Daft sciencey type questions

[seren]

This is mine:

WHY (and/or HOW) does bicarbonate of soda relieve itching skin?

[skbuncks]

Didnt realise it did. Source please, or application which contains it.
In aqueous solutions it is a mild base (pH~8ish), and on reaction with an acid will liberate CO2, cant see how this would help though.

skb

[Trinoc]

Possibly something to do with the fact that bicarb is one of those odd compounds that can neutralise both acids and alkalis, so if the itching is caused by either of these then bicarb is likely to help it.

When bicarb encounters an acid it releases CO2 and becomes an alkali, forming the sodium salt of the acid. When it encounters an alkali it behaves as a weak acid, exchanging its hydrogen ion for an ion of whatever metal the alkali is made from, producing, say, sodium potassium carbonate.

A bottle of bicarb solution is always handy to have around when messing with chemicals. If you drop something corrosive on your hand you can pour bicarb on it without having to think first about whether what you spilled was acid or alkali.

This also means bicarb is good to rub into insect stings, and it doesn't matter whether the sting is acid or alkali.

[niggle]

My favourite from The Last Word:

Why do mirrors reverse left and right but not up and down?

[skbuncks]

My favourite from The Last Word:

Why do mirrors reverse left and right but not up and down?

[pedant mode] Concave mirrors will invert the image for objects further away than the focal point (S>F) [/pedant mode]

skb

ETA: I refuse to answer the actual question as it would cause my brain to explode from the stupid

[Mulder]

My favourite from The Last Word:

Why do mirrors reverse left and right but not up and down?

Because they are a flat surface and not a point ...

[Trinoc]

My favourite from The Last Word:

Why do mirrors reverse left and right but not up and down?

They don't. They reverse front and back. It is only our conventional assumption about the location of left and right on a human body that leads us to think left and right have been reversed.

[seren]

When bicarb encounters an acid it releases CO2 and becomes an alkali, forming the sodium salt of the acid. When it encounters an alkali it behaves as a weak acid, exchanging its hydrogen ion for an ion of whatever metal the alkali is made from, producing, say, sodium potassium carbonate.

Fascinating stuff, thank you! As for my source: my doctor!

My next question: please explain the cause of the Doppler effect, and (this is really the question I want answered) which is the real pitch of the siren/sound? The one you hear first, or the lower pitch heard afterwards?

Thank you!

[niggle]

I'll have a go if you don't mind.

Sound is waves in the air which our ears pick up and trasmit to our brains. The waves travel outwards from their source. So if that source - say, a siren - is moving, it is moving "towards" part of the waves and "away" from another part, as shown here, with the source mving towards the left:



You can see that the waves are smaller or narrower at the left and wider at the right. The pitch or "note" of the sound we hear is determined by its frequency - that is, the distance between the waves. So as the source approaches us we get the narrower waves we see at the left of the pic, which sounds higher to us, then once it passes us we get the wider waves at the right of the pic which sounds lower.

Neither pitch is the actual sound of the source - you can only get that if the source is stationary or it runs you over, giving you an instant of correct pitch.

This is a simplified explanation, but I hope it helps!

[seren]

Thank you.
So presumably this effect happens to some degree with any moving sound-emitting thing, but natural things don't move fast enough for it to make an audible difference?

Simple version of what I'm asking: is the speed of the sound-emitting thing a factor in this?

[Tomolac]

it more like waves on a beach, if your moving thought the waves they seem to be more or less frequent depending on your direction even though the frequency and speed of the waves remains constant, if your moving out towards the sea the waves will seems very close together. its the same effect with light and radio.

Thank you.
So presumably this effect happens to some degree with any moving sound-emitting thing, but natural things don't move fast enough for it to make an audible difference?

Simple version of what I'm asking: is the speed of the sound-emitting thing a factor in this?

its the relative speed between the emitter and observer, and when it comes to very small movements our ears simply are not sensitive enough to detect the very slight changes.

[niggle]

Here's another fun one if you're bored:

What's the largest number?

Well it's infinity, right? Obviously. But here's a problem: how many positive whole numbers are there? Well, there are an infinite number, we all know that.

Now how many positive whole numbers plus and minus a third are there - one and a third, one and two thirds, two and a third, two and two thirds and so on? Well there are an infinite number of those too. But you can quickly see that there are twice as many of those numbers as there are whole numbers.

In other words, there are lots of different infinities and some are bigger than others. In fact the largest infinity of all may well be the number of different infinities that exist!

[Mongrel]

Here's another fun one if you're bored:

What's the largest number?

Well it's infinity, right?

Not really.
Unless your working with it mathematically infinity is more of a concept, not an actual number unintuitive, as that may be.

Summed up nicely by Hilbert's Hotel paradox

[Matt]

Here's another fun one if you're bored:

What's the largest number?

Well it's infinity, right? Obviously. But here's a problem: how many positive whole numbers are there? Well, there are an infinite number, we all know that.

Now how many positive whole numbers plus and minus a third are there - one and a third, one and two thirds, two and a third, two and two thirds and so on? Well there are an infinite number of those too. But you can quickly see that there are twice as many of those numbers as there are whole numbers.

In other words, there are lots of different infinities and some are bigger than others. In fact the largest infinity of all may well be the number of different infinities that exist!

Infinity is not a number. There are not twice as many of "those numbers" as postive whole numbers. That is to say that they are both countable sets. However there are other infinite sets that have greater cardinality but that's not to say they are any larger, after all infinite means that you can't get larger. Add a member to an infinite set and it's still and inifite set, double the members of an infinite set and it's still an infinite set. Spawn a unique infinite set from each member of another infinite set and take the union of all sets so created and you still get an infinite set though this time it MAY be of higher cardinality, just not any larger.

[Trinoc]

In other words, there are lots of different infinities and some are bigger than others. In fact the largest infinity of all may well be the number of different infinities that exist!

There are lots of different infinities of different sizes - an infinite number of them as you say - but infinities like integers plus or minus a third, or even all of the rational numbers, are the same size as the infinity of integers. You can prove this easily by devising a one-to-one correspondence between the numbers of these types and the integers. The number of real numbers, including irrationals (or even just the irrationals), is a larger infinity, though.

As far as I know, the number of definable infinities is a countable infinity, since each infinity is 2 (or any other integer) to the power of the previous one, and the number of times you raise this power is an integer.

However, it has been shown (see Continuum Hypothesis) that either the presence or absence of other infinities between the known ones is consistent with the rest of set theory, so perhaps the number of infinities could be a non-countable infinity in some systems.

See Georg Cantor for details.