h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

i feel like stumping my physics teacher on monday, give me "THE" hardest physics question you can think of, and an answer so i can feel special and smart

um, hello? this is the longest ive ever had to wait for an answer from the comunity, i mainly posted this message so that my question will got the top of the list and more people will see it....hopefully

Don't panic, 50 minutes isn't that long! h2g2's often a bit slow on Saturdays

I'm afraid I know pretty much nothing about Physics.

darn

It's not quite what you mean probably, but a recent quiz night featured the question "Which of these items would work on the Dark Side of the Moon?" ("work" was further qualified as "fulfilling the purpose for which they're normally intended")
The items were : a kettle, a solar panel, a magnetic compass, a pendulum clock and a distress flare.
The debate was, shall we say, interesting.
Suggestions, anyone?

How long is the lead for the kettle?

Liam.

Okay, a hard physics question... explain why metals are highly reflective at optical wavelengths (i.e. shiny ).

Not quite sure what the answer is, I just nicked it from a final year exam paper ... I imagine you can find it if you look hard enough on the internet, though...

Does water boil at a higher or a lower temperature as the pressure decreases... I can never remember. If it's higher, then an ordinary kettle might not be able to boil it. If it's lower then it would boil much more quickly... assuming that lead is long enough

The solar panel would work for half of the Moon's orbit - the half when the Moon is in its new phase as seen from the earth, and the 'dark' side is lit by the Sun.

As far as I know, the Moon has a magnetic field, so I guess a compass should work, although the needle may not point to what we'd call 'north'.

A pendulum relies on gravity, so it would work if the clock was geared to allow for the lower gravity of the Moon.

If it's an ordinary distress flare which burns oxygen, then it wouldn't work. If it's the kind of flare which divers use underwater and which burns without oxygen, then it would.

How'd I do Pinni?

The kettle will work if you have a suitable power source for it - say if you had an exercise bike wired up to a dynamo and pedalled hard enough

The solar panel won't be much good on the DARK Side of the Moon. Even if the Earth were up, you wouldn't get much juice.

The distress flare, not too sure about that, surely if there's no air, it won't burn? Or is the oxidiser contained inside the flare?

The magnetic compass won't be much good as the Moon doesn't have an overall magnetic field. See here http://www.berkeley.edu/news/berkeleyan/1998/0909/moon.html So your compass would work occasionally but not be consistent.

The pendulum clock would work fine, except that because of the Moon's lower gravitation, the pendulum would have a longer period. How much longer? The Moon's gravity is 1/6th of Earth's and pendulum period varies as 1/root(g) so a 1s pendulum on Earth would be a 2.45s pendulum on the Moon.

Hope that's right!

Water boils at a lower temperature when atmospheric pressure is low. The kettle would work, but it won't make you a nice hot

The solar panel would work while the moon is between the earth and the sun (assuming that by the dark side you mean the far side that can't be seen from earth). The compass wouldn't work. The pendulum clock would work, but at the wrong speed unless it was recalibrated.
The flare would need its own oxidant - don't know if they normally do, though.

Noggin

You want the answers? These seemed to be the consensus on the night (and who am I to argue?)

1. A kettle DOESN'T work. A kettle is for making water hot, to infuse , cook stuff, warm up your bath etc. With no atmosphere, water boils off instantly at ambient temperature. So although water will indeed boil in your kettle, it serves no useful purpose, lead or not. (There are no power sockets on the Moon, BTW, just in case anyone's thinking of going...)

2. A solar panel DOES work, actually slightly better than on earth, because there's never any cloud-cover. It still won't work when the sun sets, which happens every 24 hours and persists for ~12 depending on latitude, just like at home, since the Moon is in captured rotation about the Earth. The point here is that the Dark Side of the Moon is not permanently dark - just hidden from the view of the Earth.

3. A magnetic compass DOESN'T work. The Moon has no well-ordered magnetic field, at least not one that has a uniform orientation over the whole surface or indeed (so far as is known) over any extensive area. So you couldn't take a bearing with a compass. The point was also made that you'd hardly need to resort to this method. Characteristic stars are always visible. At the Sea of Tranquility Apollo 11 site, the (weak) magnetic field detected was almost exactly perpendicular to the lunar surface - you don't really need a compass to find up, not even at one-sixth gravity.

4. ...bringing us to the pendulum clock, which DOES work, but runs six times slower than the same clock would on Earth, because of weaker gravity. No matter, it still keeps time, and you could easily set it right (for the 24 hour cycle you still experience) by lengthening the pendulum. No one could remember the formula, of course, but there is one. Honest.

5. And finally the distress flare. It DOESN'T work. The points about the combustion mechanism given above are correct, but academic. We are on the Dark Side of the Moon. No-one will see it, and a quick response to a crisis is impossible. If you really needed to launch it, you're doomed, I'm afraid.

...Quite fun, I thought. Not a lot of Physics, though. Maybe that's WHY it's fun...

Pin (I'm allowed to say that - I'm a Physics graduate)

Well, I was on the right track with most of them

That one about the flare is something of a red herring though doncha think? If there's anyone else nearby (you're hardly likely to be there on your own, especially if you're an Apollo astronaut), they could see it if they were looking, if conditions were dark enough, and if it was a chemical combustion flare which didn't need ambient oxygen. If there was no-one to see a distress flare, you wouldn't need a distress flare, so you prolly wouldn't take one to the Moon

An addition to the kettle. If it is in the shade, it will be close to absolute zero, and any water will freeze long before it can boil. Heating it in vacuum will skip the liquid fase and turn it bit by bit into vapor. So you eat your with a chainsaw and fork

I would dispute a couple of your points (for no other reason than because I like contradicting people )

Firstly, the Dark Side of the Moon and the Far Side of the Moon are different. The Dark Side of the Moon is just that. Dark. Except when the Earth is up - earthlight is somewhat brighter than moonlight but still a lot less bright than sunlight. At New Moon the only side of the Moon we see *is* the dark side. At that time the Far Side is the light side.

Secondly the period of a pendulum will be root 6 (about 2.5) times longer. T = 2 pi (l/g)^(1/2) I worked it out myself, because I couldn't remember!!

Can the *star wars* defense shield the US wants so badly, WORK?

Actually, no.

The easiest circumvention would be:
load a weapon of mass desstruction on a ship, sail it to New York, or any other large place with a harbor and explode it.

Desparate people are showing daily that they would be glad to die that way.

BTW, I sincerely hope it will never happen. Killing innocents is never the right answer.

I'm sure you're all right. Like I said, who am I to argue?
Pin

re posting 7 [explain why metals are highly reflective at optical wavelengths (i.e. shiny)]:

the short answer is: electron mobility

the long one is:
light is just one variety of electromagnetic waves which consist of oscillating electric and magnetic fields which are coupled with each other through Maxwell's equations . The E-field causes the electrons within the metal to wiggle up and down (or left and right), which is an easy job because of the low resistance, or high electron mobility in a metal. These wiggling electrons, in turn, cause a new e/m wave which is radiated from the surface and carries the energy away from it (that's what 'reflection' is all about). The whole thing doesn't depend on wavelength, optical or other; thus a metal mirror is a mirror for radio and microwaves too.


good luck for Monday!

As far as I'm aware, Electric Dragon, the Dark Side of the Moon *is* the far side... by definition. It just happens to be a screwy definition

Whats the answer to the theory of everything?