h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Try teaching general relativity to a 13-year-old...that's why.

B

MOST* 13 year olds, i would love to have learn about it at school when i was 13

Master B, I too am chemist. Never used it though. But I have to admit that I find it strange that everytime we move potential scientists on a gear, GCSE to A-Level, then A-Level to Degree, we often get told that most of what you learned before is wrong. The idea of electrons orbitting the atom is a prime example. From the idea that they're solid objects circling the nucleus in circles all the way to wave-particle duality theory and quantum mechanics. It can be very dis-heartening

You just killed my faith in physics. You really did just burst the bubble that had me happily thinking of electrons as zipping bits of energy (golf balls) going in perfect circles round the nucleus (plasticene and other balls).

The only way I could handle electrons was to think of them as a regular energy pulse around an even bigger energy pulse that was the nucleus.

In case Kat's question was to me:
Imagine a ball with mass (like a planet). By Newton's law of gravitation (yes, superceded by Einstein but still good to first order), the field of gravity exerted by that object (ball, planet) is proportional to its mass and inversely proportional to the distance away from it that the field is "probed" - V (G-field) = Gm/(r*r) (G is the constant of proportionality). To get the force, you have to include the object subjected to the field: F=G*m(1)*m(2)/(r*r). Now this equation has spherical symmetry. That is, all points on a sphere (shell) the same distance (r) from the ball have the same field strength. The lines representing the direction of the gravitational pull converge at the center of the ball. On the other hand, in the elevator, the acceleration field is uniform; that is, all the lines of force are parallel (in the opposite direction to the acceleration of the lift).
That's the difference between a gravitational field and an accelerated "reference frame". However, there is no way "locally" (where "locally" for a physicist is very close indeed) to distinguish between them since 2 observers would have to communicate over a distance great enough to sample the divergence (or lack thereof). This introduces clock rate problems and other "curvature" effects.

Was that the question?

Science is all about providing a hypothesis for what we understand:

As GCSE students cannot reasonably be expected to appreciate wave-particle duality, energy levels, exclusion zones, tunneling and the like, we use the 'planet-sun' model as a hypothesis. It works well, because it explains everything required of a chemist/physicist at that level (and, frankly, at just about all levels in the real world). Similar explanations apply to all sorts of science, including phase transformations, imaginary numbers, mechanics, photosynthesis, rainbows and thermodynamics.

The big problem at A-Level is often having to 'unlearn' what was drummed in at GCSE and learn a more accurate form of explanation. This process becomes second nature by degree level, where students really learn to examine the fundamentals behind said processes.

B

Master B, I feel depressed...help me out. I'm just about to launch into AS Physics. I'm going to die aren't I?

Alitni thank you I got at least half of that.

Erm, erm, erm...

I don't know about the 'new' A-Levels, but everyone I knew said Physics was one of the hardest step-ups from GCSE (I never did it myself). The flipside to that depressing bit is that you can concentrate on a few modules which really appeal to you, and the exams are marked at degree-levels (40% pass mark, 70% for an A).

Don't get down, it's a damn useful A(S)-Level to have for many degree courses.

B

"Why do we still talk about Newton when he was superceded by Einstein."

Because it's easier to do the calculations probably and in many instances the results are close enough such as sending space probes to Neptune.

AS physics is basically all about understanding the maths. As to whether its easy or not, I'll tell you tomorrow when I get my results.

Physics has nowhere near the step up from GCSE that maths does (but then chemistry is a doddle compared to physics (NB take this with a pinch of salt and the knowledge that I got a B in Physics a C in Maths and a D in chemistry...although I blame the heavy coursework component and my I can't be a*sed attitude)). *But* the crossovers between maths and physics reduce the overall workload by about 1/6th because mechanics is about that weighitng in both subjects

You all might as well be talking Greek now, but that's okay I guess.

Physical science is difficult I think because it's hard to visualize stuff that's expressed in algebra basically. In fact it got so difficult after awhile that Einstein thought he needed to get back to the basics and he was probably right because Newton was visualizing a universe which really didn't resemble the universe we have to live and work in especially at the limits of our perception.

It's one thing to say that the motion of an apple falling from a tree and the motion of the moon circling the earth are related. It's quite another thing to extrapolate that into some universal clockwork that is totally predictable once we know all the initial conditions and can specify a formula, a Lagrangian, whereby those conditions change.

Seems like a no-brainer at first but then some chaotic behavior intrudes seemingly out of nowhere and the thing's toast.

But you can show algebraically if not visually how repetitive or cyclic processes can become chaotic, that is unpredictable in principle as well as practice. So there's really no Lagrangian after all even if we could know all the initial conditions which we can't really.

I'm not taking maths though, so I have to have little extra lessons, but I can't see when as I only get 4 frees a week and 30 minutes for lunch a day!

Awww, I just found my old physics exercise book!

*copperplate writing*
Newton's Third Law *underlined neatly*

Action and reactional forces are equal and opposite. If body A exerts a force on body B, then body B exerts an equal and opposite force on A. These forces do no cancel each other out, as they act on different objects.

Having done further maths (if anyone is considering taking this, don't do it to yourself), basically anything vaguely tricky in my Physics AS was covered in maths classes.

Although if anything this made last year even more unbearable. If you taking into account that Physics is maths, I was doing 13 hours of number pushing lessons per week. How I'm going to face going back I don't know.

Well, there you go then. What you're feeling is the non-canceled aspects of the equal and opposite forces - gravity acts uniformly (at this scale, anyway) on all of your body, while the "Earth pushing back" force pushes only on your feet. The structure of your body has to do the rest of the pushing-against-gravity in order to keep you from collapsing into a puddle, and that is what you feel.

...although if you're one of those infuriatingly annoying people who finds maths easy, double maths and physics is a walk in the park...

You all gonna help me when I start physics and cry?

A walk in the park that will drive you over the edge of insanity. Unless you actually enjoy maths. Yes those people actually do exist

Let me ask a general perceptual question. How many people here can feel the weight of their pancreas, or the fifth lumbar vertibra? Or the index finger?

How many have noticed that when your arm goes to sleep, it feels heavier maybe?