its bottom and cool higher up (at least for the first 10 miles or so). It is true even though, as we know, hot air tends to rise! It all happens because THE BOTTOM OF THE ATMOSPHERE IS WHERE AIR RECEIVES ITS HEAT.

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

http://www.phy6.org/stargaze/Sweather1.htm

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?

Reply

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.

Reply

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:

http://www.phy6.org /stargaze/Senergy.htm

and have a question. Is kinetic energy available to do work later?

Reply

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:

You zoom on your bicycle down a valley and gain kinetic energy. That energy can help you rise again on the upslope on the other side. Rising against gravity is doing work.
Your car has to stop at a red traffic light. If it is an ordinary car, you press the brake pedal, which pushes brake pads against wheel disks or drums. Friction converts the kinetic energy into heating of the disks or drums, but you can hardly convert that to work.
If however your car is of the new "hybrid" type with electric motors on the wheels (like the Toyota "Prius" or the Honda hybrid), by braking you connect the motors to the car's batteries. The motors act as generators and charge the batteries, turning your kinetic energy into chemical energy of the battery, which can be reused.
The space shuttle in orbit has a tremendous kinetic energy. Upon re-entry, its orbit is nudged into the atmosphere, the shuttle turns so the underside of its wings (covered with heat tiles) faces forward, and air resistance, helped by a shock, converts the energy into heating the air. Not much can be done with that.
If however you link the shuttle to a conducting tether, as was done once (with some problems), the motion of the tether across the Earth's magnetic field lines creates a voltage, which may be tapped, e.g. for charging batteries. See http://www.phy6.org/Education/wtether.html.
Accelerators of high-energy particles used to depend on pulsed magnets. I remember visiting one such accelerator around 1960, the "Cosmotron" in Brookhaven, Long Island. Its magnet needed a large electric current, which created a great amount of magnetic energy. It made little sense to waste that energy every time the magnetic field was allowed to decay.
The solution was a large flywheel, connected to the generator providing the current. When the current decreased, the generator acted as a motor (a bit like that of the hybrid car) and spun up a flywheel weighing a few tons. The next cycle, the flywheel provided most of the energy for generating the magnet's current, slowing down again; only a little extra power was needed to make good friction losses. Thus the energy bounced back and forth between magnetic and kinetic.
Someone pointed out to me that the way the flywheel rotated was carefully chosen. In case the flywheel's bearings somehow gave way, its rotation was such that it would roll through the wall of the building and out into the field--not in the opposite direction, which would have brought it into the crowded accelerator hall.
I hope you get the idea by now.

122. Does any location get the same number of sunshine hours per year?

Is the period of light and dark the same for every place on earth over the course of a year? Example: Do Nome, AK and Rome, Italy get the same number of hours of light and dark per year (ignoring intensity differences).

Big question in our family recently!!
Thank You !!

Reply

A good question--and congratulations for having a family with such wide-ranging interests! Intuition seems to say "yes", but the challenge is to demonstrate it without any calculations. Below is one try.

Certain approximations are necessary. You ignore the actual size of the Sun's disk--but count as "light" times when the center of the Sun is above the horizon and as "dark" times when it is below the horizon. Also we ignore the ellipticity of the Earth's orbit, which causes the speed of the Earth's motion around the Sun to vary, but assume that it moves at constant speed. (About that variation, see Skepl2A.htm.

With those assumptions the Sun, too, seems to move with constant speed around the ecliptic over the course of the year. For the purpose of deriving average sunshine over the year, we can therefore replace it by an "average Sun" spread out evenly around the ecliptic. The question then becomes does this "average Sun" give equal hours of light and darkness to any point one Earth.

Assume first that the Earth DOES NOT rotate, so that your position--Rome, Nome or Pennsylvania--does not move. Your view from that location is then determined by your horizon, which in turn is determined by the plane tangential to the surface of Earth wherever you stand. You can call this "the plane of the horizon." Anything above that plane--exactly half the celestial sphere--is visible, anything below it is not. (It may be useful for you at this point to fetch paper and pen.)

The plane of the horizon cuts the plane of the ecliptic along SOME line of intersection, passing through wherever YOU are standing. In the plane of the ecliptic, the "average Sun" is a circle around your position, and any line through its center (=your place) is a diameter. This includes the above line of intersection. That line therefore cuts the "average Sun" into two equal halves--one above the horizon (which you see), the other below the horizon (which you don't see).

Over the year the Sun occupies equally every little bit of the circle. So half the time you see it, half you don't.

But the Earth rotates, you say. True, and this causes you to occupy not one position on the sphere representing the Earth, but many--an entire circle around the axis. But ANY point on that trip (performed daily) obeys the "half-and-half" rule. So it also holds for any place on the rotating Earth.

123. Speed of toy car rolling off an inclined ramp

Reply

Every object has a WEIGHT, the force by which gravity pulls it down. A big stone has more weight than a small one. Weight is one way of measuring the amount of material in the stone, or its "mass." A big stone has much more mass than a small one.

However, if you drop them together, you will find that they fall equally fast!! This is, because each object also resists motion, and the resistance (called "inertia") is ALSO proportional to mass. That is why on a horizontal surface it takes much more force to get a bowling ball rolling than a tennis ball. The motion is horizontal, gravity is not too much involved, but the bowling ball has more mass and therefore much more resistance to being set in motion.

Say the big stone has 10 times the mass of the small one. It also has 10 times the inertia, and that inertia does not allow it to move any faster, even when the Earth pulls it down 10 times as strongly.

Model cars on a ramp (I suppose they are moved by gravity, like soap-box racers) obey the same rules. A heavy car is pulled more strongly, but also has more inertia, so the two should roll at the same speed. Try it! Put two toy cars--big and small--together on a slanting board, and let go. All other things being equal, they should move together.

To make the car roll faster, the only thing you can do is reduce friction (and at very high speed, air resistance). A car with well-oiled axles may move faster. Try it!

124. Acceleration due to gravity

What are values of acceleration due to gravity at different locations on Earth and what is the value closest to my school?

If you could help me out because I am really interested in finding out the answer, I would be greatly appreciative.

Reply

I am not really supposed to do your work--but look at http://www.haverford.edu/educ/knight-booklet/accelarator.htm Actually, the interesting questions are not "what is the number" but "why does gravity vary from place to place?" and "how do we know?"

It varies because the Earth rotates, adding a centrifugal force to the forces felt locally. That does two things: it makes the Earth bulge at the equator, so that points there are more distant from the center of Earth. And it adds there a force opposing gravity, so that the acceleration is smaller. Newton proposed that around 1690.

The process was experimentally studied by comparing the time kept by pendulum clocks ("grandfather clocks") at different locations. The period of the pendulum depends on gravity, and a clock which keeps correct time in Pennsylvania will probably run slow at the equator.

Response

You actually were a great help for me. You explained the subject better than my teacher. I hope you continue this because physics and other sciences really interest me.

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Last updated 9-17-2004