« first day (2323 days earlier)      last day (29 days later) » 

Avi
12:00 AM
in elementary school they gave us batteries
 
The online labs have built-in "experimental error". As in, they have imperfect numbers that come out exactly the same way every time.
did the batteries power anything?
 
Avi
uhhhhh
i nearly burnt my teacher's fingers
 
in AP chem we made batteries
 
Avi
because I used a paperclip and connected one end of the battery to the other
thus creating a short circuit
 
not smart
 
Avi
12:02 AM
because I didn't know what a short circuit was
wisely i was scared enough of what I had done to not touch it
 
what did the teacher do?
 
Avi
but then by the time the teacher got there, it had heated up significantly
and they touched the battery
turns out, they didn't know what a short circuit was either
or they'd have realized instantly that that was a bad idea
 
did they curse? my elementary school classmates loved it when the teacher accidentally cursed.
 
Avi
nono
i've never heard a teacher curse
quinapalus.com/cgi-bin/qat?pat=*%5B%21p%5Dickle&ent=Search&dict=0
ah, it didn't work correctly
it was the qat query for *[!p]ickle
was definitely trying to turn my sickles into pickles
 
why is the entire starboard at 2 stars?
 
Avi
12:08 AM
"Weapon loses its edge after..."
or "Can weapon losing its edge after ..."
@bobble that's because you are a star-2
 
I like doing reviews. I wish there were more. On the other hand, it would be good if posts didn't need reviews. Did I mention that I like doing reviews?
 
Avi
D: I gotta run
 
byebye
 
1:11 AM
0
Q: Swapping 3 rooks in a 3x3 grid

Dmitry KamenetskyThis puzzle was inspired by this one: Swapping rooks in a 4x4 board What is the least number of moves required to swap black and white rooks? Rooks move using standard chess rules (vertically or horizontally), but you do not need to alternate players.

 
1:36 AM
0
Q: Logic puzzle: 19 balls and a scale

Samo MoYou have 19 balls that are identical in all ways, except one ball is either heavier or lighter than the rest. With a balance scale, how could you determine in three weighings which one the odd ball? and whether it’s lighter or heavier?( in a systematic way)

 
2:26 AM
0
Q: Math Puzzle #01 (remade from Math Tapestry Puzzle)

Player1456 The Puzzle is just above this message. The check mark will be whoever can get the lowest pair possible. The smallest pair will be decided by the average of the numbers.

 
 
1 hour later…
3:45 AM
Okay, I've finished all parts of the Making Of post except "Logistical Steps"
Time to look in my grid's revision history :)
 
 
10 hours later…
1:20 PM
0
Q: 19 balls and a scale

Samo MoYou are given 19 identical looking balls and a two sided scale. One of the balls ism different either lighter or heavier. Using 4 weighings of the scale How can you determine which ball is odd and whether it's lighter or heavier?

 
1:45 PM
0
Q: 2 weird problems I came across

cristi0p A man accused of murder hires a lawyer who is always believed by the court to be telling the truth. The prosecution's attorney says that if the accused man committed the murder, he had an accomplice. The defence lawyer says that this affirmation is not true. Did the lawyer help his client by say...

 
2:35 PM
1
Q: The ID of my next Zoom meeting

Bernardo Recamán SantosMy friend is inviting me to a Zoom meeting. Rather the sending me the ID for it, he sent me the following 10 IDs. "Each of these numbers has precisely two digits which are identical, and exactly in the same position, to two digits in the correct ID for the meeting", he told me. My friend claims t...

 
3:33 PM
@GarethMcCaughan, any updates on the CCCC?
 
 
3 hours later…
6:42 PM
@bobble same, my biology labs have just been things like "count the things in 3 random squares and average them all and then count them all up" and "make an arch out of ice cubes"
@bobble apparently not
 
7:26 PM
CCCC: Quaint rubbish, the tail end of someone's stuff from long ago (8)
Sorry about the slowness. @bobble The problem is scarcely ever that I've forgotten about it.
 
no problem! glad to see it up
 
7:39 PM
hmm
 
 
4 hours later…
11:15 PM
Hm, "quaint" and "from long ago" can have awfully similar meanings
 
11:28 PM
Physics help, again: how can you measure the velocity & acceleration of an elevator, while inside it with a spring, meterstick, stopwatch, and set of weights?
 
Well, famously you can't measure its velocity.
 
well, then what about acceleration?
I need to calculate v_max and a when at v_max
 
Acceleration is indistinguishable from gravity. How would you measure the gravitational acceleration?
 
and my teacher has gone over literally nothing about springs
Calculate F_grav with the weight of the objects & g
 
So, just to return to the thing about velocity, because it's important: if you are inside a box that's moving with constant velocity v then there is literally nothing you can do to distinguish that from being inside a box that isn't moving at all. There actually isn't any physically-detectable difference -- aside from interactions with things outside the box that might be moving in different ways.
If the elevator has a glass window or something so that you can see the outside, of course it's a different matter.
 
11:32 PM
I know that the elevator will start at rest (v = 0), accelerate to v_max, and then decelerate to rest
the question specifically says no windows
 
(You're then measuring the velocity relative to what's outside, but that's all you can Oh, wait, you have extra information about what it's doing.
)
 
This is feeling like a puzzle where I wasn't taught the necessary background information :(
 
What do you know about springs?
 
There's something called a spring constant which is N/m
 
And do you know what it means?
 
11:34 PM
but again, my teacher hasn't taught us ANYTHING about springs, but this workbook is due tomorrow
and no, not entirely sure what the spring constant is
 
That's OK -- you don't need to know very much about springs.
Did you ever learn something called Hooke's law?
 
Nope
Should I google?
 
OK then, so that's the one thing you need to know about springs :-).
You could ask Google if you like, or I can just tell you.
 
tell me i'm lazy :)
 
So the behaviour of a spring (at least as long as the forces on it aren't too large) is completely determined by two things. (1) Its length when nothing's stretching or squeezing it. Call that L. (2) Its spring constant. There's probably a standard name for that but I'm not sure what; let's call it k, which might even be standard.
 
11:36 PM
Okay
 
If you stretch the spring to have length L+x, then the force you have to exert to do that (= the force with which the spring pulls the ends inward again) equals kx.
The fact that there is such a k -- i.e., extension and force are proportional -- is Hooke's law.
 
so delta-L = x, and F = delta-L * k
 
Yes.
For real springs, if you stretch or squash them too far/hard then this law breaks down in various interesting ways.
 
I'm assuming this is an ideal spring
 
And of course some spring-like things don't work in compression but do in tension. (Think about, say, a length of elastic.)
Yup.
So, if you hang a mass m on a spring with given values of L and k, what happens to the length of the spring?
 
11:39 PM
F = mg, F = delta-L * k, mg = delta-L * k, delta-L = mg/k
probably should have made those separate
 
OK. Congratulations, you have invented the weighing scale.
 
cool!
 
Now, suppose the elevator is accelerating upward with acceleration a. What happens to your spring while that happens?
 
I don't know, this is the part I have trouble reasoning about
The forces present are confusing to me
 
OK. Forget about "doing physics" and answer this: When you're in an elevator and it starts moving upwards, what does it feel like?
 
11:42 PM
Like I'm being pushed down
(I think. haven't been in elevators recently)
 
What if it starts accelerating downwards?
(Just trying to build a little intuition here.)
 
Like gravity is less, a bit like floating
 
But why? Gravity isn't actually less
 
So in fact there's no way to distinguish, from inside the elevator, between (1) gravity acting in a particular direction and (2) accelerating in the opposite direction. This is called the "Principle of Equivalence", and it was first formalized by a chap you might have heard of called Albert Einstein.
 
11:45 PM
Okay, I'm with you so far
 
So. Suppose the elevator is accelerating upward with acceleration a. What do you reckon is the equivalent thing that could happen to gravity to make it feel the same?
 
g becomes g + a
 
(It's a bit unnecessary to formulate all this in terms of what would happen to gravity, but I think it's not a bad way to think about it. We'll talk briefly about another way later.)
Right. So what's the length of your spring now?
 
delta-L = m(g + a)/k?
 
Right.
What about when the acceleration goes the other way (when it's slowing down from v_max to 0 again)?
 
11:48 PM
delta-L = m(g - a)/k
 
OK. So, which of these things can you measure?
And which do you already know?
 
m, k (outside the elevator), delta-L, and g is constant
 
Are you allowed to do preliminary experiments outside the elevator to measure things?
And are you told the value of m?
 
I think. The weights have known mass, and I calculated the spring constant (not really understanding what it was) beforehand.
 
Do you mean some earlier bit of the question said that you did an experiment that ends up finding the spring constant?
 
11:51 PM
Yes, there was a previous question that was just weights, meterstick, and spring. I got k from there
(didn't think it was important to mention. sorry)
 
OK. (You can actually do without that, by being a bit sneaky with your measurements inside the elevator. But apparently there's no need :-).)
Right. So, it seems to me that from the things you know and the things you can measure it's pretty straightforward to find out the value of a at any moment.
There's a bit of a difficulty around the fact that when the acceleration changes the spring can't really respond instantaneously, but I assume we're supposed to ignore that issue :-).
 
Assume long enough elevator ride for evening out, I guess
 
Are you told that the elevator accelerates with constant acceleration from v=0 up to v=v_max, and then decelerates with constant acceleration from v=v_max down to v=0 again?
 
Nope
 
(I'm guessing yes.)
Oh.
So in that case do you have a way of recording from instant to instant what the acceleration is? Because you're going to need to do some numerical integration if you want to get velocities out of this...
That seems slightly unlikely to be what they have in mind; you might want to double-check whether constant acceleration is implied somewhere.
 
11:55 PM
^^ you can read the problem
 
I think that phrase "the upward acceleration a that the elevator has as its speed increases to maximum" implies that a is constant.
 
Okay, that is good. I wasn't sure if it counted.
 
OK. So it seems to me you have everything you need to find a.
 
So I start stopwatch, then measure initial a.
Once a changes (the spring goes haywire), I record the time
And then measure second a.
 
It looks to me as if they haven't actually asked you to measure what happens as it slows down again.
 
11:57 PM
measure a = take delta-L and calculate
why do I have a stopwatch?
 
So I'm not sure you need to measure the second a. (But if you didn't already have the value of the spring constant, but knew that the acceleration and deceleration were of equal magnitude, then you could have used that to figure out the value of a. You don't need to care about that.)
"why do I have a stopwatch?" is a good question. Here is another question: does your procedure, so far, compute all the things they have asked you to compute?
(hint: no)
 

« first day (2323 days earlier)      last day (29 days later) »