Cambridge Uni interview question

[abskebabs]

Ok, I have a question for all of you that a teacher asked me, and he told me it was an old cambridge interview question. I took a minute to think about it, but I was able to answer it correctly the first time.

 

Ok, I drill a hole through the Earth directly from one side of the Earth to the other. I drill it in a staright line that passes through the centre of the Earth and the length of the hole is the diameter of the Earth(ignore things like a hot core of magma etc). Now I drop a ball bearing down the hole which experiences no friction or air resistance as it falls down the hole. Describe to me the motion of the ball bearing.

 

It's sounding quite easy to me now:-)

[JustStuit]

It will speed up until it passes the center when it will start to slow down. Neglecting air resistance and any other forces (only gravity is counted) it would go to the same distance it was dropped but on the other side. My teacher explained this to our class one day. It is, however, impractical becuase there will always be some retarding force.

[Neil9327]

There is another factor I believe, which is that the ball bearing will be forced to the side of the tube as it falls (unless it is pole to pole tube) by the rotation of the earth.

[matt grime]

It's just simple harmonic motion. It's not just a 'cambridge interview question', it is a very standard simple exercise in mechanics.

[abskebabs]

You're right it is simple harmonic motion, that's the same answer I gave. I suppose you're right, it is pretty basic, and I shouldn't be feeling smug about it just because I got it. I was wondering, if I didn't understand simple harmonic motion so well as well as other similiar physical effects, would I have been able to work out what would have happened?

 

Also Neil, that's a good point you made about the Earth rotating, I didn't think of that, but I'm unsure about the magnitude such an effect would have(unless of course the hole was drilled throught the poles as you mentioned)

 

P.S. Matt(or anyone actually), please could you have a look at my solution to this thread.

http://www.scienceforums.net/forums/showthread.php?p=276580#post276580

I've sent a PM ot you about it as well... I'd just like to know if what I did was correct

[matt grime]

I have a simple rule of thumb: if it is mechanics and I can do it it's easy. In general I am useless at applied maths having no intuition at all for it.

[ecoli]

here's another one a friend got at her cambridge interview:

 

"In WWI doctors spread jam over the wounds of soldiers... what was their rationale behind this?"

[Genecks]

To ecoli:

Antiseptic and perhaps closing up the wound a bit?

[Nevermore]

To stop bugs from getting in?

[ecoli]

both incorrect, sorry.

[abskebabs]

I don't think this is right, but perhaps it was to keep the wounds hydrated. Jam is a colloid/gel. After a while it undergoes synersis. Perhaps also because it prevents air from reaching the wounds, therefore preventing potentially pathogenic bacteria from respiring.

[ecoli]

I don't think this is right, but perhaps it was to keep the wounds hydrated. Jam is a colloid/gel. After a while it undergoes synersis.

 

no, wrong again. But don't worry, this is a hard one, I was surprized at the answer. The trick is, you have to think like they did in WWI. The science doesn't necesarily make perfect sense in terms of our thinking.

[Louise]

I'll take a stab at this jam problem.

 

I know they used alcohol a lot for cleaning wounds, because they didn't have a lot of access to antiseptics during the first World War. I'm assuming jam was used the same way, however, Genecks has already guessed that, so I don't know how right I am. Oh well, here's for some redundancy, if so.

 

I'm not sure if they had a lot of access to bandages back then, otherwise, they would probably use those on the wounds. See, they needed something to put on the wound to seal it and prevent infection. Jam is quite sterile, if it wasn't, it would just go off in the container (and that's not good for eating, certainly not good on a wound). I know some jams contain enzymes, as well, so much the better.

 

They had food, at least. It's not like they would stuff bread into a gushing wound by a bullet, so jam would make the most sense for what was available to them.

[ecoli]

nope, that's not it either.

[Callipygous]

no, wrong again. But don't worry, this is a hard one, I was surprized at the answer. The trick is, you have to think like they did in WWI. The science doesn't necesarily make perfect sense in terms of our thinking.

 

to attract insects which then clean the wound by eating all the infected gunk? : P

[Genecks]

To keep the wound moist?

[ecoli]

both no.

[GutZ]

hmmmmmm. Maybe as an indicator? To know when the wound stops bleeding or to know who has been taken care of?

 

...lack of supplies?

 

It's hard to tell because it could be anything.

[Psycho]

Because it fills the wound making a barrier and therefore stop the blood from coming out of it?

[abskebabs]

After thinking about this, I would like to ask what kind of wound was inflicted, and how? Or is this not important, and was this carried out wholesale regardless of the kind of injury personnel received?

[scicop]

I'll take a stab,

 

so thinking about WWI, I imagine the soldiers were down in the trenches in the mud, were gangrenous bacteria survived. Gangrene was a often a cause of death in the trenches, perhaps more than bullets I don't think that's been substantiated. There was a lack of antibiotics on the field. Thus I gather they may have spread the jam to promote the growth of fughi/mold that produced antibiotics (pennicillins), which would combat the gangrenous infection.

 

And thats my guess..promote fungus/mold formation as a means for antibiotic treatment!

[Genecks]

Is this not suppose to act like honey? What am I missing?

[ecoli]

all wrong so far, so I'll drop a hint. It has something to do with preventing infection.

[clarisse]

here's another one a friend got at her cambridge interview:

 

"In WWI doctors spread jam over the wounds of soldiers... what was their rationale behind this?"

 

1) So that the soldiers would lick the wound?

 

2)Because jam had a very high concentration of sugar so when a microbe stands on it water molecules move to the jam because of osmosis?

[ecoli]

2)Because jam had a very high concentration of sugar so when a microbe stands on it water molecules move to the jam because of osmosis?

 

 

DING! DING! DING! Congradulations Clarisse! As you guessed the correct answer you get first prize! Acceptance to Cambridge university on FULL scholarship!

 

good job.

 

Yeah, they hoped to kill pathogens via osmosis.