# Can't understand physics

1. Feb 12, 2014

### piisexactly3

I'm currently studying A level physics independently as an older student and I'm finding it hard to make sense of some the concepts. For example, its amazing how many textbooks tell you that F=ma but make little or no effort at explaining why even though the simple ice-skating analogy is sufficient.

So I read through a textbook on something, and then I might go online to get a good explanation (because that's what I love about physics, the explanation!). This sometimes works but the free internet doesn't actually have all the information like some people say.

Let me talk about how, as another example, electromagnetic waves confuse me:
take the graph function for a transverse wave. So what does the displacement axis represent? Is it the displacement in physical space a path of energy is taking? Is it a displacement in energy? Then you get this equation for intensity I=P/4pi(r^2). Ok so the waves come out in spheres? Are the waves pulsing spheres of energy? Then you go into diffraction or reflection and the spheres turn into rays or beams? Then you get told light is a wave and a particle at the same time? So I look further into this online and it seems you get experiments that say that light is a wave not a particle and experiments that say light is a particle not a wave. So it must be neither right? But the maths says its both?

So maybe you can see why I feel like giving up, but at the same time, I reckon as you get more advanced and closer to doing actual science and being an actual scientist, it HAS to make more sense right? (with the exeption of quantum mechanics:tongue:)

2. Feb 12, 2014

### Dishsoap

First of all, I respect you for trying to learn physics on your own. I can't imagine how difficult that would be.

Second of all, I think you ask some very good questions about EM waves. Hopefully someone who knows some more about E&M will come across this thread soon, because all I understand about it is that it is related to the Poynting vector's magnitude.

Every physics student struggles with the particle/wave nature of light. Light is both. Sometimes it appears to have wave-like properties (diffraction, etc.), and other times it has particle properties (photoelectric effect). As the mathematics says, it is both, however depending on which experiment you do, different attributes of the photon or EM wave appear more prominent. In fact, all matter is like this, since at the smallest scales an electron acts like a wave. Not sure if you have gotten to this yet.

You would think that as you get more advanced, the physics begins to make more sense. In my experience, though, the closer you get to being a physicist, the more you realize that the universe is absolutely insane, can never make up its mind, and that you really don't understand a single thing about science.

3. Feb 12, 2014

### ZapperZ

Staff Emeritus
This is why, to really get a good understanding of physics, and have a coherent, working knowledge of it, you cannot learn physics via bits and pieces! You'll read things that appear to be contradictory, or appear out of nowhere. What you have done is simply to get snapshot of things, without all the underlying "stories" that lead up to that snapshot.

Unfortunately, there are no shortcuts to learning physics, as opposed to learning ABOUT physics. Learning Physics involves a series of progression of knowledge, where after you understand something, only then can you progress to something more complicated and sophisticated. For example, your point about light having a wave-like and particle-like (note how I phrase this) properties is one clear instance where, unless you actually learn quantum mechanics and quantum electrodynamics, we can't help but simply have to just TELL you its behavior via all these phenomena. The alternative is for you to fully learn basic physics, learn QFT, and then learn QED and THEN realize that this all has only ONE consistent physical description, not two, and certainly no "duality" of any kind (see the General Physics FAQ subforum).

Now, the question is, are you willing to put in THAT much effort in learning it if you are simply doing this as a hobby? Do you expect to be able to play the violin as well as Itzhak Perlman simply by just reading books on how to play the violin? If not, then at some point, you have to settle with the idea that you have to take our word for it, that the information and knowledge are out there for you to learn, but it might be beyond your reach to fully comprehend them.

Zz.

4. Feb 13, 2014

### piisexactly3

Thank you for the replies so far. I hope what I'm about to say doesn't appear to have a hostile tone because it doesn't and I'm actually very appreciative of the time people give to help other people on these forums, I just like to be a bit frank in order to keep the discussion clear and concise.

@samnorris: so physics works more like magic than science?

@ZapperZ: lets forget I mentioned anything about wave-particle duality and look at the much simpler questions that preceded it. Would you then still maintain that I needed to learn QFT and QED to have some adequate grasp on EM waves, say the kind of grasp that physicists had before QFT and QED came along?

5. Feb 13, 2014

### ZapperZ

Staff Emeritus
No, but you do need to know Maxwell equations! The "wave" in EM waves is the solution to the wave equation by combining two of the Maxwell equations. What is oscillating are the E and B fields. I can show you the detailed mathematical derivations, but this isn't something you want, is it? Yet, there it is!

Again, the problem I see with what you want can't be done in bits and pieces. In this example alone, it is very clear that a lot of "homework" has to be done to be able to fully understand classical E&M to know what the oscillation is in EM waves. I can simply tell you (which is what I did), but what actually have you learned in terms of having an understanding of EM waves? Practically nothing. You have a piece of information, but you have no "frame of reference" to place that information in. You can win at Jeopardy with that information, but you cannot build a waveguide or an antenna with that information. It is useless.

Zz.

6. Feb 13, 2014

### piisexactly3

Thanks Zz. Yes at the moment, I don't need to know Maxwell's equations or how to build an antenna for my exams but I can't wait to get to that stage!

So the displacement axis shows the change in electric (and magnetic) fields over distance. So is this field strength oscillating between a negative and positive charge? This puzzles me particularly when I think about gravity. I originally thought of a body of mass having a gravitational field around it which decreased in strength as r increased. Now it seems that gravity comes in waves (I won't go into gravitons), which I can only interpret as a body of mass whose field strength varies as r changes but we still get the overall effect of E increasing with r. The graph would look like a sine wave with increasing amplitude?

I'm studying physics course that is typically for 16-18 year olds in the UK, where I will be looking to study physics at university after. I'm becoming increasingly aware that isn't really physics in the sense that we are given a deep enough understanding to manipulate the information and overcome small engineering problems. So it puzzles me why, for example, we learn so many formulae to plug with but derive so little. Wouldn't it make more sense to learn the science in a more chronoligical fashion and work with the more fundamental conclusions in physics at this stage?

7. Feb 13, 2014

### mal4mac

F = ma is Newton's second law of motion, there is no explaining why, just accept it, it's fundamental to classical physics. Do you demand an explanation of why 2 + 2 = 4, or are you just happy to accept it and carry on? Then again, I'm told that Russell and Whitehead produced hundreds of pages proving 2 + 2 = 4. If you can't accept F =ma , maybe philosophy of mathematics/physics would be more up your street than physics?

8. Feb 13, 2014

### ZapperZ

Staff Emeritus
You are asking this in the wrong forum. Topics with actual physics content must go into the physics subforums, not here in the Academic Guidance forum.

To paraphrase Jack Nicholson, you can't handle the derivation!

Zz.

9. Feb 13, 2014

### lisab

Staff Emeritus
You're too young for a deriver's license.

:tongue2:

10. Feb 14, 2014

### piisexactly3

So you think its right that we learn to plug with suvat before deriving it with some simple differentiation?

Last edited by a moderator: Feb 14, 2014
11. Feb 14, 2014

### ZapperZ

Staff Emeritus
You are beginning to learn about the rules and how to use them. When you were growing up, you were told certain things you can and cannot do. The rational for such rules were never explained to you because you were not capable yet to comprehend why.

You will have PLENTY of opportunity later on to discover the origins of many of these ideas, when you are equipped to understand them.

Zz.

Last edited: Feb 14, 2014
12. Feb 14, 2014

### Staff: Mentor

Just as a reminder: This forum (Academic Guidance) is for questions about school or university (which modules/classes/courses should I take, which universities should I try to get into, general questions about studying, etc.), not for specific physics questions. Those belong in the actual physics forums. In this case, General Physics would probably be the best place. Make sure you remind people there of what level you're at, because they may not be reading this thread.

13. Feb 14, 2014

### piisexactly3

'discover' that's the word! That's what's so appealling about science. Surely you must recognise that as a fundamental reason for why people get put off it at an early stage when it is taught more on authority like a religion. It makes it mundane and stamps out independent thought (which should be practiced from the start not just when you get to university)

Last edited by a moderator: Feb 14, 2014
14. Feb 14, 2014

### ZapperZ

Staff Emeritus
But you haven't considered the fact that if we try to present this to you the way we understand them, it MIGHT SCARE YOU OFF! That's also an unintended consequence if the material presented is too complicated for the level that the students can understand it. If you don't believe me, look at what happened to the class Richard Feynman taught at CalTech! How many freshman do you think were left by the end of the semester?

If you think you know better, you are welcome to write a proposal to change the pedagogy of how the schools in your part of the world teach the subject matter. Otherwise, I'm done with this.

Zz.

15. Feb 14, 2014

### piisexactly3

If you don't believe me, look at what happened to the class Richard Feynman taught at CalTech! How many freshman do you think were left by the end of the semester?

Quote: Zz

You expect me to believe you after providing 1 anecdote? Is that scientific? You're welcome to disagree with me if you just want to plain disagree with me, I just expected something more than 1 anecdote after opposing me for so long in this thread.

Secondly, I could write out a proposal for different teaching methods but in fact the onus is on people who uphold the current system to justify why it is better than all other proposals. It's an important issue and so those who uphold it better have some pretty good evidence.

So likewise, you're welcome to show me how how letting pupils discover some basic science for themselves ends in disaster.

16. Feb 14, 2014

### ZapperZ

Staff Emeritus
Please note that you haven't provided a single evidence in support of what you are proposing. So read what you had written and apply those same requirement on yourself.

My other evidence is the fact that we have trained numerous physicists out of the current educational system.

Zz.

17. Feb 14, 2014

### WannabeNewton

Instead of wasting time complaining why don't you start learning more advanced things on your own and sate your own curiosity? If you wait for everything to come to you then you'll be waiting for a long, long time.

18. Feb 14, 2014

piis3exactly, make sure first that you have a good grasp of GCSE physics before you look at the more advanced A level stuff. Also,if you can find it have a look at the Nuffield approaches to both O level and A level. The Nuffield emphasis leaned more towards learning by discovery and by thinking stuff through.You may not have access to the experiments but the books should give you the information the experiments reveal. As an example at O level students used to show That F= Ma by doing experiments with trolleys and tickertimers.
Also a lot of the equations and derivations are mathematical descriptions of things which are obvious. Think of pushing something with a certain mass which accelerates as a result . Now imagine doubling the force on that mass.Now imagine using the same initial force but doubling the mass. How's the acceleration likely to change in each case?

19. Feb 14, 2014

### piisexactly3

@ Zapperz: I think you've misunderstood what I said in that you must have solid reasoning for opposing a mere idea and the notion of thoroughly testing that idea.

@WannabeNewton: Had it occured to you that I am considering teaching physics as a profession? Am I still wasting time discussing teaching methods?

@Dadface: Thanks for the helpful post. I should mention that F=ma does indeed make sense to me but its interesting to see how many students know a good explanation.

Lastly, I too have gotten tired of this thread, so unless anyone has anything genuinely constructive to add, I won't be saying anymore.

20. Feb 15, 2014

### mal4mac

I second that! I did Nuffield 0 and A level and they were great courses, the A level textbook was really good.