Anyone good at physics?

[Sandstone-Shadow]

I'm currently taking AP Physics C (if that means anything to anyone; if it doesn't, it's an advanced physics class basically) and struggling with it. It's an early class which starts before everyone else's school day, so it's difficult to concentrate to begin with, and my teacher's teaching style does not help me at all. I'm going to explain how the class is set up because I think that's part of why I can't keep up with it; it's just such a detrimental structure for me. We have online homework assignments pretty much every night that we print off (which, by the way, leaves very little room for writing, a fact that is infuriating because I need space to organize my work) and work on. We submit the answers online as well; it tells you instantly if the numbers are right or wrong, but it does not give you any sort of hint if you got the wrong answer; it just spits a red x in your face.

We go over some questions in class, but pretty much only the day before it's due (all assignments are due at midnight the day we would go over problems) but this requires having done the assignments ahead of time so you know what problems are hard and require going-over. Working ahead is something that just doesn't work for me in this class; there's too much to do. There is a forum component to the online homework, but this is ridiculous since the teacher preaches to us to use the forum, but then she rarely checks it herself and when she does, her "helpful replies" are confusing and very, very concise.

Now, she has started pulling us out of our study halls into physics class to do assignments if we didn't get a 100% on them that night. I use my study hall for other homework; I'm not getting the other homework done at night because I'm working on physics that I don't understand, and I can't work on it during study hall because I'm being called out for physics that I still don't understand.

Moral of this huge post: is anyone good at physics that would be willing to help me out? We're doing angular motion, energy, and momentum right now. I really need someone who can help explain some of this, or at least someone who can give me some tips so I can keep up with this class better.

 

[shadow_lugia]

I might be able to help with a couple of things, but probably not. I'm only getting into high school next year, and have never studied physics except in my free time from astrophysics books, which I enjoy reading. So, I doubt I could answer any questions unless they involve, say, black holes or something, but I may as well try to help.

 

[Sandstone-Shadow]

We're doing stuff like calculating moments of inertia and torque and angular velocities and stuff like that. So if you know anything about that... if not, thanks for the attempt, I appreciate it. =)

 

[shadow_lugia]

Oh... No, I'm not any good at that, sorry. Usually when I see "physics" I think "atoms," which I kind of get. Depending on what torque is, I might know something (probably not, since I have no idea what torque is :P). Otherwise, sorry for not being able to help.

 

[Sandstone-Shadow]

Thanks anyway! =)

 

[Minnow]

If you'd posted this earlier, then I'd probably suggest dropping it, if the work really was too much and you wouldn't lose much by dropping it (I don't know if you're taking it, basically for fun, or because of some physics requirement thing you're going for), but since the year is almost over now, I think it's best to try to tough it out. Perhaps you could talk to the teacher a bit about the problems you're having?

How do the other students seem to handle it?

 

[Sandstone-Shadow]

The teacher doesn't really ever help. =/ She explains things but the way she explains is so confusing and crooked. She makes mistakes all the time, too; it's hard to really listen to her explanations because you also have to be checking to make sure she's not making a mistake. Everyone in the class pretty much agrees with me. =/

Ha yeah I wish I had dropped it though.

 

[Murkrow]

I might be able to help you. With bits of it at least, I might not have done the same things that you are. (I wasn't there when we did torque in physics but I got the question right in the exam so I think I'm okay at that)

What sort of questions are they, like what information is usually given and are they usually in diagrams or sentences?

 

[Kali the Flygon]

Is anyone good at physics? Boy are you lucky...

I'm currently working on my doctorate in physics, and could be done this year. I already have a bachelors and masters. I suppose that would qualify me as 'good'.

How much math have you had also, by the way? And do you have a way I can talk more real-time with you, like MSN messenger or something?

 

[Lorem Ipsum]

I was going to offer my help, but then a Ph.D student turned up and stole my thunder! No worries, I'm only up to GCSE level Physics anyway, which probably wouldn't help you.

 

[Sandstone-Shadow]

Awesome! I'm currently in Calculus AB, but I feel that as far as math goes I'm fine, unless calculus deals with some physics stuff as you progress in it. The problems are usually word problems, such as this one:

A particular tower clock similar to Big Ben has an hour hand 2.70 m long with a mass of 51.0 kg, and a minute hand 4.20 m long with a mass of 100 kg (Figure P10.40). Calculate the total rotational kinetic energy of the two hands about the axis of rotation. (You may model the hands as long thin rods.)

So what I did is I used KE=.5Iω² for each clock hand and added them together, using I=(1/3)ml² and ω for the minute hand = 2π radians/60 minutes, and for the hour hand, ω = 2π radians/720 minutes. This didn't get me the right answer, though, so I guess I must be missing something.

 

[Flitterbie]

I=(1/3)ml² only works when the rotation is around the center of mass. Since the axis is on the end, you want to use I=(1/12)ml².

At least, that's what I remember. Also, I'm pretty sure time for the hour hand would be 720 minutes, since it takes 12 hours to complete a rotation.

 

[Sandstone-Shadow]

Hmm, our book taught us the other way around, although I'm not entirely trusting of my book. I tried it that way though; it didn't work. =(

Ah, right, I did use 720 minutes; I reduced it later and put the wrong number up there.

 

[Kali the Flygon]

To be honest, what you wrote there now looks right. Are you sure you didn't make an error in the mathematics of your calculation?

Yes, it's 1/3 for a rod about the end, and 1/12 for a rod about the middle. One way you can check that is by imagining a rod about the middle as two independent rods about the end.

 

[Sandstone-Shadow]

Pretty sure. =/ I tried it a few times and got the same (wrong, apparently) answer each time. Is it fine to use radians/minute for the angular velocity? I'm never sure exactly when to use which units.

Hmm, okay. So if a rod about the middle was two independent rods rotating about the end, their lengths would be half... so (1/3)m(.5l)² = (1/12)ml². But wouldn't you have two of those rods? How come it isn't (1/6)ml²?

 

[Flitterbie]

To be honest, what you wrote there now looks right. Are you sure you didn't make an error in the mathematics of your calculation?

Yes, it's 1/3 for a rod about the end, and 1/12 for a rod about the middle. One way you can check that is by imagining a rod about the middle as two independent rods about the end.

...Dammit, why do I get those two mixed up so often?

Typically, angular speed is measured in radians/second, though, as long as you're consistent, it shouldn't matter. But you'd likely want to use whatever units the book gives the answer in.

 

[Sandstone-Shadow]

The answer is supposed to be in joules at the end. And joules are (kg m²)/s²... ohh, my velocity should be in m/s then, shouldn't it?

Edit: Still didn't work.

 

[Flitterbie]

What's the book's answer?

 

[Sandstone-Shadow]

I don't know. The assignments are online; they use problems from the book but change all the numbers so you can't just submit the book's answer. =/

 

[Flitterbie]

Then how do you know you have the wrong answer?