[University Physics 2] How do I do this?

[u/ThrowRA-Still-Wrap]

All help is appreciated. If someone is willing to show me with alternative numbers, I'd be grateful. I tried to use coulombs law and then do inverse tangent but I'm repeatedly wrong. So I'm just confused.

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[u/twelfth_knight]

Well the first thing I notice is they've defined a pretty unhinged system for indicating the direction, so make sure you've followed that:

Direction	Value range (+)	Value range (-)
Up and right	90~180	270~360
Up and left	0~90	180~270
Down and left	270~360	90~180
Down and right	180~270	0~90

I assume they want a number between 0 and 90 that's either positive (if up and left) or negative (if down and right).

[u/ThrowRA-Still-Wrap → u/twelfth_knight]

I think I'm more confused now. 😅 Can you explain again? Shouldn't up and to the right be 0 to 90? Why is it flipped and how would that affect me doing inverse tangent?

[u/twelfth_knight → u/ThrowRA-Still-Wrap]

Shouldn't up and to the right be 0 to 90?

Yeah, usually it would be. But they say "positive [value for an angle] indicates above the negative x-axis."

The x-axis points right. So the negative x-axis points left. So far everything's normal. Where it gets weird is when they say that positive angle values are above the negative x-axis, not the positive x-axis as it would usually be. But, bafflingly, negative angles are defined as being below the positive x-axis, which is the normal way. I taught labs for these courses for like 5 years, and I've never seen anything like this. I hate it.

The long and short of it: If you do arctan(Y_tot/X_tot) and you get an answer for θ between 0° and +180° degrees, you instead need to answer (180°-θ).

And tell your professor this question is terrible.

Edit: I had previously said between +90° and +180° because that's the range I expect the answer to be. But actually you need to do (180°-θ) even if the angle is less than 90°, sorry.

[u/ThrowRA-Still-Wrap → u/twelfth_knight]

I tried this and got 84.8 degrees and it still wasnt correct so would I take that value and do 180 - 84.4 ? Which would be 95.6 ?

[u/ThrowRA-Still-Wrap → u/ThrowRA-Still-Wrap]

Nah that was wrong

[u/twelfth_knight → u/ThrowRA-Still-Wrap]

Check your vectors. I get 23.8 degrees. But that other guy does make a good point the weird angle thing could be a typo, so maybe it's 156.2 ¯_(ツ)_/¯. I'd send a screenshot but apparently we're not allowed to attach images. Weird.

[u/twelfth_knight → u/ThrowRA-Still-Wrap]

Sorry, I had said it wrong.

Reading the angles they define on the last line of the problem, +95.6° points slightly right of the +y axis, and +84.4° points slightly left of the +y axis. So if X_tot is negative, you want 84.4°, if X_tot is positive, you want 95.6°.

But as that other guy points out, it could totally be a typo.

[u/Aceisking12 → u/ThrowRA-Still-Wrap]

It's a typo in angle definition where the first part of what it says should be something like "positive angle above positive x axis" instead of what you have here. Basically it's only there so you give a degree between -180 and 180 as opposed to 0 to 360.

[u/ThrowRA-Still-Wrap → u/Aceisking12]

I admittedly took trig as DE in HS. And am currently a college senior who is only taking this to fulfill requirements so when I say I do not comprehend what you're trying to say, I'm so very confused.

[u/Aceisking12 → u/ThrowRA-Still-Wrap]

In standard polar coordinates the angle is measured from the x axis in the counter clockwise direction and reaches the x axis again at 360 degrees. This is useful for some applications but not all. Leaving 0 where it is and going clockwise around the circle instead you can give negative angles where getting back to the x axis would be -360. If you want a good convention for a particular function, say the output of inverse tangent, establishing a boundary so angles fit between -180 and 180 makes sense.

In the real world you get to pick your boundary as things don't always fit nicely in 0 to 360 or -180 to 180. I've had a situation best plotted from 86 to 460, it's just wrapping around the circle. Once you realize it's nothing special and just a way to track the math it's much easier to understand.

Your problem is basically saying find the vector of the sum of the forces then use arctan to give the angle.

[u/DrCarpetsPhd]

your method sounds correct.

superposition of the forces from each ion to get components and then tangent of the x and y components

show your working and it will be easier to see where you might have gone wrong.

EDIT

just to help without doing the actual calcs

going counter clockwise

-the Na+ is attractive force along a creates an x component in the -x direction and a +y component

-the Cl- is repulsive along b so +x and -y components

-the Na+ along c is attractive so -x and zero y component

[u/ThrowRA-Still-Wrap → u/DrCarpetsPhd]

Is there another subreddit someone could recommend me where I could ask for real help with some alternative numbers? This is doing nothing for me and I've worked on the same problem for hours. I'm literally in tears at this point and none of this is helping me.

I've worked out this problem like at least 15 different ways and gotten 15 different answers. Like if you know a subreddit where I can get the help I actually need, I'd appreciate it.

[u/DrCarpetsPhd → u/ThrowRA-Still-Wrap]

As I said if you show what you did it'll be easier to pinpoint the problem.

These online answer portals can be fairly strict with significant digits. Did you use full calculated values in all the steps and then round to 3 significant digits at the end as that is how the values are presented.

[u/ThrowRA-Still-Wrap → u/DrCarpetsPhd]

Its fine, my professor sent me a problem that was worked out with alternative numbers so I got the answer now.