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# Kepler's Laws

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Joined: Oct 2005
Kepler's Laws

In my AP Physics class, we just went over Kepler's laws. He said that these laws were true for any object that orbits another. I asked him if these laws also applied to electrons orbiting around an atom. He said that since they are traveling at or very close to the speed of light, that other laws came into play. Does anyone in this group know of these laws and/or any equations?

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Joined: Oct 2005

In general, gravitational laws can apply to big objects like planets, but you need quantum physics to apply it to small objects. Keplers laws would also not apply to black holes. If you want the answer to your electrons, just go google, Maxwell, Schrodinger, or de Broglie and you'll probably get your answer. I don't know a whole lot about it, but I can point you in the right direction.

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That's right. When you get to the point where you're dealing with objects so tiny that the force of gravity is negligible, quantum mechanics comes into play, and the world of newtonian classical mechanics is turned upside down. It's difficult to easily understand the ideas, energies, forces, etc involved in modern physics, because they are all so counterintuitive. but luckily most high school physics courses don't require much knowledge in this field. AP physics B does require you to know about the energy, wavelength, and momentum of electrons, the de broglie wavelength, and a couple other things that escape my memory at the moment. the AP physics teacher at my school did not cover modern physics. i had to learn the stuff on my own. hopefully your teacher will eventually cover that subject for you. if not, learn it as soon as you can on your own.

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one of the formulas we use is Fc=Fg: Centripital force equals Force of Gravity

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Joined: Oct 2005

That works for planets, not electrons. For electrons, you need college level science, I don't know a whole lot about it. You can transform them into de broglie waves and find their wavelength, but I don't know much more about anything else. Sorry

You know you're an AP student if...

you think studying is fun.
you constantly find yourself saying "we had homework?"
everything you know about sex, you learned in english class.

If you try to fail, and succeed, which have you done?

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Joined: Nov 2007

Well, the things we learn in mechanics is known as classical mechanics. It works all the time except when 2 things occur:

1. it approaches to the size of an Anstrom. Then, you need another branch of physics known as Quantom Mechanics.
2. if it travels to the speed of light. In that case, you need the theory of relativity.

Other than those, you are safe to apply Kepler's Law and every other classical mechanic formulas

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Joined: Nov 2007

Well, Anton is partially wrong.

Classical Mechanic works for all motion except for 2 exceptions. One, if the object is really small, usually at the size of an Anstrom. Another, if the object travels in the speed of light.

For really small objects, it would be Quantum Mechanic. For speed of light, that would be Eisenstein's Theory of Relativity. Mainly, when an object travels really close to the speed of light(since no object can get to speed of light, but let's say 0.99c c= speed of light = 3 x 10^8) then mass and time is no longer a constant. In another word, time is delay..and mass is increase.

I just realize that I answer the same problem x2. LAmo T_T

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