# DrKrettin

Senior Members

822

## Posts posted by DrKrettin

### Length Contraction

I remember as a student having to explain this Gedankenexperiment problem:

Imagine an infinitely large and infinitely thin and infinitely smooth table with a slot in it of length L, and a rod of length L- dL just small enough to fall through it if you placed it over the slot and dropped it. Imagine the rod sliding along its axis towards the slot. When it reaches the slot it will fall through. Now imagine the same at a relativistic speed. An observer on the table will see the rod moving towards him, contracted, and it will fall through the slot. An observer on the rod will see the slot approaching, contracted. But then the slot is smaller than the rod and the rod will not fall through. Clearly an apparent paradox.

Anything happening at very low speeds must also happen at relativistic speeds so it must fall through the slot regardless of speed. Of course I can't remember the maths, but it turns out that even when the observer on the rod sees a contracted slot, the ends of the rod meet the ends of the slot at the same time so it still falls through.

I'm not sure whether that helps at all.

### Possibility to survive a building jump?

3 minutes ago, Lord Antares said:

Are you sure about that? You exert no weight on the table. Maybe if you were crouching on it so you could exert weight by raising up. If your legs were completely straight, would you be able to lower them and thrust the table downwards?

Yes, I'm sure about that. If as you say, you crouch on the table then thrust the table downwards by stretching your legs, you impart an impulse to the table and the table gives you an equal and opposite impulse upwards. This will slow you down. I doubt whether you would disagree with the situation if there were a rocket  falling backwards towards earth which then ignites its engines and provides a thrust upwards slowing the rocket's descent. It's the same principle.

### Possibility to survive a building jump?

You give the table a downward impulse and you slow yourself down in the process. I have actually seen the practical application of this, albeit not in a vertical direction but horizontal. Here we have plenty of lizards which can descend vertical walls quite quickly with the following technique: they drop off the wall and further down they flick their tails out away from the wall. The body of the lizard gets an equal an opposite impulse towards the wall, which they can then grab with all four feet and cling on. Their tails are quite large, so flicking them gives quite an effective impulse.

Same principle, anyway.

### Help needed arguing with a creationist

Why look for aliens elsewhere, when there are so many among us?

### Possibility to survive a building jump?

1 minute ago, Itoero said:

I think he means you jump on a table which is on the floor, the table doesn't fall.  If he did jump of a table on a building then the table would remain on the building.

The OP is really unclear. But with respect, if you don't use prepositions accurately there is no chance of getting clarity. Do you mean "onto a table" and "off  a table"?

### Possibility to survive a building jump?

3 minutes ago, Itoero said:

This means he jumps from a building on a table which is on the floor.

I understand it that he jumps off the building whilst standing on the table. How else who you understand it?

### Possibility to survive a building jump?

3 minutes ago, Itoero said:

The table is not in freefall, he jumps of a building, on a table and jumps of the table before impact(from jumping). Where did you read the table is in freefall?

Why do people think the table is in freefall?

Because he jumps off the building on a table, so he and the table are in freefall, unless you want to consider the surfing properties of the table

### Possibility to survive a building jump?

4 hours ago, Lord Antares said:

Stepping off the table won't do anything. You are still falling with the same speed as if the table wasn't there. You'd still die just the same.

This is not correct. If you and the table are in free fall, if you give the table an impulse downwards, the table gives you an equal and opposite impulse upwards. This will reduce the downward speed, although it won't make any difference to the outcome.

### What causes to 3rd law of motion in theory of Newton

37 minutes ago, Area54 said:

That's a bit of a harsh judgment on a non-native speaker.

Perhaps, but even if you ignore the shortcomings of the language, the underlying message is still incoherent.

### A Squirrel called Igor

12 minutes ago, geordief said:

I was going to suggest that he only felt right in Spring (so it was Stravinski,then ?)

Nice one!

### A Squirrel called Igor

3 hours ago, interested said:

Squirrel (ToF-AMS Unit Resolution Analysis Software)

Maybe there is a cartoon character, named Igor. Igor was also Dr Frankensteins side kick.

Nice try, but no he's not. He has just revealed the reason for the name: Stravinsky did not compose the Nutcracker.  I'm still trying to work that one out. I can think of plenty of people who did not compose the Nutcracker.

### A Squirrel called Igor

8 minutes ago, geordief said:

Does he anything like this fella?

Hard to say whether that is red or grey, but does look remarkably like Igor.......

### A Squirrel called Igor

My son has a house with a garden which has a resident squirrel (garden, not house). He has challenged me to work out why the family has named it Igor. Does anyone have an idea why? There must be some connection which I'm unaware of. All suggestions welcome in helping me to cheat.

### Help needed arguing with a creationist

1 minute ago, studiot said:

Surely if he admits it doesn't have to then you can challenge him as deceitful.

Ask why did he use something he knew to be false as a challenge, instead of offering proper evidence or reasoning.

His argument seems to be that evolution cannot possibly happen, therefore creationism is the only alternative. What can you do when he can't see the absurd non sequitur?

### if we hold PhD degree can we work at europea or america?

Just now, inverse said:

are patentings and written articles to  be effective ,too?

Published articles are very effective, and presumably a patent is also a positive.

### Help needed arguing with a creationist

iNow - Wow - thanks that! Just the response I needed

I find it almost impossible to discuss things with creationists because they just deny evidence. It is no good responding to a challenge like his with a question like why does it have to, because he will then use it as an admission that it doesn't. I know this is all like nailing jelly to the ceiling, but one must try now and again.

### Help needed arguing with a creationist

I'm not a biologist, so this is unfamiliar territory. He has challenged me thus, and I'm unsure how to respond

cite me one study that shows a mutation that produces an increase in the genetic information of the organism.

### if we hold PhD degree can we work at europea or america?

In theory, yes, but it does depend on where you get your PhD. The competition is great, so people with a good reference from a known supervisor stand a better chance than somebody with no or few contacts. The question is really too general to answer more accurately.

### Male rape

5 minutes ago, zapatos said:

I remember a local case quite a while back of a taxi driver claiming rape by two women he picked up. One kept up a constant physical threat to him if he didn't comply to sex with the other. Seems like a clear case of rape by physical force.

If it had been a female taxi driver and two men, I would feel very sorry for the victim. As it is, I find it difficult to sympathise with the taxi driver simply because I can't imagine that he suffered much. These days, the offenderati insist on sexual equality, so he is as much of a victim as a female would be. I'm not convinced. This so-called equality reaches ridiculous proportions when a young attractive female teacher is sent to jail for having sex with one of her teenage male pupils. Thinking back to my teenage years, the teacher is actually satisfying the wildest fantasies of the pupil and she gets into trouble for it. I just don't understand how this could be called abuse.

### For the Brits: magneti(s/z)e, magneti(s/z)ation, re(-)magneti(s/z)ation, re(-)magneti(s/z)e

44 minutes ago, Function said:

I'm writing my thesis in British English:

•

Good move! The principle is that words taken from French are spelled with -ise, for example advertise, advise, apprise, chastise, circumcise. But words taken directly from Greek follow the spelling of the Greek infinitive ending -izein, so have endings -ize.

I think therefore that the best procedure is to use -ize except for the small number which require the -ise. This issue is a minefield, and nobody agrees. If your spellchecker shows up with an error, the chances are that it is correct because the spellcheckers are all American.

### Continuos skin rash which has trigger but not known

Why do you think that somebody here could deliver a more reliable diagnosis that a medically trained skin specialist?

### Today I Learned

10 minutes ago, Tub said:

Hello, Drk. I stumbled across two new words recently: one was " catachresis ", which is relevant to " phase " and " faze "; the other is  "ultracrepidarianism "  which could apply to a few  people in these forums ( not including you, of  course..ha,ha.. but definitely including me ). You may already be familiar with these words but, if you aren't, and knowing your interest in Greek and Latin, i think you might be interested in their etymology.

Thank you - they are both interesting, but the ultracrepidarianism  (which I had come across but forgotten) has a particularly interesting etymology. I find etymology fascinating, which puts me in a very small minority methinks.

### Journey to Alpha Centauri

4 hours ago, Area54 said:

Camels are not native to Australia, which, unlike all the other continents, they were not native too.

It sound like you are saying that camels are also native to the Antarctic . (My turn to be pedantic)

### Today I Learned

2 hours ago, Area54 said:

And this new knowledge didn't phase you?

*groan* I'm rather fazed by your spelling.

### Today I Learned

3 hours ago, Area54 said:

Had we been talking about sheep rather than cattle, there would have been the option of discussing rams.

Not too long ago (but before railways) the word "cattle" could mean both sheep and cows. I'm reading a Greek novel at the moment, written about 200 AD which uses the word "probata" literally "that which is driven forwards" referring to a flock of sheep or a herd of cattle. In both cases, the English translation of around 1650 uses the term "cattle".  The device to stop livestock from walking through gate is still called a cattle grid in the UK. There are people who go round building little ladders inside them to help hedgehogs escape when they fall in. I'll stop now.

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