(The "snowballs" in those rings cannot stick together, they are too close to the planet and its gravity prevents them from sticking together to form big moons--although some small moons do exist there. Look up "Roche Limit" in a search engine.)

imprints on the Moon

before all their orbital eccentricity

has completely disappeared

The process which you propose

length of days

Seasons

not

inclination of the Earth's axis

plane of the Earth's motion

316. Forces on Comet-dwellers

Assume a habitable comet in a highly eccentric orbit. If you were living on it, would you feel acceleration as it sped up close to the sun? My thoughts:

(i) No. the comet and you are just following curved spacetime. If there was a feeling of acceleration - would it not mean that the comet would break up? We don't feel acceleration as we orbit the sun (nor do astronauts in the space station orbiting the earth). Maybe we do feel it but we are so used to it that we can no longer notice it?

(ii) Yes. There is definitely acceleration - tangential as well as radial. If you have a large change in speed, how could you not feel it?

Help! ;)

Reply

You stand on the comet, somewhere far from the Sun, and let go of an apple. Ignoring now the feeble gravity of the comet itself, the apple would move in the same orbit and therefore just hang there. It will not "feel" any force impelling it to move to another location. And neither would you (if the icy chill, so far from the Sun, lets you feel anything).

Next suppose the same comet is near perihelion, following a hairpin orbit around the Sun. Sure, you are undergoing a large acceleration, moving you in a curved path. But so does the apple. If both of you sense the same force, you will maintain your relative positions, and so would all parts of your body. It will feel the same as no acceleration at all.

But wait! The apple may experience a different force (per unit mass) than the comet, if it is closer to the sun than the comet's center of gravity--or if it is on the side away from the sun, at greater distance. That difference, in fact, could even break up the comet itself, as has been observed (with Biela's comet). For more on that, look into the

gravity gradient

317. Nuclear reactors and bombs

Why does a nuclear reactor require a moderator to slow the neutrons down, but a nuclear bomb does not?

Also: Is the uranium in a nuclear bomb doped with an element that emits neutrons - in order to start the chain reaction? Or are there enough neutrons just flying around to trigger it (assuming that critical mass has been reached)?

Reply

A nuclear reactor uses slowed-down ("thermal") neutrons, while a nuclear bomb uses "fast" neutrons, emerging right after fission has occurred. The large spacing between "fuel rods" in a reactor makes sure it cannot explode like a bomb--though it may undergo "meltdown" if the nuclear reaction becomes uncontrolled (not supposed to happen, but a few times it has).

Slow neutrons are efficient in causing fission in U-235, but any such fission produces mainly fast neutrons, and if there is U-238 around, these are likely to be captured by it first (leading later to plutonium production). In a nuclear reactor, almost all these fast neutrons escape the fuel rod and wander around surrounding material, of a kind chosen not to absorb them well (such as "heavy water" or pure carbon), losing energy until they are slow enough to evade absorption. Then, if they re-enter a fuel rod, they can fission another U-235 atom. For these reasons, nuclear reactors can tolerate a fair amount of unenriched uranium containing mostly U-238, and the original "pile" of Enrico Fermi in 1942 used natural uranium with only 0.72% U-235. See

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

Bombs

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

318. Why doesn't magnetism affect electro-magnetic waves?

I'm a 6th form student in the UK and it suddenly occurred to me while I was revising what we had learnt on waves and magnetic fields that, if EM waves, are indeed electro-magnetic waves, why is it that they are not affected by magnetic fields? And why do such waves not display the properties of magnetic fields in the fact that they loop from pole to pole?

Forgive me if this is a rather naive question, I do understand the principals behind EM radiation and waves quite well, its wave-particle nature, thus allowing superposition etc., but I'm just curious as to how there is such a distinction between an electromagnetic wave and a magnetic field. If one consists of the other then how is it not affected by the other, and how is it EM radiation can travel in a straight line for nearly infinite distances (gravity aside)?

Reply

Electromagnetic waves are linear--when several are added together, each preserves its identity and can be separated again. On the radio, or on TV, many stations can send their signals through the same region of space, and yet your receiver can pick out any of them and amplify it alone.

You could regard a steady magnetic field as a signal of zero frequency (taking forever to switch to its electric signature). It does not interact with any electromagnetic waves of non-zero frequency, at least in vacuum.

In a material medium the magnetic field may modify the electromagnetic properties of the medium, affecting the propagation of waves. For instance, the Faraday effect in transparent media may rotate the plane of polarization of an electromagnetic wave.

In a plasma (gas containing freely floating ions and electrons) many different kinds of modified electromagnetic waves are possible--depending on the frequency of the wave and how far it is from some characteristic resonances, which depend on density and magnetic field in the plasma. In particular, "Whistler Waves" (in space near Earth they are produced by lightning at frequencies like 3000 Hz) are indeed guided by field lines, sometimes bouncing back and forth from one hemisphere to the other.

319. Is humanity changing the climate, or is it the Sun and the Earth's magnetism?

Can you comment how climate change is affected by the earth's magnetic field, the sun's output, and other directly and not so directly related factors?

Where I can read about such theories? I have tried to understand the common reasoning about man being directly responsible for (however you label it) Global Warming, Climate Shift, etc. However, based on the sheer magnitude of such changes, I find that man-made causes are the most dubious direct factors. Among the other far larger factors, man's impact adds at most an assist or a catalyst to the equation.

It would seem to me that the earth's magnetic field might have a huge impact on how the sun's various and changing outputs are directed and distributed around our globe. Changes in the sun's output and changes in the earth's magnetic field seem to greatly affect global changes, with no input from mankind needed.

However, I have heard very little discussion of this highly suspect aspect of the discussion. It is almost as if the science of the sun and the earth, in areas where you appear to be an expert, have been curiously absent from the debate. Is this due to political pressure? Has the objective process of true scientific discovery started to buckle under the pressure of political correctness? Al Gore and Ponzi scheme carbon credits be damned!

Reply

I do not think magnetism has any connection to global warming. There is too little energy in the solar wind or its magnetic field, compared to the flood of energy we receive in the form of heat from sunlight.

Global warming is caused by molecules in the atmosphere, as described in

http://www.phy6.org/stargaze/Sun1lite.htm

Earth radiates its heat

Molecules

absorb and emit infra-red

carbon dioxide, methane and ozone

heat output from the Sun

320. Advice to home-schooling parent

In the process of searching for an Astronomy curriculum for my home-schooled high school senior, I came across your website.

I am interested in getting this curriculum for my daughter. Will she be able to complete the course work online? Or should I purchase the CD as well? I am not a very "technical" individual, but my daughter is! She desires to become an astrophysicist. I'm confident she'll achieve her goal because she has the passion for it as well as the understanding in math and science. Thank you for your help.

Could you please also provide a working link to the poems listed on your website, e.g. those by Bialik? I was not able to access them using the provided link.

Reply

The central linking page is

http://www.phy6.org/readfirst.htm

http://www.phy6.org/outreach/outreach.htm

April Dykes

readfirst

solutions to problems of "Stargazers,"

(but readers of this shouldn't expect them here!)

far too big

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

YOU are the teacher.

you

46 lesson plans

questions and answers

math course

timeline

Spanish

http://www.phy6.org/stargaze/StarFAQ6.htm#q100

321. Science of Clothing

Have any ideas about how to introduce some of the concepts which you discuss to 6th graders -- girls in particular? Any advice you can provide will be greatly appreciated. Thanks so much for sharing your knowledge on the internet.

(Later additional message) What I'm most interested in is the science of clothing: how can I apply angles, force, friction, color theory perhaps? All this, to show girls that there is science behind fashion.

Reply

There is science behind clothing, but it is mostly chemistry (dyes, fabrics...) and I happen to be a physicist. The physics involved is mainly related to heat insulation: why is a fluffy fabric so much better as a barrier to loss or entry of heat than a solid heavy material--bricks or metal, say? It has to do with the trapping of air and stopping it from circulating; air is a poor conductor of heat, but it can transport heat by circulating (

as it does in the atmosphere

Wool and feathers

cotton

http://www.phy6.org/stargaze/Sun4spec.htm

orange

whole lore

Tyrian purple, mauve (first artificial dye), saffron, khaki, indigo

would rather use clothing to teach language

Muslin

Calico

tabby

Damask

seersucker, pajama, dungaree, denim

distaff

hose

Marco Polo

322. Calculating a Collision

I saw your website and thought you have great answers to many questions. I hope you can help me figure out what I know is a probably a basic physics problem. I could be doing this wrong but I'd like to know. There was a story I read recently about a 1,500 lb wrecking ball that came loose, rolled downhill, and smashed into a car moving it 20 feet. The car weighs about 2,400 lbs, I would imagine. I was trying to formulate a way to calculate how fast the ball must have been rolling, using "slugs."
(A detailed calculaion followed).

Is this right?:

Reply

Your answer is probably not the right one, and in fact the problem cannot be solved without added information about the roughness of the surface. The same force will push the car much further if it is standing on smooth ice then in a plowed field!

You need to know the coefficient of friction k (my notation here, many tests use μ), the ratio (in this case) between the (horizontal) force of friction on a moving object and the (vertical) weight causing that friction. It is an observed fact that k is almost independent of speed, so whether you move fast or slowly, the friction force is the same. We assume so here--without doing so, the problem becomes even more complicated and more assumptions need to be made.

The value of k can vary anywhere from about 0.02 (car on smooth road, axles greased and turning freely, tires well inflated) to 1.00 (car with brakes applied tightly, to where it would not slide even on a 45-degree slope). Let me assume here k=0.2.

You must also assume something about the collision--most likely, an inelastic collision, in which the ball smashes into the car, after which the two travel together.

I am not familiar with calculation in slugs and feet--most of the world (especially technical people) uses the metric system, so excuse me if I solve a slightly different problem--a 700 kg ball hitting a car weighing 1100 kg, moving it 6.1 meters.

Say the ball started with velocity v, and after impact, car and ball continued together with velocity u. By the conservation of momentum (

section #18b

700 v = (700 + 1100) u = 1800 u

0.5 m u² = 0.5 (1800) u² = 900 u²

F' = mg

section #18

F' = 1800 . 9.8 = 17,640

F = 0.2 F' = 3528

section #18c

W = 21520.8 joule = 900 u²

u² = 23.912

u = 4.89

v = (1800/700) 4.89 = 12.57

Rsponse

323. The Coriolis force and more

On your website you state that the Coriolis force is too small to affect the rotation of water down a toilet, yet the water does flow in opposite directions in the Northern Hemisphere as compared with the Southern Hemisphere. What else would explain this phenomena?

The Coriolis acceleration I learned in school from Prof Wills at CCNY is described as a vector equation as follows:

A_c = 2 ω x V

where ω is the vector describing the angular velocity of the rotating earth, approx 70 micro-radians per second, V is the velocity vector and A_c is the coriolis acceleration. It is approximately 0.004 feet/second squared, which is small but perhaps large enough to start water flowing in different directions down a smooth toilet bowl.

I enjoy reading your web site just as I enjoyed reading the books of George Gamow in the 50s.

Why are all planetary bodies planets, stars, etc. round? Asteroids may be pear shaped but most bodies are round, why are some not shaped like a disk? Why does gravity exist at all? Did Einstein ever discuss that issue?

Reply

Your formula is correct, assuming V is the velocity vector in the rotating frame of reference (velocity in addition to the one resulting from being carried around by the Earth's rotation itself) and x denotes vector multiplication, which also depends on the sine of the angle between the direction of motion (vertical, for draining water) and the Earth's axis.

However, the rotation induced by the Coriolis force F = m A_c depends not just on F. It also depends on the variation of F between the two sides of the descending or rising fluid. If F is the same on both sides, the entire motion just gets deflected to one side. Rotation is produced because F is smaller on the point closer to the pole. Near the pole omega is almost parallel to V, making 2(ω x V) very small.

That is why hurricanes are affected--they are much bigger--and draining sinks are not. In Jupiter's red spot, which is still bigger, the effect is even more pronounced--and Jupiter also rotates faster than Earth. In fact Voyager's pictures of Jupiter show many additional swirls in its atmosphere.

Why are planetary bodies round? You would expect gravity to make them so, if they were all liquid or gas--any bump rising on the surface would be pulled down to its level. In a big planet like Earth, the force of gravity is strong enough to force the material into the shape of a sphere. Rocks on the surface can vary in elevation, but at a depth of 100 km (say), the weight of layers piled up above makes even rocks adjust their level as if they were fluid.

That is the reason no mountain on Earth reaches even 10 km: the weight of mountains pulls them down. On Mars, the highest volcano can rise to about 25 km, because gravity is weaker. Asteroids and moons up to about 500 km can maintain irregular shapes, but if they are larger, gravity causes them to become round. See also

question 305

reference 26 on the list on top

why does gravity exist?

http://www.phy6.org/stargaze/Sun4Adop3.htm

324. Why isn't the solar system stratified by density?

I'm a law student (sigh) from Germany, and I spend way too much time thinking about physics

A question has been nagging me some time now.

I've been thinking through the formation of the solar system: ok, so we have this huge gaseous cloud compressed into a disk by it's own gravitational pull. Seeing that the disk is still rotating I assume a centrifugal force to be exerted on it which leads the elements it is comprised of to distribute according to their specific mass, lightest at the core, heaviest at the edges. Next step, they clump into pieces. OK, so far so good, plenty of hydrogen at the core, massive planets following and then the asteroid belts that for some funny reason don't want to stick together (maybe the pull of nearby Jupiter disrupted their clumping).

BUT THEN: the gas giants. I mean. Excuse me? Is my thinking that flawed? Isn't that gas supposed to be in the sun to give us a few more years of gentle warmth? Hydrogen and helium? (I imagine some kind of dialogue of the type "hey, what is such a cute and especially light couple doing out here at this time?") Could it be possible that it's the hydrogen scooped up from deep space AFTER the formation of the core solar system? I just don't get it. Where did I go wrong? What did I miss?

Reply

You are asking intelligent questions, and it is not easy to answer them, especially without mathematics. If you go on with your studies, I wonder if you end up practicing patent law, where such questions sometimes arise.

Why doesn't the solar system separate--light stuff in the middle, heavy stuff outside? Let's look at a situation where this DOES happen: a very tall glass (tall enough so you need not worry about friction with the bottom) filled with a mixture of gasoline and water, and stirred into rotation by a long spoon, or maybe a paddle attached to a motor.

You know what will happen: the water which is heavier will gradually move outwards, and the gasoline which is lighter will be near the middle. In the rotating frame of the fluid, every bit of matter experiences an outwards-directed centrifugal force (sections #23 and #23a in "Stargazers"). If two equal globules rotate side by side, one of gasoline and one of water, the one of water is denser and senses the greater force, so it moves out while pushing the lighter gasoline inwards.

Now replace this by a solar nebula, rotating because of some systematic average motion it had when it started to pull together. No spoon or paddle is necessary--it just rotated when it came together, and there is nothing to stop it. The centrifugal force may be said to be balanced by gravity (or else, looking from a non-rotating frame, the centripetal force is supplied by gravity). If some material moves out, it slows down, but has then larger potential energy, so it regains its speed as is moves in again--as, say, Halley's comet does (see

section on Kepler's 2nd law, section dealing with energy

in this motion, materials do not separate

At early times

question 315

Earth and Venus are lucky to keep their atmospheres

Go to main list of questions (by topic)

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Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: stargaze("at" symbol)phy6.org .

Last updated 27 March 2007

Get a Straight Answer

Please note!

Items covered:

305. About mountains

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306a. "Will the World end in 2012?"

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306b. Another question about the year 2012

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307. Advice to graduating High School Student

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A similar letter, received earlier:

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308. Could a (heat resistant!) ship float on the Sun?

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309. Reducing the fuel weight of the Space Shuttle?

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310. How do Rockets Land?

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311. The Earth's Spin reduced by Global Warming

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312. Circumnavigation of the Sun

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Response

Answer to the response

313. Are nuclear forces merely gravity at very close distance?

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314. Changing the Earth's Rotation

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315. Why are planetary orbits eccentric?

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316. Forces on Comet-dwellers

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317. Nuclear reactors and bombs

Reply

318. Why doesn't magnetism affect electro-magnetic waves?

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319. Is humanity changing the climate, or is it the Sun and the Earth's magnetism?

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320. Advice to home-schooling parent

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321. Science of Clothing

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322. Calculating a Collision

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323. The Coriolis force and more

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324. Why isn't the solar system stratified by density?

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