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# Cambridge Chat (previously New Cambridge Students Entry 2004) watch

1. (Original post by Willa)
well it's a combination of gravity and intermolecular forces that hold the earth particles together. In a washing machine, it would only be intermolecular forces, since gravity is negligible. The force that stops the droplet flying off at a tangent in the drum though is contact force acting inwards. This is because of the effective "centrifugal force" thrusting outwards on the drum.

If you want the full story, then it's a little beyond me for this time of night, but this is essentially it:

Rotating objects are accelerating. Relativity says accelerating objects is same as gravity. So the rotating objects warp space-time to cause a gravitational blip outwards against their acceleration, thus causing the effective "thrust". It's like when you decelerate on a train and feel the thrust forwards. That's not your inertia acting, that's a gravitational field in front of you pulling you forward. Oh it's all very complex, i'll write a book on it for you one of these long summer days :P at least you've given me something to ponder over!

oooooooooooooook i think we are going over my head now.

the way we talked about it at school was like:

when you are in a car going round a bend, you feel like you are being pushed outwards, but it's actually your perception of the inside of your door etc pushing you inwards...

i dont really understand this though because like
have you ever been on a bus or something with slidy seats and no seat belts?
when you go round a corner you actually slide towards the window furthest from the corner...

ah so much i dont understand.
2. (Original post by crana)
mm but what about all the water in the oceans etc? and all the little teeny weeny bits of sand and soil and stuff on the Earth's surface? they don't go flying off...so I don't think it's just the intermolecular bonds etc..
Yeah, I forgot to add (although Willa did) that gravitational pull in a washing machine is negligible, whereas the Earth's core has a strong pull. Actually, the Earth's atmosphere also puts pressure on the oceans and sand to stay in place...if that disappeared, then the water would evaporate (which I think is what happened on Mars).
3. (Original post by Squishy)
I think the mutations are because people mishear. For years, I thought, "at your beck and call" was "at your beckon call".

Nope...centripetal forces cause circular motion. At least that'd better be the case, or I know I'm screwed doing Physics next year.
yes, that's what I meant.
i seem to remember quite a few from primary school prayers, hymns, songs etc.. and of course all the song lyric mishearings.

i think phrases are quite interesting though because they get "passed on" more than say mishearing a song lyric.

Ah yes I think I am muddling myself.... you are right oon the centripetal forces. I was getting confused because iwas thinking "where is the centripetal force coming from then?

So is it like this:

1) Outside bits of earth attracted towards inside: gravity
2) hence force acting towards cente: centripetal force
3) causes circular motion

?

I think we only did it in the context of "one thing going round another" at school, not something being attracted to itself.

But now I am getting confused because the inside bits are also being attracted towards the outside by gravitational attraction....!
4. (Original post by crana)
i dont really understand this though because like
have you ever been on a bus or something with slidy seats and no seat belts?
when you go round a corner you actually slide towards the window furthest from the corner...
That's because the bus changes its velocity quite quickly as it rounds a bend, but for a while, you're still stuck travelling at the old speed. So if the bus and you were moving straight, but the bus suddenly swings to the left, for a moment you're still moving forward, which is why you're shoved to the right side of the bus.

I gave up trying to understand how the world works a long time ago...as long as it keeps working roughly this way, I'll cope.
5. (Original post by Squishy)
Yeah, I forgot to add (although Willa did) that gravitational pull in a washing machine is negligible, whereas the Earth's core has a strong pull. Actually, the Earth's atmosphere also puts pressure on the oceans and sand to stay in place...if that disappeared, then the water would evaporate (which I think is what happened on Mars).
i said that, you silly person,

"the attraction between the clothes in the washing machine and the droplet is very small... "

don't the oceans and sand also "push away" the atmosphere though?

ah i have a really hard time fitting all the different bits of physics together. its a good job i NEVER EVER HAVE TO STUDY IT AGAIN apart from stuff like nerves but they make that easy for us squishy people anyway

ps. how dare you call yourself squishy, you physicist you!!
6. (Original post by Squishy)
That's because the bus changes its velocity quite quickly as it rounds a bend, but for a while, you're still stuck travelling at the old speed. So if the bus and you were moving straight, but the bus suddenly swings to the left, for a moment you're still moving forward, which is why you're shoved to the right side of the bus.

I gave up trying to understand how the world works a long time ago...as long as it keeps working roughly this way, I'll cope.
aha.....

and there was me thinking "doopey doopey doop - ooh, slid over a bit, i took that corner a bit fast - F = mv^2/r!!! go me!! - doopey doop doop"
7. (Original post by crana)
1) Outside bits of earth attracted towards inside: gravity
2) hence force acting towards cente: centripetal force
3) causes circular motion
Yup, that's right. Remember that every single object in the universe exerts a gravitational pull on every other object...my grandmother who lives in China and I are exerting a gravitational force on each other right now, but it's so small it's negligible. You just have to pick the forces that actually make a difference...if Earth orbited too close to the Sun, then the Sun's gravity would be strong enough to pull us apart, but where we are, it's not enough.

(Original post by crana)
i said that, you silly person
*whines* It's late...and I'm not really a physicist...I studied it for A-level because a teacher persuaded me to do it against my better judgement (well, she was fit)...and now I basically have to do it again next year.

The sand and stuff do push out on the Earth's atmosphere, but the atmosphere still keeps it in place. For example, when you stop a ball rolling with your foot, the ball exerts a force on your foot, but you would normally think of your foot keeping the ball in place, not the other way round.

Oh yeah, and check out www.kissthisguy.com and www.amiright.com if you haven't already.
8. (Original post by Squishy)
Yup, that's right. Remember that every single object in the universe exerts a gravitational pull on every other object...my grandmother who lives in China and I are exerting a gravitational force on each other right now, but it's so small it's negligible. You just have to pick the forces that actually make a difference...if Earth orbited too close to the Sun, then the Sun's gravity would be strong enough to pull us apart, but where we are, it's not enough..
Yuhuh, i know that much.
even if i seem not to

(Original post by Squishy)
*whines* It's late...and I'm not really a physicist...I studied it for A-level because a teacher persuaded me to do it against my better judgement (well, she was fit)...and now I basically have to do it again next year...
seems we are in the same situation AND WERE YOU UP AT 5.20 AM THIS MORNING!??! HARD AT WORK CARING FOR THE SICK???? EH???

(uh. i didnt do it because my teacher was fit though)

(Original post by Squishy)
The sand and stuff do push out on the Earth's atmosphere, but the atmosphere still keeps it in place. For example, when you stop a ball rolling with your foot, the ball exerts a force on your foot, but you would normally think of your foot keeping the ball in place, not the other way round.

Oh yeah, and check out www.kissthisguy.com and www.amiright.com if you haven't already.
hehe, i know really, i was just being picky
9. (Original post by crana)
seems we are in the same situation AND WERE YOU UP AT 5.20 AM THIS MORNING!??! HARD AT WORK CARING FOR THE SICK???? EH???

(uh. i didnt do it because my teacher was fit though)
Aww, no...not today. What happened?
10. (Original post by Squishy)
Aww, no...not today. What happened?
hehehe i work in a hospital but i normally do the shift that starts at a more....acceptable.......time for me! but i was doing the earlier one today

and i gave blood today too. so that gives me double reasons to be stupid! i lost all that......uh.........good stuff in my blood!
11. (Original post by crana)
hehehe i work in a hospital but i normally do the shift that starts at a more....acceptable.......time for me! but i was doing the earlier one today

and i gave blood today too. so that gives me double reasons to be stupid! i lost all that......uh.........good stuff in my blood!
That's really good of you... Do you do it for community service?

Ahh, joy...I gave blood last Friday. I'm still waiting for my donor card telling me what blood group I'm in though. I think I'm AB, so I might just become a plasma donor, since it's probably more useful that way.
12. (Original post by Squishy)
That's really good of you... Do you do it for community service?

Ahh, joy...I gave blood last Friday. I'm still waiting for my donor card telling me what blood group I'm in though. I think I'm AB, so I might just become a plasma donor, since it's probably more useful that way.
no, I get paid hehe.
I'm an O+, so not very special.

incidentally.... are platelets "blood grouped"? i seem to remember they are made from degraded old RBC, so do they still have the "blood group" antigens? or not?

still if you're AB+ that's the best one to be for you, if not for everyone else... O is good for everyone else, but not as good for me!

it's good i'm +, though, in case i ever had babies with a - man. I think it's that way round anyway..

rosie
13. (Original post by crana)
no, I get paid hehe.
I'm an O+, so not very special.

incidentally.... are platelets "blood grouped"? i seem to remember they are made from degraded old RBC, so do they still have the "blood group" antigens? or not?

still if you're AB+ that's the best one to be for you, if not for everyone else... O is good for everyone else, but not as good for me!

it's good i'm +, though, in case i ever had babies with a - man. I think it's that way round anyway..

rosie
Minimum wage for 5AM work, eh?

Platelets do have ABO antigens, but I don't think it matters as much as with red blood cells. I've known people who got platelets from incompatible groups and they were OK.

Yes, I'm the selfish blood recipient. But AB is the universal plasma type...

What is the difference with the +/- Rh factors anyway?
14. (Original post by Squishy)
Minimum wage for 5AM work, eh?

Platelets do have ABO antigens, but I don't think it matters as much as with red blood cells. I've known people who got platelets from incompatible groups and they were OK.

Yes, I'm the selfish blood recipient. But AB is the universal plasma type...

What is the difference with the +/- Rh factors anyway?
do explain about the plasma business... I always assumed that plasma was plasma and it didnt really matter what blood group it came from.

its a bit above minimum wage happily

Rhesus, and explaining the pregnancy bit to save me doing it:

"How rhesus incompatibility develops

People who are rhesus positive have a compound called D antigen in their blood, while people who are rhesus negative don't.

During pregnancy, and especially during labour, tiny amounts of the baby's blood crosses the placenta into the mother's bloodstream. If the baby is rhesus positive and has the D antigen, then this triggers a reaction in the mother who will start making antibodies to the D antigen.

In subsequent pregnancies, these antibodies may go back across the placenta and attack the blood of the baby. The result is destruction of the fetus's red blood cells, which may result in a condition called hydrops fetalis, and stillbirth. "

interestingly, if you have an abortion like in hospital, you sometimes have to be treated so that this doesn't affect any subsequent pregnancies - I don't really remember what they treat you with, I think it's an antibody against the antibodies you have made which stops them working, if that makes sense.

so i mean obviously an early miscarriage is not really the same as having an abortion bt you would think still there could be mixing of the two bloodstreams and it can happen that women can be pregnant and miscarry without even knowing.. so then their subsequent children could be affected. probably very rare though.

anyway, as im Rh+, i dont have to worry
15. (Original post by crana)
do explain about the plasma business... I always assumed that plasma was plasma and it didnt really matter what blood group it came from.

its a bit above minimum wage happily

Rhesus, and explaining the pregnancy bit to save me doing it:

"How rhesus incompatibility develops

People who are rhesus positive have a compound called D antigen in their blood, while people who are rhesus negative don't.

During pregnancy, and especially during labour, tiny amounts of the baby's blood crosses the placenta into the mother's bloodstream. If the baby is rhesus positive and has the D antigen, then this triggers a reaction in the mother who will start making antibodies to the D antigen.

In subsequent pregnancies, these antibodies may go back across the placenta and attack the blood of the baby. The result is destruction of the fetus's red blood cells, which may result in a condition called hydrops fetalis, and stillbirth. "

interestingly, if you have an abortion like in hospital, you sometimes have to be treated so that this doesn't affect any subsequent pregnancies - I don't really remember what they treat you with, I think it's an antibody against the antibodies you have made which stops them working, if that makes sense.

so i mean obviously an early miscarriage is not really the same as having an abortion bt you would think still there could be mixing of the two bloodstreams and it can happen that women can be pregnant and miscarry without even knowing.. so then their subsequent children could be affected. probably very rare though.

anyway, as im Rh+, i dont have to worry
Not entirely sure actually...apparently the roles are reversed for blood plasma (O is the universal plasma recipient)...maybe because AB blood hasn't got ABO antibodies in it? But yeah, it does make a difference.

Ahh...OK. Good luck with your family.
16. argh... physics gives me a headache...

about platelets - no, they do not come from RBCs. rather they come from another population of cells sharing a common semi-undifferentiated ancestor with RBCs known as megakaryocytes, which break down to form platelets. as for ABO antigens, i don't see why they wouldn't have them, since these antigens are probably membrane-bound proteins found on the surface of most cells involved in the blood (haemopoietic) system...

oh, and though the rhesus factor and the ABO factors are the most commonly recognized antigens, there are many, many more lying around - which is what makes finding suitable transplants such a difficult proposition.

and about the genetics - i just spent a day arguing about this with a friend, and we finally cleared it up. the number 50% indicates an average number of genes that you share with a sibling. and by sharing, it is not that the genes are same or different (though if you shared them they'd be the same...) - rather, by "sharing", this is more with respect to the source, i.e. mum or dad, or whichever particular grandparent. because each parent has 2 copies of each gene, and you have one copy from that parent, and a sibling would have another with a 50% chance of getting the same copy from the same source, hence over a population of 23 chromosome pairs and ~30000 genes, it averages out that about 50% of your genes will be shared (simple random chance and suitable application of probability )

ultimate frisbee is great! there is a college league that runs on weekends in the first two terms, and wednesdays in easter term. not too professional/serious (lots of beginners), which is always good for atmosphere and relaxing...

and for you gzftan, canoeing is particularly great because the 2 places that CU canoeing keep their canoes are both on caius property - at the bike shed in harvey court, 1 min from your room, or behind our boathouse, easily within reach after a rowing outing

some links you people might find interesting...

http://www.srcf.ucam.org/socs/index.html

http://www.cam.ac.uk/societies/

P.S. - why is it that when i read "fellowship of the pole", i see punts travelling in formation down the river cam accompanied by strains of the Blue Danube waltz???
17. (Original post by Squishy)
Not entirely sure actually...apparently the roles are reversed for blood plasma (O is the universal plasma recipient)...maybe because AB blood hasn't got ABO antibodies in it? But yeah, it does make a difference.

Ahh...OK. Good luck with your family.
i dont really understand why A, B, or O blood would have ABO antibodies in them though.
18. (Original post by KHL)
aand about the genetics - i just spent a day arguing about this with a friend, and we finally cleared it up. the number 50% indicates an average number of genes that you share with a sibling. and by sharing, it is not that the genes are same or different (though if you shared them they'd be the same...) - rather, by "sharing", this is more with respect to the source, i.e. mum or dad, or whichever particular grandparent. because each parent has 2 copies of each gene, and you have one copy from that parent, and a sibling would have another with a 50% chance of getting the same copy from the same source, hence over a population of 23 chromosome pairs and ~30000 genes, it averages out that about 50% of your genes will be shared (simple random chance and suitable application of probability )
To quote Steve Jones:

"99.4 percent of our genes are the same as in chimpanzees, and 40 percent are the same as in bananas. Differences between races are a question of thousandths of a percent." (from http://www.britishcouncil.org/poland...-in-poland.htm )

I'm sorry if I have missed your point but I really don't understand what you mean by "sharing" from "and by sharing, it is not that the genes are same or different (though if you shared them they'd be the same...) - rather, by "sharing", this is more with respect to the source, i.e. mum or dad, or whichever particular grandparent."

It would really help me out if you could define in a different way what you mean by "sharing" because I just don't get this, sorry.

I am taking "sharing an allele" to mean "both siblings have one (or more) copies of this allele".

You talked about "genes" but I think from the way you phrased things you might have meant alleles as well..?

If you did, (sorry if not) the 50% figure (for one gene locus) is only reached if you have the situation where this particular locus has very very many alleles that are all quite rare - as far as I can see.

I don't see how you can get a lower than 50% average chance of two siblings having the same allele at this locus in this situation, but if someone else can, please tell me!

So, until someone explains where I've gone wrong, I'm working on the basis that in this situation (with lots and lots of rare alleles for this gene) 50% is the average chance of 2 siblings having a shared allele (shared meaning they both have it) at this locus.

However for a significant number of our genes (I think - I don't have any figures for this and I don't even know if they exist), one or two alleles are vastly more common than the others, so it's much more likely that at the loci of these genes:

- one or both parents are homozygous
- both parents have at least one allele in common

Therefore at a locus in this situation, (I'll call it Case 2) the average chance of two siblings sharing an allele at this locus is much higher than 50%. For no particular reason at this Case 2 locus I'll call it 60% (which is not that much higher, but nevermind).

Even if we assumed that 99% of the human genome was composed of "Case 1: Lots of Rare Alleles" genes (I'll include segments of 'junk DNA' as genes here), and only 1% of our genome was made up of "Case 2: Few Predominant Alleles" genes, your average chance of sharing an allele at any of your gene loci with one of your siblings is still slightly higher than 50%! - and obviously this is using extremely conservative figures.

I'm not sure how to interpret this in terms of "an average number of genes that you share with a sibling", as you said, though. I do think that it (although i would refer to alleles not genes here) would be much higher than 50%, but thinking about it is getting too complicated for me at the moment.

Incidentally, it might be a very minor point, but two sisters are likely to share more alleles than two brothers...just a random thought there really.

I've just realised I have been up for over 23 hours... and yet... I actually stayed up later to finish writing this, which no one will probably read anyway. Still, if anyone does, I'd be interested to discuss it.

I'm sorry if I have got stuff wrong. I've only just done A-levels and an awful lot of A-level stuff bypassed my brain completely - I think it also bypassed several of my teachers' too, and I only managed to notice a couple of the lies they told us......!

goodnight for now! Rosie xx
19. likely i wasn't really talking about alleles, sorry... now that you put it that way, it does seem quite confusing.

okay, forget about alleles for the time being... just think of each gene's identity as being derived from the source, i.e. mum-gene or dad-gene. because we all have 2 sets of genes, there are theoretically 2 possible mum-genes, and 2 possible dad-genes. whether or not a person is homozygotic is irrelevant to our current discussion.

now, we have a mum-gene and a dad-gene for each gene locus in our cells. chances are, that a sibling will have the same mum-gene or dad-gene is 50% for each gene. (they could just as easily get the other possible mum- or dad-gene)

so, if you averaged across all the 30000 or so gene loci in our cells, we'd probably share about 15000 or so of the EXACT same genes (i.e. same source, mum or dad) as a sibling. (i.e. 50%, because the other 50% are probably copies of the other possible mum- or dad- gene that we lack in our cells, having only received one of two possible copies from each parent)

that said, considering that at most gene loci, the human species share common alleles, 99% of all the genes, and thus 99% of the differences arising from different sources (mum and dad), probably become irrelevant since they will both be the same allele anyway, if only from different parents.

this is only significant when dealing with the gene loci where there are different alleles, but because of random assortment, the percentages still hold - i.e. at gene loci where there are different alleles possible, a sibling will still share approximately 50% of your alleles, simply because they got it from the same source! and if it happens that there are other different alleles available from your parents, then chances are the other 50% will be these different alleles, if not, then this problem becomes irrelevant again.

(we are ignoring mutations, crossing over, etc.)

oh, and two sisters are not morelikely to share alleles. why? because they have 4 X chromosomes between them, but still have a 50% chance of sharing alleles (as described above). 2 brothers will have 2 X chromosomes, but again have a 50% chance of sharing those alleles. admittedly, there is a greater dispersion in the distribution of sharing, (i.e. more likely not to share 50%, but maybe 25% or 75%), but the mean value of sharing, when averaged across a population, will be 50% in both cases. you can say sisters are less likely to share more, or less than 50% of their alleles than brothers, but on average, both brothers and sisters will share 50% of their alleles.

mathematically, if allele sharing in a population is represented by a normal distribution, then the standard deviation of the population of males is higher, although the mean is same as that of females.

final point: all these things about alleles and gene loci, you must keep in mind that they are statistical, i.e. applies only to large numbers of gene loci. single gene loci follow mendelian genetics, and are usually pretty cut and dried. 30000 gene loci, on the other hand...
20. (Original post by crana)
i dont really understand why A, B, or O blood would have ABO antibodies in them though.
I thought type A blood had antibodies that attacked B antigens, etc. So it is important. But eh, the bio part of science lost me a long time ago. Just like the phys side will be losing me soon.

Updated: June 24, 2006
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