Cort's 3/4 Physics Thread

[Cort]

I didn't know if I should be asking here or in the other threads. In the other threads, I don't want to derail the author's original questions. Anyway, I'm not the brightest crayon in the pack-- so expect some kind of brain aneurysm doused with induced vomiting, face palming and enough hilarity to make yourself ask: 'you're kidding, right?'. This is complemented with me holding your hand and letting out a constant whine.
 Nonetheless, I am one stubborn bastard and I'm eager to learn. If I'm looking at things the wrong way, or you've got extra ideas/information that you'll want me to be aware of, please do tell. Tell me your secrets. Oh, good buttocks, tell me all your secrets.

Working through holiday homework, this was one of the 'common' exam questions that we were given.  I'm trying my best to understand the question, but it's these type of questions that makes me go blank. How would you work this out?  By 'describe', in physics exam, do you always must show working out if need be? Or is it all written?

[Aurelian]

Okay, so, to begin, we're told that in each of the two situations the same car is involved and the same breaking force is used.

This in turn implies that the car will decelerate at the same rate in each case.

So, to start off - what is the definition of acceleration? And, given this, what graphical feature will acceleration correspond to on velocity vs. time graphs, as we have here? (NB: We actually have a speed vs. time graph here, but I won't distinguish between velocity and speed for the time being).


EDIT: Also, "describe" in the context of VCE physics generally means words, but often it will be helpful/necessary to include a little bit of mathematics/working (that's not really the case here though).

[Cort]

Okay, so, to begin, we're told that in each of the two situations the same car is involved and the same breaking force is used.

This in turn implies that the car will decelerate at the same rate in each case.

So, to start off - what is the definition of acceleration? And, given this, what graphical feature will acceleration correspond to on velocity vs. time graphs, as we have here? (NB: We actually have a speed vs. time graph here, but I won't distinguish between velocity and speed for the time being).


EDIT: Also, "describe" in the context of VCE physics generally means words, but often it will be helpful/necessary to include a little bit of mathematics/working (that's not really the case here though).

Right, the gradient of the breaking force would be ~-33. So if that's the same as the first, does that mean the line would be just on top of the other? Or something like that.

[Aurelian]

Right, the gradient of the breaking force would be ~-33. So if that's the same as the first, does that mean the line would be just on top of the other? Or something like that.

On the right track! =)

To be clear, it's the gradient of the sloped part of the graph we're considering, rather than the gradient "of the breaking force", and the magnitude of this gradient represents the magnitude of the deceleration of the car in the first situation.

As discussed above, this deceleration is constant in both situations. That means that the gradient of the sloped part of the speed vs. time graph in each situation will also be the same.

This in turn means that we expect the sloped part of the new graph (which again corresponds to the deceleration of the car) to either be on top of that of the first graph or otherwise parallel.

Now, in the situation for which we're asked to draw the speed vs. time graph, we start at a slightly higher speed than in the situation whose graph we're already given - 65 km/h compared to 60 km/h. If we assume that deceleration begins at the same time in both situations, what will the new graph look like?

(Note, by the way, that we don't actually have to calculate the gradient to answer this question).

[Cort]

On the right track! =)

To be clear, it's the gradient of the sloped part of the graph we're considering, rather than the gradient "of the breaking force", and the magnitude of this gradient represents the magnitude of the deceleration of the car in the first situation.

As discussed above, this deceleration is constant in both situations. That means that the gradient of the sloped part of the speed vs. time graph in each situation will also be the same.

This in turn means that we expect the sloped part of the new graph (which again corresponds to the deceleration of the car) to either be on top of that of the first graph or otherwise parallel.

Now, in the situation for which we're asked to draw the speed vs. time graph, we start at a slightly higher speed than in the situation whose graph we're already given - 65 km/h compared to 60 km/h. If we assume that deceleration begins at the same time in both situations, what will the new graph look like?

(Note, by the way, that we don't actually have to calculate the gradient to answer this question).

Lovely. Now I understand. Thanks boss.

[Cort]

I've got a question concerning all past and present physics student -- I've only recently started on my physics preparation, and I've taken into account of PB's suggestion to 'conceptually' understand. I've got a bunch of resources at my disposal -- my question is : How do you know when you full 'conceptually' understand? One of my textbook for example presents a bunch of questions, and I'm worried that I won't feel confident if I don't do them. And lastly, this might be due to me being daft - is there a way to understand 'conceptually' beyond the idea of a bloody mathematical formula itself? It's irritating, but I'm trying to get myself to think more dynamically on the subject.

Edit: Another fact that comes into my struggle is my inability to properly remember events/reasons and little things that I should be aware of. Quite amusing how I survived History : Revolutions last year.

[Kanon]

Tricky question, but I'd say if you're able to teach the concepts to you mum or dad (and be able to answer their questions) then I'd say you'd have a pretty good understand of the concepts. For me personally, I look at physics equations as a more mathematical relationship for explaining concepts than anything, for example, Newton's Law of Universal Gravitation states

Rather than just saying wow that's cool, we can see that the numerator is often constant within any system, allowing us to derive

Which is an inverse proportionality relationship, stating that as the radius increases the less force we get. Hopefully this can help you answer/understand the concepts behind if something happens within the system

I hope this answers  your questions

[Cort]

Never got to thank you for that, but thank you. Now there's a little less than 2 weeks to go, but my time wasted attempting to finish my speech has taken up my studying time. I'm worried... should I be doing any form of questions at all, or should I continue learning conceptually? I'm pretty buggered up in balancing enough questions done to understand to wasting time struggling on one.

[Rod]

Never got to thank you for that, but thank you. Now there's a little less than 2 weeks to go, but my time wasted attempting to finish my speech has taken up my studying time. I'm worried... should I be doing any form of questions at all, or should I continue learning conceptually? I'm pretty buggered up in balancing enough questions done to understand to wasting time struggling on one.

Less than two weeks? What date do you guys start school?

You don't need to understand everything at the moment, so don't stress. Try and go ahead as much as you can, even if you understand 50% of something, it will help because when you learn the topic at school it will come to you immediately. As for questions, I've been doing heaps,  if you get stuck you should look for worked solutions on the internet or ask someone on here.

Just keep in mind that most people doing physics haven't even opened the text book, so as much as you read ahead, and have a basic understanding of what's to come, will be really beneficial for you.

Hope that helps,

Rod

[PB]

Hey Cort,
My apologies if I have confused you with my advice. I am simply paraphrasing the very same tip that has been given to me by Michael Li, Istafa and Nliu (lxnl) - all of whom have gotten absolutely ridiculous study scores and 99.95s.

Ok, so you DON'T want to be FULLY understanding everything at the moment. That would be impossible unless you are some kind of savant! What you want to be doing is understanding the main concepts of the course. So, do not go into the little tidbits and minute intrinsicalities of the subject, but merely grasp the basic stuff of each topic yeah? For example - momentum and collisions, just recognise that in every isolated system, momentum is NEVER lost. Thats it. Just the barest summaries of each concept.
 I highly recommend this StudyOn website thingo or the Studyon booklet which my friend lent me, which provides amazingly succinct and straight-to-the point summaries about certain concepts. For me, reading the textbook can sometimes be confusing and extremely time-consuming, having to critically sift through a whole lot of irrelevent garbage.

And by all means, if doing a few questions here and there helps crystallise a certain physics idea in your head then go ahead and do them! Don't let me or anyone else tell you otherwise.
When I said not to do questions - I simply meant not to start diving into every imaginable question to fully understand everything about the subject. You simply won't be able to get through the whole course (my mistake when I was where you are right now).

So my take home message is this - by simply confining yourself to understanding the surface concepts of the subject, you should be able 1. actually get through the whole course 2. get a big picture of the subject.   Which is ideally what you want for each Math/Science subject entering into VCE.

I hope that clears things up If you have any other questions please feel free to ask, I absolutely hate confusing people!

Kind Regards,
PB

P.S. I know the idea of understanding only the surface of your subjects is a little wishy-washy and undefined. However, it really is up to your disgression to know how deeply you should to dig into each physics topic at this point of time

[Cort]

Thank you both for shedding some light -- but it's my fault on my end for assuming so much ( I tend to do so). Nonetheless, I'll take each of your advice and use it with the time I have remaining.

[PB]

Good luck

[Cort]

Do I have to know about Kepler's Laws? the study design my teacher gave makes no mention of it.

[lzxnl]

The first law, for VCE purposes, says that orbits are elliptical (not quite but you don't need the details)
Second law isn't needed for VCE purposes at all, but one consequence is that objects move faster when closer to the sun
Third law is
I trust you know what each variable is.

[Cort]

Thank you for that Izxnl. Since I'm nearly finished doing some scouting for photonics (before I dive into a real understanding), I just wanted to ask: What is the mentality that you have/use/adopt when you're imagining and thinking about frequency waves/signals? It's a concept that I really cannot apply into any situation as of yet, because it feels so paper thin. That kinda sucks because this makes it harder for me to grasp modulation.

Thanks,
Cort.