The Evidence For What You Learn In School

I'm so tired of trying to discover what the evidence and supporting arguments for particular scientific theories. Society doesn't care about any of it, and as a result it's trying to obtain this information can be like being a damn archaeologist, sifting through the internet and textbooks and pop-sci and what few historical documents are available online trying to piece together the full picture.

So I propose a thread dedicated to bringing together all the evidence for the scientific knowledge that they teach you in school and expect you to take for granted. Ideally this would take the form of a separate web-site, but a thread will do for now.

Naturally scientific evidence isn't black and white. The value of almost any observation, or the validity of almost any argument, could be debated for hours. No amount of static prose is going to be able to provide a completely convincing argument for any scientific theory, unless it's a really trivial one. The information has to be turned over and discussed and analyzed before a person can be expected to really feel convinced. So don't worry if you can't make it perfect. We can do that discussion in a different thread.

The one "rule": write your posts as if you're talking to Aristotle. Or Archimedes, if you'd prefer a later thinker. The point is, write for an imagined audience who is intelligent, but ignorant of all of our current science. You wouldn't convince an ancient Greek philosopher by just telling him that "scientists have demonstrated that...", and you wouldn't be able to make assumptions about their "knowledge" about the physical world. Don't explain optics in terms of waves unless you're also prepared to prove that light is indeed a wave. Don't explain electronics in terms of electrons unless you want to describe the basis for atomic theory as well.

This is not a thread for explanation of phenomena! This is a thread for the evidence for scientific theories about those phenomena. Don't say "combustion happens because oxygen combines with a fuel", say "scientists claim combustion happens because oxygen combines with a fuel, and here is the evidence in support of that statement...".

Anyway I'm too tired to post anything right now, and I probably won't be much use anyway. The only subjects I could really talk about are classical mechanics and a few basic chemistry topics.

I can strongly relate with your frustration, this is why I focused entirely on mathematics until recently. Anyway, this is a great idea for a thread.

That matter is made from atoms: I think it was one of Einsteins 1905 papers on the pressure produced by Brownian motion that proved this. (On the Motion of Small Particles Suspended in a Stationary Liquid, as Required by the Molecular Kinetic Theory of Heat) we might like more demonstrations of the atomic hypothesis, though

That light is an electromagnetic wave: Maxwell develops a unified theory of electricity and magnetism based completely on experiments (which are described in full) then calculates the velocity of propagation of EM waves from the electric and magnetic "permittivity" constants, it is in agreement with the velocity of light. volume 1 volume 2

Atomic theory began with John Dalton, who justified it at least in part by the law of Multiple Proportions.

Newton's First Law
A force is defined as that which causes an object to accelerate, and it is also implied that a force is exerted when two objects collide. In the absence of a force, the object does not accelerate. This is simply based on the observation that any time an object accelerates, we can find a source for a force that might be acting upon it, such as the earth's gravity, a magnet, or a collision. In the absence of such possible acting forces, objects always continue at constant speed in a straight line. Rocks continue to fly even when released from the hand. Balls continue to roll after being kicked. There really isn't any way of "proving" this law to be true. It's simply a hypothesis: thinkers looked at the world and came up with it to explain what they saw. It turned out that we never found any contradicting evidence, so it stuck.

Newton's Second Law
This law is really just a definition of mass. We find that exerting the same force on different objects has different effects, and conversely, different forces are required to produce the same effects on different objects. Lifting a large rock is harder than lifting a small rock (we can justify that how tired our arms are is a good measure of the strength of a force by noting that shoving an object harder causes it to accelerate more). And if you find a way to always generate the same force (such as by dropping a weight on a pendulum from always the same height - and noting that when applied to the same object the object always accelerates by the same amount); you will find that applying it to different objects causes them to accelerate by varying amounts.
Thus it is obvious that how much something accelerates is due not only to the force acting on it, but also to some intrinsic property of the object. We call this property "mass". We define a way of measuring mass by taking some unit mass and some other mass, and saying that if a force F produces an accleration A in the unit mass, and an accleration A/k in the other mass, the other mass is k times larger than the unit mass. Thus we have DEFINED mass such that F = ma.

Newton's Third Law
This law is sometimes called "trivial", but in fact the reverse is true. This is the only law that is actually somewhat non-obvious and requires careful measurement and experimentation to be proven. I actually cannot find any descriptions of experiments that might do this. Two are mentioned in a textbook I have, but one involves magnets, and one involves springs. The magnet one is unsatisfactory because for all we know, the third law might apply ONLY to magnetic forces, and the spring one is unsatisfactory because analysis of the results would likely involve Hooke's law, the proof of which requires the third law as far as I know, so it would be circular reasoning. (http://www.lightandmatter.com/html_books/lm/ch05/ch05.html#Section5.1)

Hooke's Law
Take a vertically-suspended spring and attach various masses to it. You should be able to find this equipment easily if you have access to a school physics lab. You should find that the length of the spring is proportional to the mass. The force of gravity on an object is proportional to its mass (justified by the fact that gravity accelerates all objects at the same rate, coupled with Newton's second law), and the force on the spring is equal to the the gravitational force on the object (this is justified by the assumption that the spring is the only thing exerting a force on the object other than gravity. We then note that the object is not accelerating, and that therefore the Spring->Object force is equal to the gravitational force on the object, and that therefore by Newton's third law the force of the object on the spring is also equal to that). Thus the length of the spring is proportional to the force on the spring. Flip the spring upside down to verify that this works for compression as well.

Request: an explanation of how a scale works, and proof that this provides an accurate measure of an object's mass.