/sci/ - Science & Math

he is even drawing autistic diagrams about his alleged reasoning
He really believes his crap

it is 200 you brainlet

>>9324247
Take another look at the FBD. As far as I can tell, strings don't resist shear forces, and that "bar" with the F1, N, F2, would show bending.

>>9324310
We don't know that. We can't calculate it.
If you look for the exercise on any mid to serious physics webpage, you will always see the correct answer being marked as 100N.

It is the only good answer that one can get with the information provided.

Ask him to stretch the rope pulling only one end of it

>>9324317
Draw an FBD on the pulley. Or pretend that one end of the scale is blocked

i've done the experiment before, the answer is 100

Tell him that when you are holding something from an "immovable" object such as a wall, you are in essence just having something that provides you with a counter Force to whatever you put. Example, when you hang your dynamometer from a ceiling your object pulls the spring in it which is connected to the whole apparatus so it pulls it, but then the wall acts on this force and maintains the appartus fixed after some time. For it to be fixed also in position only the spring can stretch and will change the reaction force in terms of the reaction force given by hooks law. This kinda looks like the chicken or the egg, but that's because the simplification we assumed in the machine, and the fact that adding an immovable object is a really strong hypothesis, so in the most autistic way possible it's no exactly true, but after a perido of time the equilibrium will happen because at that position the forces are balanced.

Start at either mass. Since it's not moving, the downwards force of 100N must be exactly cancelled by 100N upwards to the next immediate piece of rope holding it. Then just keep repeating this along the entire rope until you get to the other mass.

>>9324389
it doesn't take a whole lot to see it's in equilibrium, but that's not what the problem is asking

>>9324397
>How can I show a fucking idiot that the tension in the rope/scale is of 100N?

>>9324247
>>9324252
I will clarify, since I forgot to give his 'numbers', and maybe it is necessary to understand his mental disability.
He believes that the scale will read (which is the tension) 200N.

Because if it was 200 the weight would fall on the floor

>>9324364
Makes sense. It's fundamentally no different from hanging it from a heavier object.

could i get 1 (just one) written out, non picture, sum of forces that shows that this is 100N?

>>9324247
>How can I show a fucking idiot that the tension in the rope/scale is of 100N?
Get him to replicate the experiment.

>>9324247
it reads zero you retards

the system is in equilibrium so the forces balance each other out, 100 on the left and 100 on the right

>>9324460

>>9324460
how does it feel to be this retarded

>>9324460

>>9324460
t. Brainded

>>9324489
>>9324484
>>9324483
>>9324465
is the scale moving?

no.


therefore the net force on it is zero.


simple stuff...

>>9324494

>>9324460
my right arm weights about as much as me left arm therefore they both weigh nothing

>>9324247
cover up the right side of the page and say "pretend the scale is attached to a wall, what is it going to read?", if he's only mildly autistic he'll say 100.

Then add the right side of the page back in and explain that since the system is in equilibrium, it should be no different since instead of a normal tension with a wall, its just replaced with an equal and opposite weight.

>>9324460
you do realize it's asking for Tension right? not Net Force?

>>9324247
if you draw the free body diagram of one weight attached to a wall on top of the table, the force in the scale will still be 100N, and the wall will be pulling on the string with 100N. You can represent that force the wall exerts with another 100N weight.

>>9324489
>>9324484
>>9324483
>>9324465
none of you can provide the result, what are you people on about?

why this board fucking sucks this bad? isn't there like one fucking physicist who can adress the OP's question? what the actual fuck?

>>9324864
Everyone has already answered the question itt. There's really nothing else to it and it will measure 100 because at that force applied to the spring (which ks then applied to the other weight) you get equilibrium. Brainlet may be just a fucking meme, but it's not that difficult to picture even without much knowledge fr mechanics. Saying it's 0 is just being dumb.

>>9324323
This

>>9324864
Jesus christ this board does not exist to help you with your grade 4 homework, you could determine the answer to the ops question any of the following ways: thought experiment, inspection, google, read a book, and finally perform the ops experiment yourself. Given the incredible ease of conducting any of these research methods constructing an argument becomes moot as ops friend is clearly a retard beyond critical thought.

>>9324247

>>9324460
does that mean if I grab a random person our weights will cancel out and we will float, nice.

>>9324975
honestly the only thing that actually helped me understand it itt wasn't any of the intuition, etc. it was >>9325098

>>9325229
peak pure mathematician

What about this situation? What will scale read?

>>9325233
50 Newtons.

I'm a physics brainlet, my instinctive answer was 200N. The problem lies in how your friend thinks the scale works, not how forces work.

>>9325233
50, at least once it's in equilibrium assuming the scale doesn't fling onto the floor

>>9324247
Shouldn' the tension of each string be 100N and the scale read 200?

>>9325235
But the 100N answer is still true if you remove the scale.
No information about the scale is provided, anyway.

>>9325237
No. If that was true. It would mean that a free falling system of a string and a weight (ignore atmosphere) would experience 100N of tension. Think of it.
I provide you with that mental experiment

>>9325238
I know. He probably thinks the scale adds the opposite forces into a scalar value.

>>9325247
Let's rewrite what I just said: he thinks that the scale can measure two vectorial forces by just adding the norms.

>>9325098
this is wrong
in one equation you're saying J = ma
in the other you're saying a = 0
and finally you're saying J is somehow 100, even though it is a result of multiplication by 0
lmao

>>9325280
I wrote F = ma as a starting point before resolving in the horizontal direction towards the right. Tension acts towards the left whereas the weight acts towards the right.
As for a = 0. It has to equal 0 since the system is in equilibrium

>>9325237
Think of it this way.
First the right weight is on the floor. You start pulling the string 10N, 20N, 30N. The weight doesn't move until you reach 100N, then you start lifting it and you are stuck at 100N even if you pull harder.

>>9324975
>this board does not exist to help you with your grade 4 homework
a grade 4 homework which apparently can't even properly solve. i'm not here to read your half-arsed answers based on your nonexistent physics skills you retard. nobody here needs your dumbass input on how easy it is to solve a question you can't even fucking explain why.

>>9325233
>>9325234
>>9325236
it wont be 50, it will me
T=2g m1 m2 /(m1+m2) where m1 and m2 are masses of the weights.

>>9324247
Ronnie Coleman grabs your right arm and screams in your ear "ERRYBODY WANNA BE A BODYBUILDER NOBODY WANNA LIFT NO HEAVY ASS WEIGHT" then Jay Cutler approaches you and grabs your left arm and whispers to you "Come on kid, suck it up." They both proceed to pull on either half of your body in opposite directions. You hear the sound of your shoulder bone snapping out its socket. You can finally feel it. Your body is starting to actually rip apart into two pieces. Luckily there is a physicist nearby and he examines the situation. It is obvious to him that the tension from both bodybuilders cancels the other out and just as your spine snaps in half he explains how it is physically impossible that the sum of the tension would be 200 newtons.

>>9325346
>Hello academic advisor this is anon, another anon called my physics skills nonexistent on the internet i'm dropping out.
>>9325280
You cant identify f=ma and you cant perform basic algebra, is this the power of american education?

>>9324247
think about how that scale works.

First you have to hold it, only then you can hang a weight and measure it.

Therefore on the right side there is 100N weight or there could be 200, 500, 2000N weight or it could just be attached to wall, or you could be holding it. t doesnt matter, what matters is the weight on the left
the answer is 100N

>>9325407
No, that's the tension of the rope in an atwood machine. It will read 50. Jesus Christ the brainlets.

>>9325490
look here brainlet, both weights are 1 kg, scale reads 9.8 which is in accord with what i said,
IF YOU CAN PROVE ME WRONG POST EXPERIMENTAL PROOF AND CALCULATIONS

>>9325539
yeah 1kg is 10N(or 9.8N if you are autistically precise) are you retarded?

>>9325233
obviosuly 250 brainlets
first imagine hanging a 300N weight on a wall
then imagine adding a 50N weight on the opposite side
the forces counteract, so the reading is 250N

>>9325570
of all incorrect answers possible 250N is the most brainlet one

>>9325580
i think saying "0 since it's in equilibrium" woul dbe the most brainlet response

>>9325233
If the scale is free to move, it won't show the real weight of the object, since it's moving. If the scale is isn't free to move, the scale will show the weight of the object that is on the side which measures it.

>>9325539
Nah, it's fixed. You set up the scale beforehand to read -10.

>>9326340
>t. I spend half of my pension on tin foil hats

>>9325539
>hurr durr the equilibrium position is in acording with my shitty formula
No shit retard, so does the other models, the point is off when one weight is heavier.

>>9325098
Well done Sr. Isaac

>>9324247
Fuck the rope and use symmetry.

>>9324247
this retards had the same problem, and not happy with obviuos lab results, they destroyed some cars :^^^)
https://www.youtube.com/watch?v=-W937NM11o8

Long time since A level physics, come at me bros:

For every action there is an equal and opposite reaction, right? So if one end of the scale were attached to a wall not a weight, (and the scale would then read 100N, yes?) the wall would be resisting the pull of the weight with a force of 100N. Am I right so far?

So if you replace the wall with a 100N weight, you are not making any material difference to the force acting on that end of the scale. So the scale still reads 100N.

How did I do?

>>9325233
the scale will move untill it is stoped somewhere (sbd sshould calculate) and there it is going to have a different tenson probably

>>9326394
Better than the average engineer. Grats anon, you are correct.

>>9325484
if the weights are not equal the system is not in equilibrium and thus accelerates toward the side with more weight which will reflect on the measured force

the scale obviously works symmetrically as you can attach a weight to either end

my hypothesis is that the scale divides the sum of opposing forces on both ends by 2 so if you attach 100 N to one end and 200 N to the other end the system will accelerate toward the end with 200 N and the scale will measure 150 N

I am curious, can anyone here confirm or deny my guess?

>>9326417
Tell me does the scale reads something when you don't have a weight in one end?
This >>9324323 should have ended the debate.

>>9326420
in an idealized scenario it will not

in a realistic scenario it will due to the acceleration and the inherent mass of the measuring device

>>9326425
I just want to mention there are obviously other factors too but that would be the most significant in a realistic scenario

>>9326425
The spring may oscilate a bit because of friction and aberrations in the floor yes, but if you can actually calculate that without using an idealized friction (which in turn will only end afecting the acceleration of the apparatus and not the ideal measurement) then by all means, give a better model. But if you aren't autistic and want a concrete and measurable answer, the dynamometer will read 50 newtons if the apparatus holds to just slide at a constant acceleration .

Fucking brainlets, it's all because of relativity.
>attach first 100N
>attach second 100N
>The moving part of the scale moves relative to the stationary part of the scale.
>Only one end of the rope experiences a force (relative to the other)
>So only 100N is applied to the scale.

>repeat from the perspective of the moving block
>see that the scale still reads only 100N

>>9326459
Your answer is correct but you cannot use relativity per se because you still need to explain why a part of the scale is stationary which is not a hypthoesis or can be considered a priori. It's just balance of forces considering the scale has a fucking spring inside it. Your method would break for
>>9325233
As you cannot fix your frame to the base as it will not be inertial.

Being a time-traveler I can solve this easily.
The scale will read 100N, as it's identical to a merchant's scale used from ancient times up to the late 20th century. The main types were the scales for goods where you had goods on one side and weights on another. The other type was used till the 90s for weight measurements (the last time I encountered it was at a pediatric clinic in 1996): to weigh a person you had them stand on the scale and you had to move a weight along a nearly horizontal bar to adjust the lever-force against the person standing (the lever allows for far lower counter-weight, as a multiplicator is introduced). The second type was later also used for kitchen scales, my grandmother still used one around 2005, she has switched to a digital scale since then, though.

>>9325233
The forces are not equal, so there will be acceleration. Acelleration depends on the masses.

The net force is 250N.
F = m * a
250 = (m1+m2) * a
a = 250/(m1+m2)
This is the acceleration of each block.

Considering the block on the left:
F = m * a
T-50 = m1 * a = m1 * 250/(m1+m2)
T = m1 * 250/(m1+m2) + 50

Considering the block on the right:
F = m * a
300-T = m2 * a = m2 * 250/(m1+m2)
T = 300 - m2 * 250/(m1+m2)

The scale will read T.

>>9326578
It's not a three block systen, there's a fucking spring in between them.

>>9326582
Does the spring have a non-negligible mass? If it doesn't have a mass, you can ignore it and consider the system as two blocks connected by a rope.

>>9326435
Thanks to your input I have realized my hypothesis is bullshit. Updated hypothesis: the measured force is [math]x\cdot (1+\frac{x}{x+y})[/math] whereas x is the smaller and y is the larger of the forces attached to the scale.

>>9326578
Continuing, considering that:

m1 = 50/g
m2 = 300/g

We have that T = 85,71 (approximately)

>>9326587
No, the spring as it streches mantains the left block tense and in equilibrium while the other block just pulls everything (the first box and the measuring apparatus) with a resultanr force of 250. But the scale reads 50. Again a spring is only tense if it's being pulled by both sides.

>>9326591
Scratch that, I fucked up my formula.

>>9326595
There is no spring. It is just an ideal device to measure force. You are over-thinking it.

>>9324247
>pic

Neither, the pulley's remove some of the weight. A tiny amount, but its there. Seems like this shows that, >>9325539

You guys are over-thinking the problem. It is a textbook problem. Everything is idealized. So, there is no friction, rope and scale have no mass and don't stretch.

>>9326600
>let's us assume some bullshit dynamometer that is physically
Tell me how would you built an appartus that can measure forces (mecanical without) without some pivot or rest? Also considering is was a continuation of OPs and you are basically throwing the explanation of why it's 100 out of the window. The point of these problems is to get a proper conceptual understanding of Newtons law, not to show us that you can solve (incorrectly considering that it's not the same tension in each rope for a three block system) some shitty intro problem.

>>9326613
see
>>9326611

>>9326597
Here my updated hypothesis formula, this time it does not matter which force you choose for x and for y.
[math]\frac{2xy}{x+y}[/math]

If x = 100 N and y = 100 N then the output is 100 N. If x is arbitrary and y is 0 N then the output is 0 N. If x is 50 N and y is 300 N the output is ~85.714N which is the same as >>9326594

>>9326620
I think this time my hypothesis is accurate. Fuck me, how did it take me 3 tries to get it right? I really have become rusty and retarded.

>>9326618
Then it's a laughably ill posed problem. What the hell is an "idealized" measuring apparatus? If you go to the lab and try with a dynamometer you will get 50 in that configuration. Forget the blocks, just consider you ar holding it with your hands, no gravity or friction or whatever, and you are some sort of god an can pull with one end 50 and the other 300, what do you expect to read? Obviously if have no understanding of how the measuring apparatus works you fan have different answers. If you said a ok, I have some bullshit that let's me measure the tension in rope in a direct non invasive way then ok it's just a retarded way to pose the problem, but if you tell me dynamometer that uses a spring is in between the two blocks, then the apparatus will measure 50 which a lot of people find difficult to understand. Otherwise the problem wouls be solve the atwoos machine.

Generalization:

F = m * a
F2 - F1 = (m1+m2) * a
F2 - F1 = (F1+F2) * a/g

a/g = (F2-F1) / (F1+F2)

F2-T = m2 * a = F2 * a/g = F2 * (F2-F1) / (F1+F2)
T = F2 - F2 * (F2-F1) / (F1+F2)

T-F1 = m1 * a = F1 * a/g = F1 * (F2-F1) / (F1+F2)
T = F1 + F1 * (F2-F1) / (F1+F2)

So, if F1 = F2 = 100, T = 100
if F1 = 50, F2 = 300, T = 85,71

>>9326642
Let's say you have a scale and a 50N block on it.
If it is at rest, it will read 50N.
If it is accelerating up, it will read more than 50N.
If it is accelerating down, it will read less than 50N.

>>9326648
this post makes me want to smash your face

>>9326650
No, that's the whole point. To make it easy, that's why you can think of a basic dynamometer as a spring with known hooks coeficient etc etc. The spring streches only if it has a force being applied at both sides. Usually the second force is not considered because you just hang it from a ceiling, but the ceiling acts as a way to apply an oposite force. In this case you have 300 newtons going right and 50 going left so the spring will strech till the point where there's an equilibrium with the left forces, then you have 250 newtons still pulling the whole thing but not stretching it. You can try it, go see if you can strech a spring by just pulling from one side.

>>9326666
Nice quads.
If you pull anything 300N going right and 50N going left, it will accelerate to the right. It may stretch, it may oscillate, but it will accelerate.

>>9324460
This is bait

>>9326677
Yea, bur the question is what's going to be measured by the fucking apparatus. As I mentioned, ita painfully obvious that it's a dynamometer in OPs pic. And yes, the thing will accelerate to the right being pulled by a force of 250 newtons. Now, you can even add mass to the apparatus and solve it.

>>9326682
Yes, I solved it. See:
>>9326648

>>9326687
Are you just fucking with me at this point?

>>9326692
What is your answer for the 50N-300N problem? How much will the scale read?

>>9326702
50 for the third time you fucker. It then will move as if the whole apparatus is being pulles by an external force of 250 newtons to the right.

>>9326705
How about 50N-50N? The block would be pulled by a force of 50N and stay at rest (or constant velocity). Right?
So, 50N makes the block stay at rest. To make it accelerate, you gotta apply more than 50N.
A scale that accelerates does not measure the same thing as a scale at rest. It is a non inertial frame of reference. There are fictitious forces involved.

>>9326716
50n was discussed previously, and yes you get that the thing doesn't move. Also, the measuring apparatus is in itself accelerating yes, but my analysis comes from a lab frame that is inertial. If you have a spring in a frictionless table and you pull it, will it expand? No the whole thing is going to move uniformly as a rigid body, the fact that it's in a non inertial frame doesn't mean that it magically read all forces, that's not how the apparatus work. If you are thinking about an accelerometer, also attached, then of course. But a dynamometer is supposed to measure load forces directly, it can not measure it's own acceleration.

>>9324247
>my face when free body diagram

>>9324247
if you don't know how to explain this, then you are also an idiot.

>>9324247
just show him action reaction you stupid piece of wigger trash

>>9324465
>>9324483
>>9324484
>>9324489
>>9324517
>>9324531
falling for obvious bait

you are the real retards

>>9325233
AHHHHHHHH
DYNAMICS
AHHHHHHHHHHHHHHHHHHHHHHH


the scale only reads whatever loads its resisting, in this case it'll be in max tension of 50N before it starts sliding with down with the 300N weight

finding out what the scale will read in intermediate instances will require more assumptions or more knowns such as friction coefficients, masses, etc.

how does TWO things p ulling on it only count as 1 thing? WTF

>>9327506
for a simple intuitive ideea, turn the whole shit upside down

>>9327594
so each 100 n only produces 50 n of force??

pic related this thread

>>9326966
>im just going to attack a first post i see to look cool in front of 4channers and post a picture of a girl who i jack off to but im too beta to ask her out

>>9327597
No, imagine a 100N weight hanging from the ceiling on a rope. The rope will have 100N tension, correct? So the system shown in the OP is really the same thing, but you can imagine either weight as the 'ceiling' from which the rest of the system is hanging off of, because the system is in equilibrium.

>>9324323
damn. i was cconviced it was 200N. Finally all this shit makes sense. a wall would also pull with 100N

>>9324460

>>9326281
this

>>9324247
Scale reads 100N
Brainlet-tier explanation:
Imagine the same scales attached to the ceiling. Now attach the 100N weight to them. The scales will read 100N, hopefully I don't have to explain why.
Now you can write down the forces affecting objects in both the original scheme and the "ceiling" scheme. You will notice that forces affecting the scales in those two schemes are literally the same.

>>9324402
>He believes that the scale will read (which is the tension) 200N.
It will read 200N.

>>9325539
>the masses weigh 5 each

>>9324603
Are you sure about that? I think the fact that the weight is hanging off the table, forced downwards by gravity, adds force that wouldn't be replicated by a weight attached to a wall.

>>9327909
look at this retard

name the tard
blame the tard

>>9325233
It gets dragged to the right and falls onto the floor

>>9328084
Are you deliberately trying to sound unenlightening or...

>>9327917
Why the fuck not? I wouldn't have so much trouble explaining if tou actually gave your reasoning.

remove the left side and replace with immovable object, then ask what the tension is.

then replace with the equal weight object, and ask what the difference is.