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?



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

Jam it back in, in the dark.

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).

There's nowhere I can't reach.

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.

This thing is sticky, and I don't like it. I don't appreciate it.

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 am a dolphin, do you want me on your body?

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.

I was speaking idiomatically.

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

What kind of toxic man-thing is happening now?

This happens.

Most amazing jew boots

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.)

How ya doing, buddy?

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.

Jam it back in, in the dark.

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



There's nowhere I can't reach.

Originally Posted by katchum

We place this 10 m long plastic thing of 1cm diameter..

We place this 10 m long plastic thing of 1cm diame..

We place this 10 m long plastic thing of 1cm di..
We place this 10 m long plastic thing of 1cm..

We place this 10 m long plastic thing of 1cm diameter

Oh the science..

This thing is sticky, and I don't like it. I don't appreciate it.

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

I am a dolphin, do you want me on your body?

Are we doing your homework here, by any chance?

I was speaking idiomatically.

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.

What kind of toxic man-thing is happening now?

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.

How ya doing, buddy?

Well, Giro?

Does it explode?

DOES IT EXPLODE DAMMIT???

What, you don't want my bikini-clad body?

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

Jam it back in, in the dark.

Originally Posted by Giro0001

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.

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.

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.

There's nowhere I can't reach.

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



This thing is sticky, and I don't like it. I don't appreciate it.

Originally Posted by katchum

It has something to do with material science and all... agglomerates, crystals. I'm no expert really.

I am.

I am a dolphin, do you want me on your body?

Originally Posted by GRUN-2

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

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 speaking idiomatically.

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

Spoiler:

The gyroscope: the sum of all moments = w x L. w is the precession of the man. L is the rotation of the wheel. Both vectors are perpendicular to each other so the sum of all moments is perpendicular to w and L. This moment points to us. This means that the moment this man is applying to the stick is positive which will cause a certain precession w. If the wheel doesn't spin it won't cause this precession w and the man will turn slower with the same muscle power. I'm not entirely sure if I'm right, but you are Giro? Of course, I suspect your name has something to do with Gyroscope.

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

Spoiler:

You need to steer to the left. Again: sum of M = w x L. L is the spinning wheel vector pointed to the left (while sitting on the bike). w is the precession = you lean out to the right. Precession vector is pointing forward. w x L is pointed upwards. So you have to apply a moment in counterclockwise motion on the steering wheel. The answer is steering to the left. Note that the only way to fall off your bike is to steer to the left or the right. Try it out! (if you don't include the moment between the two wheels and the ground...)

What kind of toxic man-thing is happening now?

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

FELIPE NO

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

What, you don't want my bikini-clad body?

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

@Katchum: been to Walibi recently?

Jam it back in, in the dark.