# Some dumb Physics questions

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janesixposted 10 years ago

Please, someone who knows more about physics than me, will you help me with a few "dumb" questions?

The reason I say they're dumb is because I should be able to grasp it, but can't.

Here's the first one:

Are inertial mass (as in p=mv where p is momentum) and "rest mass" the same thing? I know they're somehow "different", but I'm not sure how. I can see how rest mass is equivilant to energy. But I don't understand inertial mass.

Light has energy and momentum, so has inertial mass. (Right? or is that wrong?)

But light doesn't have rest mass.

Is inertial mass just another way to say "moving energy", while rest mass is a way of saying "still energy"?

I hope I'm making some kind of sense.

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Rest mass is just mass at rest in a particular reference frame. Inertial mass is mass that is being accelerated by a force that is not accelerated by gravity.

Yes, they are both particles, so they will exhibit energy and momentum.

Light is never at rest, it always travels at 'c'.

Those terms are not used because the cause confusion.

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wildernessposted 10 years agoin reply to this

How does light change direction 180 degrees without curving and without stopping at the point of reflection?

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For the most part, light follows Fermat's Principles. Although, at the quantum level, photons can be absorbed and emitted to follow various paths, depending on the surface.

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wildernessposted 10 years agoin reply to this

And the next question is once the photon is absorbed into the silver on a mirror, how does it "know" what exact angle to leave it at?

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Beth37posted 10 years agoin reply to this

Are you testing him? I hope you are only using nongoogleable questions. (I want full credit for that word.)

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wildernessposted 10 years agoin reply to this

Somewhat.  Someone else (or the same person) said that a while back, but I always understood that photos DO have a rest mass, and that they can be at rest.

Time marches on, though, learning advances and I certainly have not kept up the studies in physics.  Always open to learn new things.

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Impossible, photons are never at rest. The instant they are emitted, they travel at c and never stop traveling at that speed until they hit something and are absorbed.

If they have any mass at all, it is the derivative in which the energy of the photon would have to be transformed into mass.

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I'm not sure if photons actually "know"anything. But, to answer your question, they take the path of least time which also happens to be the least distance path. When reflecting on a very smooth surface, the least time path is where the angle of incident (incoming light) is equal to the angle of reflection (outgoing light).

If point A is the source of light and point B is the point somewhere beyond the angle of reflection, we set the derivative of L with respect to x = zero. Here we get the pathlength from A to B:

L = 1/2 sq/rt. a^2+x^2 + sq.rt. b^2+(d-x)^2

a is the distance from mirror surface to A
b is the distance from mirror surface to B
x is the distance from A to point of reflection along mirror surface
d-x is the distance from B to point of reflection along mirror surface

If we follow through we get:

dL/dx=1/2 2/xsq.rt.a^2+x^2 + 1/2 2(d-x)(-1)/sq.rt.b^2+(d-x)^2 =0

d is the distance from A to B along surface of mirror

This reduces to:

x/sq.rt.a^2+x^2 = (d-x)/sq.rt.b^2+(d-x)^2

which is:

sin0i = sin0r

which further reduces to 0i=0r

Law of Reflection

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MelissaBarrettposted 10 years agoin reply to this

That's crystal clear. Thank you ED.

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My pleasure.

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MelissaBarrettposted 10 years agoin reply to this

Would you believe me if I said that reading that was like reading a Chinese manuscript using a Russian dictionary while underwater...and drunk...and sleep deprived...and juggling...while carrying on a simultaneous conversation with Richard Simmons by text?

I don't do hard sciences. The only reason I have achieved a high-school level understanding is because I have to teach my kids.

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janesixposted 10 years agoin reply to this

OMG. I hope I'm NEVER that smart:) That looks totally painful.

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MelissaBarrettposted 10 years agoin reply to this

I was pretty sure he caught the sarcasm. He knows I'm intellectually limited.

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I'm not sure if I ever mentioned this before, but you too could easily understand that stuff if you wanted. It just takes the same methodical process to get there as anything else. You just start with the basics and work your way up. No problem.

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One thing to remember about physics is that most of it is counter-intuitive and is difficult to grasp unless one let's go of their "intuitive" nature.

An example of this is the earth itself, it appears flat from our perspective, yet counter-intuitively, it is a sphere.

"Counterintuitive means contrary to what seems intuitively right or correct. A counterintuitive proposition is one that does not seem likely to be true when assessed using intuition, common sense, or gut feelings."

http://en.wikipedia.org/wiki/Counterintuitive

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wildernessposted 10 years agoin reply to this

No, the law of reflection merely says that the angle of incidence = angle of reflection.  I'm asking why, as you said that the photon was absorbed.  After being absorbed I cannot see any reason it cannot be emitted in any direction.

In addition, imagine a half sphere, radius B center at point of reflection.  Your math works for every point on that sphere, does it not?  Plus sinOi (whatever angle that represents, you didn't say) - sinOr does NOT mean that angle oi=or.  There are two angles with the same sine function value.  Or, looked at another way, the same angle in one plane and an infinite number in another plane in the 3D universe.

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MelissaBarrettposted 10 years agoin reply to this

Yeah, same thing as with ED, except this time the manuscript is in Yiddish and I'm talking to that kid that played Urkel.

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wildernessposted 10 years agoin reply to this

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So, you didn't want to see the math and find out why it is a law? Did I waste my time showing you that?

I just showed you that.

Sorry, but I don't what you're talking about.

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wildernessposted 10 years agoin reply to this

Did you understand the comment with the sphere?  There are an infinite number of possibilities, each of the same length, to various B's.  What makes one more likely as the same math works for all?

And ED, I'm not one of your religious basketballs.  You want to discuss this, I'm game; you want a punching bag, I'm outta here.

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janesixposted 10 years agoin reply to this

Thank you. I appreciate it.

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Beth37posted 10 years agoin reply to this

I've always said he was a retired science teacher in GB. Maybe someday he will reveal himself.

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janesixposted 10 years agoin reply to this

Lol. Maybe. I know he's got a brain.

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Way off base.

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mbuggiehposted 10 years agoin reply to this

Inertial mass is determined by momentum not gravity. It results from force not gravity (Newton's Second Law).

You can think of it as resistance to acceleration.

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janesixposted 10 years agoin reply to this

Thank you. It's still hard for me to grasp though. There's "gravitational" mass, and "relativistic" mass too. It's just hard for me to decipher what each thing means, whether they are ultimately the same thing in some way (looked at in different perspective, or under certain conditions).

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mbuggiehposted 10 years agoin reply to this

You're welcome. These are tough concepts.

I try to think of gravitational mass as more/less like weight and inertial mass as trying to move that weight. Not only do you have to move X number of pounds, but you've got to get it moving.

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Beth37posted 10 years agoin reply to this

Maybe you are a multi-dimensional learner. Maybe using your eyes and ears at the same time would be helpful. Please don't think I presume to know any of this stuff... I just googled for you.

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janesixposted 10 years agoin reply to this

Thanks Beth. I think I probably am. I will check out your links later tonight. I did watch a couple last night on the Higgs, as it's related to mass, and that was pretty cool.

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Relativistic mass is when an object is moving a near light speeds. In order for any object to move at those speeds, it takes a huge amount of energy to accelerate it, this energy is actually added to the mass of the object making it appear "heavier or larger". However, when the object slows down, the energy to accelerate it is no longer needed and the object reverts back to its original mass state.

With relativity, it works both ways from both reference frames.

For example, if you were in a ship moving towards the sun at very near light speeds, the sun would take on a lot of relativistic mass. If that were the case in reality, the sun would have so much mass it would turn into a black hole.

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janesixposted 10 years agoin reply to this

So, would that be like what takes place in a particle accelerator> Or do they not go quite that fast? I think they accelerate the particles to like 99.9999 percent the speed of light?

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Exactly! However, with accelerators, the energy used to accelerate it is not part of the mass, but instead is coming from the accelerator itself.

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janesixposted 10 years ago

The term “pure energy” is a mix of poetry, shorthand and garbage.   Since photons have no mass, they have no mass-energy, and that means their energy is “purely motion-energy”.  But that does not mean the same thing, either in physics or intuitively to the non-expert, as saying photons are “pure energy”.   Photons are particles just as electrons are particles; they both are ripples in a corresponding field, and they both have energy.  The electron and positron that annihilated had energy too — the same amount of energy as the photons to which they annihilate, in fact, since energy is conserved (i.e. the total amount does not change during the annihilation process.) " Matt Strassler

"Today, if one wants to talk about the world in the context of our modern viewpoint, one can speak first and foremost of the “fields and their particles.” It is the fields that are the basic ingredients of the world, in today’s widely dominant paradigm.  We view fields as more fundamental than particles because you can’t have an elementary particle without a field, but you can have a field without any particles. [I still owe you a proper article about fields and particles; it's high on the list of needed contributions to this website.]  However, it happens that every known field has a known particle, except possibly the Higgs field (whose particle is not yet certain to exist, though [as of the time of writing, spring 2012] there are significant experimental hints.)

What do “fields and particles” have to do with “matter and energy”? Not much. Some fields and particles are what you would call “matter”, but which ones are matter, and which ones aren’t, depends on which definition of “matter” you are using.  Meanwhile, all fields and particles can have energy; but none of them are energy." Matt Strassler

..............................

A couple more questions:

Is a field a "thing", a physical structure?
Is energy a physical thing?
Are particles created by interactions between fields and energy?

I have read different things, such as that fields are created by the particles, not the other way around. Or that fields are not real, just mathematical structures that explain the properties of particles and energy.

..............................

EDIT:  I'm trying to get at, what are the real, constituent parts, at the lowest level, that don't depend on other things to exist. What can exist, alone, without having to have some other "property" to be formed.

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Of course, it is, it has to be.

Absolutely.

Since fields are generated by energy, that would be a bit of a misnomer as the energy itself can create particles.

Fields have to be real. The entire universe is one giant gravitational field, for example.

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janesixposted 10 years agoin reply to this

So the basic building block of the universe, that doesn't depend on anything else, is energy? That's what I'm trying to go get at here.

It's the only thing that can exist by itself, without anything.

So, everything else, particles, fields, spin, mass, momentum, are formed by energy doing something(maybe vibrating in different ways or something?)

Am I getting close?

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mbuggiehposted 10 years agoin reply to this

That's what String Theory suggests.

We have these tiny "strings" of stuff and those strings of stuff are vibrating.

The result of the vibrating stuff is the universe (or multiverses) in which we live.

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janesixposted 10 years agoin reply to this

I've heard of string theory, but haven't actually read anything about it. Maybe I'll check it out though. But not now, as I am having trouble just with basic concepts. Thank you for answering:)

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Think about just shortly after the Big Bang, the universe was nothing but an opaque ocean of electromagnetic radiation and all things formed from that.

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janesixposted 10 years agoin reply to this

a kind of radiation including visible light, radio waves, gamma rays, and X-rays, in which electric and magnetic fields vary simultaneously."

So, an energetic field.

?

Light and it's field, basically, are the basic "thing"?

Energy is the power or force, what moves things around, and the field forms the "shape" And everything else comes from from that.

Slowly, slowly, I may at some point get it;)

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