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2:57 AM
Hey why does when sticking tape on above another on table and taking out each piece they have opposite charge ?
I am really confused
Oh I understand now
when i stick tape on table and unstick it steal or donate proton and if i do the same using another tape over the tape it steal or donate proton which will make first tape lack or have proton and the first tape must be opposite charge since first tape stole or donate proton
 
sticky tape donates protons??
are you sure?
goodness
"The sticky-tape X-ray machine is also baffling others in the field. "You wouldn't have thought that so much of the mechanical energy would come out as X-rays," says Ken Suslick, an expert in mechanoluminescence at the University of Illinois at Urbana-Champaign. "The adhesive on the tape is an amorphous liquid, not crystalline. What's causing the transfer of charge, of electrons or protons, what the accepting and donor groups are — these things are much less clear.""
 
3:17 AM
Sorry I was just assuming since I got 0 knowledge in physics.
@antimony cool
 
 
5 hours later…
8:14 AM
It's fascinating it takes approximately 58 N for 2 proton to be near each other in small distance
I was curious about why will proton even be close to each other since they repel
it's still confusting
 
Do you mean why are protons close together in a nucleus?
 
@JohnRennie yes
My background knowledge is equal to child so I don't know enough physics to know that
 
In a nucleus the strong nuclear force holds the protons together with the neutrons. The strong nuclear force is much stronger than the electrostatic force.
In nuclear physics and particle physics, the strong interaction is one of the four known fundamental interactions, with the others being electromagnetism, the weak interaction, and gravitation. At the range of 10−15 m (slightly more than the radius of a nucleon), the strong force is approximately 137 times as strong as electromagnetism, 106 times as strong as the weak interaction, and 1038 times as strong as gravitation. The strong nuclear force confines quarks into hadron particles such as the proton and neutron. In addition, the strong force binds these neutrons and protons to create atomic nuclei...
The strong force is very short range. It's range is limited to about the size of a nucleus.
4
 
I was thinking about that :-)
 
The reason we don't get nuclei much larger than uranium is that for large nuclei the range of the strong force is too short to hold them together and the nuclei fall apart.
 
8:24 AM
What about lead-208
 
Lead 208 isn't much larger than uranium ...
The number 208 is the number of particles in the nucleus i.e. the number of protons + the number of neutrons.
The most common form of uranium is U-238, so it has more particles in the nucleus than lead does.
 
glS
8:54 AM
@Semiclassical wow... had no idea that latex symbol existed
@Semiclassical kinda.. my personal rule of thumb is that if it involves more quantum info concepts (e.g. channels, povms, entropies or other information measures, entanglement etc) go on qc; for more standard physics things (e.g. for me most quantum optics, phase space stuff, etc) go here (and obviously, non-quantum stuff here as well)
 
 
3 hours later…
11:42 AM
morning
 
 
2 hours later…
1:18 PM
morning
 
2:00 PM
I was struggling with the problem below:
In particular, it caused me great angst as to how to account for the length of the ropes when solving for their tension.
And then I opened the solution manual:
Why is the length of the rope not important to finding its tension? Then I might as well have an infinitely long rope and it wouldn't make a difference in the tension!
Are we assuming the rope is massless? What does that even mean? And if a rope is massless, how could it have tension in the first place?
If anyone could help me understand this, that would be great.
 
 
1 hour later…
3:18 PM
@rb3652 if you use a rope to suspend an object, then technically the tension will rise as you go up the rope. It has to, because the rope at the top has to support all the mass below it—including the rest of the rope. The rope at the bottom doesn’t have this concern
The easiest way out of this is to assume the rope has no mass. Then there’s no difference between top and bottom, and the tension stays the same along the rope. So that makes the lengths irrelevant in that sense
Where the lengths would matter is if you weren’t told the angles directly and thus had to infer them from geometry
I could see a problem where that information is useful. Here it seems irrelevant
@glS makes sense. On those grounds I’d lean to QC, since the question can be framed as “when do measurement bases count as quantum information?”
In a Bell test, it doesn’t count at all. In the “Choice = Signal” paper, though, it seemingly does
 
3:51 PM
@Semiclassical Wow, this makes some sense. @Wolgwang from Problem Solving Strategies also helped me understand that a rope isn't similar to a spring in the sense that a spring is harmonic motion -- it stretch back to equilibrium, but a rope doesn't.
In this problem, the lengths aren't as irrelevant as they are deceiving
 
anyone familiar with intro quantum computing?
going to an online meetup for quantum computing at 1pm and want to brush up on the basics
or at least get some idea of what it is
 
 
2 hours later…
6:20 PM
does a unitary transformation always guarantee isometry?
 
6:39 PM
Suppose the body moves in 1 dimension. s=16t^2. s is distance in feet and t is time in sec. At 5 sec it travels 400 feet. At 5.1 it travels 416.16 feet. Speed is 16.16 feet/.1sec or 161.6 feet/sec. So is this the speed at 5 sec or 5.1 sec or at 5.05 sec?
 
compute the derivative and find out
 
@Bohemianrelativist No, only a unitary transformation that is a symmetry of the system would be an isometry, right? For example, if a system is not translationally invariant then the operator that generates translations in space would still be unitary but it won't correspond to an isometry.
 
@Semiclassical No ,without derivative!
 
to be a bit less annoying: what's been computed is the average velocity from time t=5s to 5.1s.
if you want to find the instantaneous velocity, that's differentiation
if you know the formula for velocity as a function of time given constant acceleration, then you can use that instead. but ultimately one is relying on v=dx/dt
so better to get used to that
 
@Semiclassical So there can be different velocities in between 5sec to 5.1 sec depending on the body. Body can speed up and slow down in the interval. We cannot be sure.
 
6:52 PM
if you're only told the positions at t=5s and t=5.1s, yes. But you're told s=(16ft/s^2)t^2, so you know the position at every time in between those two moments
so you can perfectly well say what the instantaneous velocity is from t=5s to 5.1s. you just need to differentiate
 
if you're not comfortable with v=dx/dt here, then the rest of physics is going to be very tedious
 
@Bohemianrelativist I see, I guess the definition is simply that it preserves the inner product and thus, unitary transformations would be isometries. I was thinking in the following sense: in GR, when we say that a transformation is an isometry, it is supposed to be a non-trivial physical symmetry -- only then the metric is unchanged. I am saying this in contrast to a general coordinate transformation which changes the metric itself to keep the inner products invariant.
Now in QM, a unitary transformation by itself is more like a general coordinate transformation that keeps inner products unchanged but doesn't mean that it is a physical symmetry of the system.
 
Hi @Semiclassical I really appreciated your explanation of tension in a rope
 
6:58 PM
So, isometries don't convey the same meaning in QM (physically speaking) as they do in GR, I guess. Yeah, I am a bit confused now, would be great if someone else can clarify a bit.
 
No, it reminded me of something
 
you see this kind of thing outside of ropes, of course
 
The tension increases as we go up the rope.
 
right
 
The pressure increases we go down the sea!
What a beautiful connection.
 
6:59 PM
hm. yes.
 
And I suspect that the increase is convex for both.
 
pressure also increases as we move down through the atmosphere, but usually we don't notice it
 
@Semiclassical Well, actually.
I once read a beautiful analogy about eggs.
Imagine there to be eggs stacked up from our ground all the way to the atmosphere.
Well, of course, the egg on the ground will feel much more pressure than the egg on the top
 
ah, yes
the version i was going to say was if you have a bunch of blocks touching and you push on them from the left
 
There was a great picture associated with it too, but now I can't remember where I read it.
@Semiclassical I don't quite follow? Is it that the one of the bottom moves most?
 
7:02 PM
they all accelerate to the right at the same rate, but the contact forces between them decrease as you go from left to right
i meant blocks all lined up horizontally, not stacked
 
Ah, I see
So the contact forces decrease because of heat, sound, etc., correct?
 
no. the reason the last block has the lowest force on it is because it's not pushing on anything to its right
so all of the force from the block to its left goes into accelerating it, and thus doesn't have to be as big
by contrast, the block to its left must also push to the right
 
Ah, I think I see what you mean
 
so the force on it from the left must be greater, since the block to its right will be pushing back
the net force on each of the blocks is of course the same (assuming equal masses)
 
Yes, yes, the first block has the "most responsibility" -- it's got to exert sufficient force to move all its siblings.
 
7:05 PM
right
of course, this goes away if the blocks are very light
or, well
becomes negligible
 
I came to the h bar to ask a question that's been bothering me since morning.
See, I still don't understand what tension exactly is. I've seen answers on PSE explaining how tension is some sort of EM phenomenon (way over my head), and other answers explaining Tension is just the "pull" a rope experiences. Maybe it's just me, but it makes me uncomfortable -- or I find at strange, at the least -- that something massless (i.e., our rope) can experience a pulling force (i.e., tension). For is not force itself defined to be inversely proportional to mass?
So how then can a massless thing have tension?
I understand that it's of course a mathematical convenience to assume massless ropes due to the variability of tension across the rope.
 
I tend to think of tension less in terms of what model we use for it, and more about what it accomplishes.
namely, tension is a force which accomplishes certain distances remaining the same
just as the contact force from a table accomplishes the task of keeping objects from falling through it
you could equally well ask: how does one model the contact force from a table? how does a table push back if it's perfectly rigid
and the answer is that the table is not perfectly rigid. it does deform ever so slightly when you push on it
and thus there's an elastic response
 
Right
 
it's just that this response is so large that the amount of deformation is effectively negligible
so too do ropes, in fact, stretch under enough force
we just usually pretend that it's so hard to stretch them that this can be ignored.
an ideal rope has to fulfill two things: it should be very light but also very strong
 
So Spider Webs.
 
7:13 PM
@Semiclassical I am reading why newton developed the idea of derivatives to define speed. Why instead of using tables of motion we use derivatives to find speed.
 
because measuring where something is every second is a heck of a lot less convenient then using derivatives
 
Can I think of a rope as a spring with a high constant k?
 
Really?!
 
infinitely stiff, in fact
 
7:15 PM
Holy ****!
Wow, for some reason, that makes a whole lot more sense now.
 
basically, if you replaced all your strings with springs, ran your calculation, and then tuned all their spring constants to infinity
you should get the same answer as just using tension in the first place
 
That's amazing, but I have one question --
A spring with infinitely high k wouldn't stretch in the first place ...?
 
in real strings, the two conditions sure do compete against each other i suppose
 
Two conditions? Can you clarify?
 
3 mins ago, by Semiclassical
an ideal rope has to fulfill two things: it should be very light but also very strong
 
7:17 PM
Ah, right.
 
it seems like the lighter something is, the easier it'll be to deform
 
Why is Lipschitz continuity a notion of "continuity"? What's the similarity with the usual notion of continuity in the sense that we can draw the graph without lifting the pen?
 
and that's probably true, but not enough to be a problem
 
So the tension in a rope would be mathematically equivalent to the spring constant in a spring ... that's the moral of the story?
 
not to the spring constant, but to the spring force
 
7:17 PM
Ah, right.
 
that said, there's a reason why I said it the way i did above: you start with a finite stiffness, run the calculation, and then at the end take k -> infinity
that avoids any issues with infinity along the way
 
I see, that's very interesting.
But can you please explain how a spring with infinitely high k would stretch in the first place?
 
...again, i'm not saying that
 
Oh, you're starting with finite stiffness
 
right
that one would stretch
 
7:19 PM
And then "taking" it to infinite stiffness
 
yeah. it's an idealization
 
So weird.
 
and a very convenient one at that
 
Thanks so much @Semiclassical. I have a better understanding of Tension & Ropes in general. I've learned:
 
you could also just say that the stiffness is finite, but it's so large that for any practical experiment the stretch is unobservable
 
7:20 PM
* A Rope = A Spring with Finite k --> Infinite k
* A Rope can never be fully taut, because it's own mass will weight it down
 
right. mathematical ropes are infinitely stiff when taut
of course, there's no such thing as an ideal spring either. pull a spring hard enough and it'll break
same with a rope
 
* A Real Rope's Tension increases (exponentially? quadratically?) as we go "up" the rope. This is analogous to pushing a bunch of boxes in contact or going under the sea, where the pressure increases, or similar to atmospheric pressure
 
linearly
assuming constant mass density
same as hydrostatic pressure increases linearly with depth
 
Ah, I see.
 
of course, you could imagine a rope where the mass density wasn't constant
but yuck
 
7:24 PM
ha ha, right
* We idealize a rope to be "massless" because then the tension will be same everywhere along the rope.
 
right
for real-world applications that may or may not be a wise idealization, of course!
 
Right
OK, well, that's it! Thank you again @Semiclassical
 
like if you're using a...huh. what's that called. when you take a bunch of metal loops and link them to get a 'rope'
i guess just a steel chain
in that case, the mass could definitely matter. in particular, it'd mean that if you used the chain to suspend something, the most obvious point of failure is at the top of the chain
it won't be carrying -much- more weight than the other links, but it will be carrying more
@ManasDogra intuitively, Lipschitz seems like a 'speed limit' condition. as in, take the example of a function x(t). then Lipschitz amounts to saying that the average speed of this function is bounded, no matter what pair of times you pick
to get the connection to standard continuity, one probably has to think about what happens when the pair of points you choose get closer and closer together
if the function wasn't continuous, then i think the average speed would ultimately have to diverge as you move the points closer and closer together
like, suppose x=1 for t>0 and x=-1 for t<0. then the average speed over [-t,t] is 2/t, which blows up as t->0
so having a step discontinuity at the origin ensures that the average speed can't be bounded
 
8:20 PM
Brilliant! Thank you so much
 

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