Science quiz
 

I've had an assessment today and I'm impressed of the creativity they had in those tests.

Here's one:

Two barrels are filled with water and they have holes in it, which one is the right one?

http://two.xthost.info/katchum9/Test.JPG

I picked the first one, but I still don't know why and if its the right choice...

I say the bottom one. Notice that the furthest down hole is spraying the water further. I'd gather that to mean there's more pressure at the bottom than in the first one, in which the lower two holes seem to be "gushing" (exit with lower pressure) rather than "spraying" (exiting with high pressure).

Damn, damn, damn. You're right, I just figured it out.

Aargh.

Explanation:

The top hole has potential energy: rho. g. h. But has also pressure energy also rho. g. (H-h). H = length of the barrel and h = length from ground to the hole.
The bottom hole has the same energy because it has half the height/potential energy but double the pressure energy.

So basically they have the same energy! So they have to SPURT the same LENGTH of water. Which means you're right and I'm so dumb... But I got only 30 seconds to think about it, not ten minutes!


I still am not sure of the answer, can someone try it out?

I tried it out, and the experiment wasn't a success, because of too much friction. But I think it's safe to say that the bottom one is correct.

We have two similar cars on a 45 ° hill standing still. There is no friction between wheels and ground.

Now in the first car we place a fat guy of 200 kg.
In the second car we place a skinny guy of 25 kg.

Who has the highest acceleration when letting the cars roll?

I would say that the car with the fat guy in it would accelarate more since the mass is greater even though the resistance is the same.

Okay, it is a common mistake to say that the mass is greater => greater acceleration.

You have to take in account that the greater the mass, the greater the inertia of the material.

An example: with the same force it is more difficult to move a standing car than moving a bike.

So basically the greater mass makes the acceleration higher, but the greater mass has higer inertia. They cancel each other out so that leaves the answer to be:

They both have the same acceleration. (without the friction included)

Think about this too: when you are on your bike, would you rather be a fat guy or a slim guy to have the advantage of a high speed when riding off a mountain? I always thought that the fat guy would be faster but it's the slim guy! It has to do with friction. The fat guy has more friction on the wheels. That's also why cyclists want their bike to be light weight.

Edit: I was wrong, it's the fat guy who is the fastest, because he has more inertia and is less influenced by the wind. The friction of the wheels is less powerful as the friction of the wind.

Some of you could ask why cyclists put this added weight in the back wheel. It's just to reduce wind vortices with the plates and reduce turbulence of the wheel by adding weight.

New problem:

We have a rectangular magnet with North-South pole. You sprinkle neutral ferromagnetic particles in the middle of the magnet. What happens?

1) The particles spread over the entire magnet
2) The particles stay in the middle
3) The particles go to the North Pole
4) The particles go to the South Pole
5) The particles go to both poles

http://img66.imageshack.us/img66/8216/magnet0873ob9.png

This happens.

We're not talking 2D/3D here. I'm talking about the alignment on the surface of the magnet. (Gravitational force is still stronger than magnetic force at those distances. The particles will fall down instead of floating in the air.)

Both poles. And that drawing is of ferro-magnetic fillings scattered over a white sheet of paper resting on a magnet (or a magnet on it with the result photographed) Nothing floating in mid-air.

That's right, the flux is greatest at the poles and is zero on the mid section of the magnet's surface. All the flux is situated inside the magnet not outside the magnet. Answer 5.


New problem:

We have a wooden barrel with water. We place this 10 m long plastic thing of 1cm diameter in a little hole on top of the barrel and fill it with water.

The barrel can withstand 1kg/cm^2.

What will happen:

1) The barrel explodes
2) The water gushes out from the top
3) Nothing happens

http://two.xthost.info/katchum9/barrel.JPG

Oh the science..

Yeah I know, when you say that on a job interview, they shoot you down... Let's say tube then.

Are we doing your homework here, by any chance?

heh, even if we are it's sort of fun..

My guess, read:GUESS, is that nothing will happen, i don't think 0.10Pi x 0.5 is enough volume to cause more than a kg of pressure.

Basically the original problem comes down to finding the velocity at which the water leaves the hole and then you simply use kinematics to figure out where each lands. Now the question comes down to finding the velocity at which it leaves the can. Unfortunately neither of those pictures are completely right. Lets pose the problem well.

We have a canister filled with water sitting on top of a table. The canister is filled with water to a height of H and there are holes on its side of equal size at intervals of h down from the top of the water.

Then v(velocity of water out from a hole h down from the top of the water) = sqrt(2gh)
You can use the correct application of Bernoulli's equation to get this interesting result.

Now we get v = 1.26 sqrt(h).

Now the problem is down to kinematics.

Find t using y and g.
0 = H - h - .5 (9.8) t^2
t = .45 sqrt(H-h)

Now use an equation of x using v and t and the fact that a_x=0
x = 0 + 1.26 sqrt(h) (.45 sqrt(H-h))
x = .569 sqrt(H h) -.569 h

So now as you can see it's a parabola. Graph it if you're interested in what it will look like. Remember to fix H though, H is the height of the water. h is your variable.

This equation is absolutely consistent with what we would expect. The very top where h = 0 has x = 0 and the very bottom when h = H , x = 0. Also I myself suspected that in the middle the distance x would actually reach it's maximum, which it in fact does.

Well, Giro?

Does it explode?

DOES IT EXPLODE DAMMIT???

Hehe I understand, nice, nice! Thanks!

There is still one thing I don't understand: the velocity is the same when the hole gets bigger? This means only the mass flow gets higher with a bigger hole then...

Well, it explodes. I think. Because:

rho.g.h is the pressure and its: 1000kg/m^3 . 9.81m/s^2 . 10 m [Pa]

This would be the pressure at the bottom of the tube. And it's higher than 1 kg/cm^2. It's not the volume that makes the pressure, it's the height!


New problem:

Two cold glasses of -20 degrees °C are put in 90 °C water, which cracks?

The famous glasses problem. You have to take in account many, many factors!!!!

(mechanical strength, microscopic allignment of the glass, warmth transfer both in time and place, volumetric expansion)

http://two.xthost.info/katchum9/glasses.JPG

Think about it another way, though. Say you put the canister on a really fucking tall table. So tall that the height of the hole in the can relative to the height of the table is insignificant. In that case, the difference in acceleration due to gravity once leaving the can shouldn't matter, and all that really matters is the initial velocity in the x direction.

Also, it should be easy to guess the trajectory will be a parabola because it's moving like a projectile under gravity. :p

As for the glasses, there's a turning point in which would crack. In the limit of the 90°C glass equaling the volume of the -20° glass, you shouldn't have any cracking since the volume of water trying to heat it up would be zero. In the limit of the -20°C glass being extremely thin, then there wouldn't be enough of a temperature gradient across it in order to produce thermal cracking no matter which situation we were in.

Right, the thicker the glass the higher chance of it breaking. But sometimes when I wash glasses with warm water, it's always the thin glass that breaks first. Here it is the fault on microscopical level, it is difficult to make strong surfaces when you make thin glass. The same goes for steel. A big steel tube is stronger than a thin steel tube. It has something to do with material science and all... agglomerates, crystals. I'm no expert really.

I'm a bit out of ideas...

Hey:

I've stuck a wooden pipe inside a man made hole in the center of a fan. Now when I push the wooden pipe really hard as stated in the following figure, what will happen visually? The fan is spinning at maximum speed.

1) Nothin happens
2) The fan will tip over and fall down

http://two.xthost.info/katchum10/Fan.JPG

I am.

Missed the point. When I mentioned the parabola I was speaking of the equation which is a function of the distance h from the top of the water at height H which represents the distance x from the canister where the stream of water hits the table. I didn't have the actual equation of the trajectory anywhere in my calculations.

Obviously when considering special conditions things may change. For example it changes slightly with variances in temperature. It has a great change with a temperature less than 0 C or greater than 100 C. Also the assumption made for Bernoulli's equation that atmospheric pressure is exactly the same at the top and the hole is not really true, it's actually slightly different. Many many things will change it. Certain things more than others. Other things are assumed, like that the table is relatively close to sea level (Not 10^30 miles above).


Now for the new problem it depends on how the force is applied. Clarify.

I was afraid it would make too much confusion.

I'll make a better problem now:

The man is turning clockwise around himself while holding a spinning wheel. Will this man rotate faster or slower or equal when the wheel isn't spinning at all? (while the man is using the same muscle power)

http://two.xthost.info/katchum10/manneke.JPG



Another thing to think about, based on the same mechanics:

When you ride a bicycle and this bicycle is riding really fast. Then at some point you want to go to the right, what is the best thing to do?

1) Steer to the left
2) Steer to the right
3) Leaning out to the right
4) Leaning out to the left

*Makes note to self to read the above posts more thoroughly before commenting

Before I forget to post it, I'm on a roll here.

New Problem:

Everyone has gone to amusement parks. You know those little round boats floating in the river. And you trying not to get wet. Now when you get out of those boats you always walk on this rotating platform to get back to the exit in the center, all wet.

Ever thought of this? When you walk in a straight line to the center while the platform spins. Will you most likely fall to the left or the right?

http://two.xthost.info/katchum10/Radja%20River.JPG

Won't that be dependant on the speed of walking towards the center and the speed of rotation?

@Katchum: been to Walibi recently? ;)

Yeah, Six Flags, it wasn't as fun as I expected... Bobbejaanland is better.

It doesn't depend on speed, either way you will fall, if you don't apply a reactional force to the ground. But which direction?!

Whoops, my bad. I had seen a slightly different question a few days ago and assumed you were going at it the same way that we had approached that one.

Here's a good one I heard today.

How many times do I whistle, at which time intervals? Does echo reflection count? And how good is the dog's hearing?

Dog can hear infinitely far, you can whistle as fast as you can in real life, and sound propagates infinitely far.

Just imagine as though you're on an infinite plane with nothing else there other than the dog and you.

I suppose it's about 8 times (edit: more like 7 times - depends on how you count ;P *valid excuse* ) the dog will hear you -

Since it doubles its speed every time, you'll have to count 2^x - and I tried it out...

It reaches sonic speed (which is 343 m/s under normal conditions) at about 2^8.45 (349.706)

That's my guess here... =(

Nope, neither guess is correct.

My guess would be 15
8 times till he's faster than the speed of sound at which time he'll overtake the sound waves of your previous whistles.

I'll assume he'll accelerate the same instance he hears the 8th whistle so he won't hear it twice.
Otherwise: 16

Yeah, 15 is the answer. Everyone I've told that one to had that, "Man, that's a great question" reaction upon hearing the answer. I liked it because it had a good trick.

Hmm, intruiging.

When you use Doppler for a moving observer and standing source. You would see that the frequency will change.

This is the formular for an observer going away from the standing source:

f'=f(c-v)/c
c=sound velocity of source
v=velocity of observer moving away from source

Now for the dog problem we have something analogical:

f'=f(v-c)/v (not entirely sure)

v=velocity of the dog
c=velocity of the whistling sound

f'=1000(350-343)/350=20 Hz (approximately, assuming ideal temperature, pressure)

Too bad... the dog can hear this sound. So yeah 15 times. Unless the whistle is of a lower frequency, like the lower sol on a piano. ~400 Hz?

Yeah - that's a nice trick; Too bad I haven't thought about it that way!

... I need to learn thinking twice ... the right way.

Try with this one, less physics more logical thinking.

You have two strings whose only known property is that when you light one end of string it takes exactly 5 minutes or 10 minutes to burn. The rate at which the strings will burn is completely random and each string is different. How to measure with them 7,5 minutes ?

Hmmm, interesting question.

I'd try to light both strings at the same time.
When the 5 minute string is burnt up, light the other string also at the other end. When that string is wasted, it should be exactly 7.5 minutes?

Yes! The main idea was to light the string on both ends. Notice that it's even easier to burn the first one (lighting on both ends) which is 2,5mins, and then the second one on both ends, which gives 5 minutes, and a total of 7,5 minutes.

Next one:

Find the next object in the following sequence

o, t, t, f, f, s, s, e, n, t, ...

Good luck=D

No!!! I got those tests on my assessment, and I'm still having nightmares about them...



Another one they used was this:

d, o, m, i, s, o,...

I mean, that's just lame... people who don't play music can't possibly know this answer. I hate those consultants who make these tests.

Does o-f-f-s-e-t have anything to do with the answer?

No, 'offset' has nothing to do with it. The answer is incredibly simple, but requires an idea.

btw I ensure you that the answer doesn't require any major knowledge.

e

That I got that as soon as I looked at it possibly says something about how my brain works.

Excellent. Next one:

A man entered a pub. He sat at the bar and started a talk with the bartender. After a few minutes bartender said he had three sons. "How old are they?" asked the man at the bar. "Product of their ages is 72." replied bartender. The man started thinking. "I need more information" he said. "The total of their ages equals the number on the building which is opposite to my bar." said bartender. The man left, saw the number on the building and came back to the bar. "I still need more information" he said to the bartender. "The youngest one LOVES ice cream" said bartender. "Now I know the answer" said the man and smiled.

Yes.. what is the answer?

6, 6, and 2 yo?

Can you explain how you get to the answer, either Shin or Lambert? You just moved right on to the next question without any explanation. Some of us aren't geniuses. :(

Nice=) I have another one:

Fish breeder Peter bought three ponds: one with silver carps, one with rudds and wone with both of them silver carps and rudds. Th previous owner put wrong all the signs (saying which spieces are in a corresponding pond). Now Peter would like to put them back in a correct order, but he don't want to fish to much fishes. He came up with an idea how to do that fishing only one fish at all. How? =)

neat!




Next one:

Assume you're in front of two doors. One door leads to a treasure and the other one to instant death. The doors are guarded by twin brothers. One of the two brothers always speaks the truth and one of them always lies. Since they're identical twins you don't know which one is honest and which one a liar. Which question will you ask to know which door you should take? (to the treasure of course!)

"Hey, shorty, what door does your brother day is the one leading to death?"

And you walk into the that one.

Additional Spam:

This is a common warmth transfer question:

I'm making some tea and I have to add cold milk and sugar to it. So I boil the water and put it in my cup. But suddenly someone is calling me on the telephone (for approximately 10 minutes) and I don't want my cup of tea to be cold when I come back. What do I do?

1) poor the milk in it now
2) poor the milk in it after 10 minutes

If you know the answer, don't tell it too fast, let other people discuss first.

Hmm, not sure about my answer.

I'm not sure about this one:

I'm on earth now and see a big glass box (10 meter x 10 meter). Inside the box is vacuum.

Now someone inside the box (probably a robot) holds a feather in one hand and a metal ball in the other hand. He releases both materials. What happens?

Didn't they actually do this on the moon (or one of similar style)?

We have a flame. (coming out of the cooking place with gas fuel)

You'll see a little bit of a blue color and much of the yellow color. Which one is the hottest part?

And if you have a flame, is it hotter at the bottom, at the top or in the middle?

When you warm up a metal in the dark, which color does it have? Red? Can it also turn into blue?

To be specific, it was a geologist's hammer too iirc :P

To be honest I don't know the answer this time... I always thought blue was warmer.

I also think the color you see is a function of the temperature. For a perfect black emittor. But yeah, metals are all but perfect black emittors.

The color of a flame can change due to a variety of things, one of the most prevalent being the material being burned. Adding certain chemicals to a flame can radically alter its color. I'm sure temperature has an influence, but it certainly isn't the sole factor.

Two glasses of water are at the same temperature. In one of them you dissolve 2 spoons of salt.

Then you throw in both of the glasses some ice cubes. In which glass will the ice cubes melt the fastest?

I've got one. If the time is 1:11, how long in minutes and seconds will it be until the hands cross again on a standard, analogue clock?

I'm not sure about that. It seems that the water will be colder in the glass with salt. I'm going to do the experiment myself, as I don't know the answer. :eagletear:

I believe you, but I'm also sceptical.

And: one hour and six minutes? I don't understand the question really. Otherwise it also could be one minute.

EDIT: I CHECKED IT!!!!

The glass with salt water melts SLOWER!!!! Anyone who can give an explanation?

I think the question is posed incorrectly. I looked around a bit, and found this slightly analogous situation. It would have to be the same weight in both cups. One containing, say, 1 kg of water, and the other containing of 1 kg of water + salt.

I'm sorry but you're looking at the wrong direction. I'll explain the answer.

The salt in the water wouldn't make any huge difference on the temperature, so don't search in that area.

What you have to think about is convection!

When you put an ice cube in the salt water, the cold water stays above and the warm water stays underneath, because salty water has a higher density! => ice cube doesn't melt fast.

When you put an ice cube in normal water, the cold water goes down and the warm water rises up => ice cube melts faster.

Actually the convection wouldn't matter, I think what would matter is in salt water more of the ice cube would float over the top of the surface (less would be submerged), so less water touching it means it melts slower. I think for normal water it's something like 4/5 of the ice cube would be submerged (actually just had that value on the final I took yesterday), so when it's salt water, maybe only 3/5 would be submerged. Since the side of the ice cube touching the water would transfer heat faster than the side touching the air, the one with more ice in the water would melt faster.

I think the convection currents within the water itself should be approximately the same between the two.

I found this explanation satisfying:

General Chemistry Online: FAQ: Solutions: Why can adding salt to ice water make the ice melt slower?

No pyrrhus, that experiment is done while stirring! In my case I just put the ice cube in the salt water, without stirring.

See:

"Watch how the temperature of the ice water falls after the initial addition of salt. What does a plot of temperature vs. time look like? (Stir constantly and completely!) "

I still think it's convection. Because you can easily imagine that this ice water layer around the ice cube will be motionless, therefore it gives isolation around the ice cube. (when put in the salt water)

In normal water there is natural convection, so no isolation around the ice cube.

When you do the experiment yourself you can even SEE it! There are these little transparant lines flowing all over...

Here is my reference, it's in flemish though. Pyrrhus will confirm it's true!:

NWO - De Quiz van 2000

Quote: "In het bekertje met zoetwater smelt het ijs. Het koud smeltwater is zwaarder dan het oorspronkelijke water, zodat het naar de bodem zakt. Het warme water stijgt juist op, waardoor er weer ijs zal smelten. Kortom, er treedt circulatie op in het glas. Er wordt steeds warm water aangevoerd dat het ijs snel doet smelten.

Anders gaat het toe in het glas met zout water. Daar is het smeltwater van het ijsblokje zoet en het omgevingswater is zout. Zoetwater is veel lichter dan zout water. Het koude zoetwater zal niet zakken en het ijsklontje blijft omgeven door het koude smeltwater. Er treedt geen convectie op en daarom zal het ijs in het glas met het zoute water veel langzamer smelten."



I'll post some more nice questions from that site next time.

Shouldn't there be even more currents in the salt water one since you'll be getting a gradient in the concentration of salt with less salt surrounding the melting ice cube and more salt towards the bottom of the glass?

You mean like diffusion? There is, but the effect of density is bigger, I think.

Hmm, looking up the density of seawater (~3.5% salinity) the density is 1.025g/mL. That's 35g of salt in 1 kg of water. I figure if we put two heaping tablespoons of water we'd have maybe 14% salinity, so just guessing that makes it around 1.1g/mL. So, yeah, only 10% more of the ice cube would be poking out of the water. So instead of 80% of the ice cube being underwater it would be closer to 72% touching the water.

I still think it could go either way. :(

It's some time ago, but I know I've learned somewhere that if there's a high difference in temperature or salinity of two solutions touching each other, diffusion is more difficult, unless you of course go stir the solution.

So diffusion won't play that much here.

Since nobody answers Shin's question I'll post another one.

What's the coldest: -40°C or -40°F?

They're about the same, as Fahrenheit and Celsius coincide around that point.

What happens when you cook an egg at 65° C for 15 minutes?

Sounds like a soft boiled egg, to me.

Nope! There is something spectacular going on in the egg.

The egg won't harden, but it will become safe to eat.

I don't know why you'd want that, though.

It's very useful for Rocky reenactments.

Nope, that yellow thing inside the egg becomes hard and the egg white stays fluid. Try it yourself. Use a thermocouple.

Why we'd want that? For science's sake!