# The Classic Physics Problems.

Beer-monster
The "Classic" Physics Problems.

In my physics education/career I have often heard people refer to the "classic" such and such a problem or to the "typical" or "archetypal" physics problems. These are the problems/models that many/most university physics problems seem to reduce down to, no matter the particular context of the problem.

I thought it'd be interesting to ask a forum such as this, with so many physicists of different levels of experience and areas of specialization, what a list of these problems would like.

So imagine if a student stopped you and asked "What are the classic problems of physics?"

My own thoughts (so far) have offered:

1) Collisions between two bodies.

2) Classical simple harmonic oscillator

3) Quantum harmonic oscillator

4) Body moving through a central potential.

5) A "Blackbody" type of calculation (like a Fermi gas)

Look forward to seeing some interesting suggestions/discussion.

## Answers and Replies

Naty1

You may have to define 'classic' in whatever context you are considering.

To keep things simple, consider the equations in freshman physics, like FT =mv for example
and pick whatever you consider significant. Or consider the titles of a freshman physics textbook...those will be your fundamentals.

Aristotle's 'classics' were probably not the same as ours.

Mentor

- Motion with constant acceleration
- Double slit experiment
- Thought experiments with relativistic trains
- Hydrogen atom

Like this?

Beer-monster

You may have to define 'classic' in whatever context you are considering. .

True, but that is often difficult, as 'classic' used in the sense I am using it can be very subjective. Let me try an analogy.

A 'classic' rock station plays an array of hits, usually determined by their age and/or agreement with the standard characteristics of rock music from the 60-80s.

However, if you went around asking people what THE 'classic' rock songs are: You'd get a range of answers based on taste but you'd probably find that people would gravitate on certain songs like "Stairway to Heaven" and "Back in Black." There would then be some discussion about which of these is really "classic," which is sort of what I'm trying to foster here but for physics.

Another way to think about it is in terms of "archetypes". In various conversations (admittedly usually with theorists) people have seemed to think there are only a limited number of "distinct" problems/models in physics. Most of the other problems are variants or extensions of these archetypes.

The harmonic oscillator is a gold example. I, and some others I know, have noted that many physics classes contain (at some point) a review of the SHO. This is because the essential physics of so many systems and situations can be modelled/approximated/understood in this paradigm. However, the little details, such as the origin of the restoring force, are dependent on the context.

I thought it would be interesting to discuss this idea of "typical" or "achetypal" problems her at PF, and see what various people would suggest those problems are.

Beer-monster

- Motion with constant acceleration
- Double slit experiment
- Thought experiments with relativistic trains
- Hydrogen atom

Like this?

Yes. Though, I might generalize the double slit to interference of waves from multiple sources. That's a matter of opinion though.

I'm curious as to why you would mention though experiments on a train? As an exercise in understanding relativity it is undoubtedly useful, but would you say it could be used as a more general model for a range of systems?

DragonPetter

The plate capacitor derivation, including the small gap to plate area approximation.

DragonPetter

Another good one with capacitors is shorting 2 capacitors in parallel and determining their steady state voltages that shows an apparent violation of the conservation of energy/charge.

http://www.physics.princeton.edu/~mcdonald/examples/twocaps.pdf

It is referred to at least in my intro physics book (resnick,walker,etc.).

DragonPetter

Another one that I have encountered in multiple intro physics courses is the escape velocity problem. Also the velocity needed to acquire orbit (variation was shooting a cannon ball fast enough that it would travel around the earth and hit you in the back before falling enough to hit the ground). Also, the terminal velocity problem seems to be common (I studied it in both intro physics and differential equations courses).

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There's too many to list. Parabolic trajectory. Field of a charge distribution with some kind of symmetry using Gauss's law. Field of a charge next to a conductor. Orbits around a central potential. Particle in a box. Carnot engine. I could go on and on.

Beer-monster

I suppose a key element is a degree of crossover i.e. How applicable is the model, and it's key physical insights, to different areas of physics or different systems.

For example, though Gauss' law for symmetry charge distributions is a common problem in undergraduate physics course, how applicable is the physics outside of electrostatics? Even in electrostatics it only works for a few select systems.

That said, there is Gauss law for gravitational fields so maybe...

The model of a particle in a box, however, can be extend to many systems (a first approximation to anything with discrete energy levels) such as quantum dots or photons in a cavity. But the particle in a box could, perhaps, be considered an extension of a standing waves problem?

What do you guys think?