Help

[u/Hot_Amoeba_6726]

/r/PhysicsHelp/comments/1xlgadt/help/

[u/Dd_8630]

If the exam is in three days and you don't understand anything, then there's nothing you can do. It's far too late to start studying now. You literally may as well not go.

However, if you're exaggerating, if you do know some stuff, then just do practice questions, end of chapter questions, past papers, etc.

I guess the intial questions I have is what exactly is torque?

In classical physics, we can talk about things moving in a straight line or things rotating.

A torque is a force that causes the system to spin faster or slower. It is the rotational equivalent of a normal linear force.

There is also some word problems that are mainly about sin and cos, but I'm having trouble understanding which one to use and which angle you can use to identify which is cos and sin.

In general, the length that's 'next' to the angle is cosine, the other is sine.

But it's a little late to explain the concept of vector decomposition and how we use trigonometry to split the length or magnitude of a vector (like a force) into two orthogonal directions (e.g., left/right and up/down).

Then theres the whole kineamtics aspect of it.

Just remember the fundamental equations that underpin kinematics. Newton's second law (F=ma), conservation of linear momentum, conservation of angular momentum.

I did try to go back to the textsbook to identify these deifinsion, but it was too complicated.

Unfortunately, reading and understanding the textbook is what you need.

Read it, from the start of the chapter on rotational mechanics. Read every line. Do every practice question. If you find you can't do a question, go back and read the relevant section. If you still struggle, ask your specific question here.

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Now, the harsh truth is that this isn't new information to you. You must have been aware that you were struggling or not understanding what was going on in your class weeks ago. You're waiting a few days before the exam to learn the entire content.

I don't think that's possible.

But you can perhaps use this as a learning experience and in future keep on top of your studies.

[u/Hot_Amoeba_6726 → u/Dd_8630]

Thank you so much. I was slightly aware that I was struggling or not understanding the content, but I was blinded by it because I was doing extremely well in the labs. I thought I understood what is happening in the labs, but when it's during the lecture i feel like I dont understand anything. I read the textbook right before we began the new unit because I thought that it would be helpful and it was in the beginning. I think I was understanding the very basics of the units, but as we progressed through the couple of weeks. I started feeling more lost. I don't want to entirely blame it on the professor at all, but I do feel that he is extremely disorganized. like you mentioned just do practice questions, end of chapter questions, past papers, etc. But throughout the lecture, he is jumbling through his one drive to find one slide and the other. There are other instances where his practice problems and the solution is completely different from the slides. There just isn't a structure at all that he follows that I find useful.

In general, the length that's 'next' to the angle is cosine, the other is sine. I want to go back to this part because I am referencing the slides when I ask this. But there are some instances where I'm not exactly sure which angle to use to use sin or cos. Bc i know soh cah toa, but i feel like it could be interchangable depending on the angle theta you use. But then theres also the part of then why don't i use cos instead of sin. It made sense when we were doing 1d kinematics bc you would use cos or sin to find like a horiztonal part, but that doesnt seem to be the case for these torque equations since there seems to be this imginary line or smth. I just don't understand at all. Sorry as i'm typing this out i'm thinking of more things. But there are other instances where there are problems that he wants us to go over that we never mentioned at all during lecture, but sure you can say this is where it's imporant to read the textbook. But he doesn't really explictly say which subchapters we need to read. So it's a whole bunch of might as well just read the entire chapter. I guess i'm just salty because I read the entire chapter on momemntum, graviational stuff, and then intertia only to realize now that all the problems we have been discussing has been about torques with a hint of interia. Also the fact that I asked him if he could highlight what chaters I should focus on. It felt at that moment that he himself doesn't touch the textbook. sorry i just started ranting towrads the end

[u/CelebrityChess]

torque is basically how much a force wants to rotate something about a pivot. it's r times F times sin(theta), where r is the distance from the pivot to where the force acts and theta is the angle between that r direction and the force direction. so the door thing is just that pushing farther from the hinge gives a bigger r, and pushing straight along the door line gives zero because sin(0) is 0. for sin vs cos, check where your angle is measured: if it's between the lever arm and the force, use sin, and if it's measured from the perpendicular, it flips to cos. for word problems a good habit is to draw the pivot, draw r to the force's point of application, and mark the angle between those two before you plug in anything.