Worst TAing/Teaching experience.

[Redbelly98]

You are incorrect. It will work with any units, as long the as the graph is not stretched or squished, because slope is a ratio.

I think you misunderstood what I was saying.

For example, we could have been graphing displacement vs. time, and want to calculate the slope in order to find the velocity.

Suppose the plotted line goes 2 grid boxes upward for every 1 grid box to the right, and each vertical grid represents 0.1 m and horizontal grid boxes represent 0.5 seconds each. The slope of that line would be

(2 × 0.1 m) / (1 × 0.5 s)
= 0.2 / 0.5 m/s
= 0.4 m/s​

However, my student would just use the grid boxes and said the slope is 2/1 or 2.

 

[junglebeast]

Suppose the plotted line goes 2 grid boxes upward for every 1 grid box to the right, and each vertical grid represents 0.1 m and horizontal grid boxes represent 0.5 seconds each. The slope of that line would be

(2 × 0.1 m) / (1 × 0.5 s)
= 0.2 / 0.5 m/s
= 0.4 m/s

However, my student would just use the grid boxes and said the slope is 2/1 or 2.

Yes, that is a stretched graph...but if each grid cell was X units wide and X units tall (for any X), then you could just count the number of boxes for rise over run to get the slope. This will work for any choice of units as well.

 

[Redbelly98]

Since one axis is measuring displacement and the other axis measures time (in my earlier example), it is impossible for them to be the same unit.

 

[junglebeast]

Since one axis is measuring displacement and the other axis measures time (in my earlier example), it is impossible for them to be the same unit.

I was not talking about your graph specifically, just pointing out that the method of determining slope by counting grid squares will work for graphs with any combination of units; so for example, not only will it work if the unit in both axis is "0.1 meters" but it will also work if the vertical axis has units of "0.1 meters" and the horizontal axis has units of "0.1 seconds". In both of these cases it will give correct slope in terms of "meters per second." Additionally, it will work to give a correct slope even on "stretched" graphs, although the units are then different. This was the mistake made by your student: he calculated a numerically meaningful value for slope, but it was no longer in the units you were expecting.

 

[MATLABdude]

I thought I'd take it nice and easy one semester, focus on research, and not TA. Then they asked me to mark for a computer interfacing course I'd never taken. "It won't be too bad, there's a marking guide, only 5 or 6 assignments [about 1 per fortnight] and it's right up your alley! -ish. Plus you're the only one remotely capable who isn't assigned to something else."

Turns out the prof was teaching 2 other courses, and dealing with various personal issues at the time (sickness / death of a relative, etc.) So no assignment solutions. And the course wasn't quite as easy (based on what I'd done previously) as I had hoped. Long story short, I ended up showing up to class, making model solutions, obsessing over model solutions to try to make sure I was right (to be fair, the prof would glance over my model solutions to make sure I was on the right track), and then marking 40ish assignments. That sucked.

So a few days before the final, I get some e-mails asking if I can get the prof to post the model solutions? I ask if he can post his model solutions (he'd said that he'd take care of it) and then he responds by asking if I can give him my model solutions! Not totally unreasonable, but I was caught off-guard by this. So after hauling butt and getting the solutions corrected and typed up that day, I give them to him for posting (approximately a week before the final).

Finito, it leaves my mind. So all of a sudden, I get a few more panicked e-mails asking when the solutions will be posted, so I e-mail him suggesting that maybe he should send an e-mail out to the students pointing out that the solutions were posted on my website. During the weekend before the final (Saturday night, or Sunday afternoon, I forget which, final on Monday), they show up on his website.

Moral of the story: know what you're jumping into! And nice and easy is never correlated with jumping into the unknown. I think I ended up putting in 3 or 4 times my allotted 3 hours per week (they paid us by the hour, but you only got so many hours) as a result of that course.

 

[humanino]

$ds=cos(x)\rightarrow \frac{ds}{s}=co(x) \rightarrow d=co(x)$

I think it's a rather comic situation. While in class, a student made quite a decent calculation on the board and came up with the result
$$\frac{\sin\theta}{\cos\theta}$$
and thought he was done. I loved my math teacher, I swear he was wearing the same clothes every single day, I'm sure he had 5 or 10 identical red shirts. Anyway, obviously he was expecting a tangent, so he insisted on the last step, but the poor student did not remember it. So the teacher took the chalk, and wrote
$$\frac{\sin\theta}{\cos\theta}=\frac{in}{co}$$
For a few moments there was despair in the student's eyes. Few of us burst into laughs, and the teacher apologized.

 

[humanino]

Oh there was also this other time when the physics teacher was drawing a solenoid, a current flow, the direction of the field, erased this and that, then draws the expected trajectory, and ends up with a huge smiley on the black board. As soon as she realized why we were laughing, she erased it promptly.

 

[Moonbear]

There are some quantities that must always be positive. One cannot have a negative number of apples, for example.

Hmmph! Of course you can! You make it a positive number by planting apple seeds. (Afterall, if the spreadsheet can have negative dollars, surely one can have negative apples for the ones owed to someone else.)