J.C.MacSwell

 

I’m trying to understand what light is to provide a commonality across all of these' date=' and not simply in example of photoelectric effect consider light as a particle, in reflection light is a wave, and have so models for the same thing just try to explain a particular concept.[/quote']

 

Noone has succeeded yet.

That's not what I am saying. I tried to make it clear. The net force on me is 2F' date=' and the net force on him is also 2F.

 

My maximum punch can deliver F.

His maximum punch can deliver F.

 

All I am saying is that if we punch each other simultaneously, the net force upon either of us is given by F+F=2F.

 

If only one of us punches the other, then we each have an external force of magnitude F applied on us.[/quote']

 

OK, here is the problem. If you punch me then in laymans terms you are the one using force. This is not the correct use of the term in physics. (or engineering)

 

So let's be very specific. In physics you cannot exert a force on me without me exerting an equal but opposite force on you. If you want to add "additional" force to make it 2F thats fine but it also has to come from both you and I.

 

So that's 2F from you and an equal but opposite 2F from me. Not F from me and F from you.

Vector analysis will help.

 

The earth pulls me down' date=' and creates a reading on a scale which we will call W. If I slowly varied the mass of the earth, I could change the reading of the scale. For example, if I take my scale, and me to the moon, and stand upon it there, the reading on the scale will be less than what it read on the earth. So that W is affected by the mass of what is underneath the scale.

 

The earth pulls things towards its center of mass, via gravitational force.

 

Let the x axis be horizontal, and let the y axis be vertical. So j^ points into the sky, and -j^ points right at the center of mass of the earth.

 

The planet earth pulls my body down, and the harder it pulls, the greater the reading on the scale. This force is being denoted as W. Specifically we have:

 

[math'] \vec W = w_1 \hat i + w_2 \hat j + w_3 \hat k [/math]

 

 

The earth isn't pulling me horizontally in any direction in the tangent plane. Therefore w1=0 and w3 = 0. The only component of W points in the negative j hat direction. Hence we have:

 

[math] \vec W = -w_y \hat j [/math]

 

where [math] w_y [/math] is a positive number.

 

Ok so that covers my weight, which has been handled as a force. Now, the tangent plane is fixed to the earth. There is no acceleration in the -j hat direction, because the surface of the earth is there, and it is pushing me up in just the right amount, to cancel out my weight. Let us call this upwards force the normal force, and denote it by N. We have this now:

 

[math] \vec N = n_1 \hat i + n_2 \hat j + n_3 \hat k [/math]

 

And there are no components of N in the tangent plane. Therefore we have:

 

[math] \vec N = n_2 \hat j [/math]

 

And [math] n_2 [/math] is a positive number.

 

Assume these are the only forces on me. Therefore, the sum of all the forces upon me is W+N:

 

[math] \sum_{i=1}^{i=2} \vec F_i = \vec W + \vec N [/math]

 

And this vector sum is equal to ma, where m is my inertial mass, and a is my acceleration in this reference frame, which is attached to the earth.

 

I am not accelerating, I am at rest in this frame, therefore:

 

 

[math] \vec W + \vec N = 0 [/math]

 

That is:

 

[math] \vec W = -w_y \hat j [/math]

 

+

 

[math] \vec N = n_2 \hat j [/math]

 

is equal to zero, from which it follows that

 

[math] w_y = n_2 [/math]

 

You say that the floor is pushing me upwards with a force of 200lbs. Yes, that is the normal force, the floor is reacting to my weight.

 

My weight is 200 lbs.

 

The net force upon me is zero.

 

You ask if the force is 400 or 200, that isn't a perfectly clear question, but the force is not 400 lbs simply because I am not accelerating in the frame. The net force upon me is zero.

 

My weight is 200 lbs, but that isn't the only force upon me. The other force has the same magnitude, but opposite direction.

 

As for which of the two my sensory perception detects, that is a good question.

OK, let's get back to the point. Out in space if you push against me with F then I will push back with F and there will be no 2F resultant force on either one of us. If a bar is in between us and the bar does not move it works the same way. You push with F, I push back in the opposite direction with F and cancels it out for the bar . You are accelerated one way at F=ma and I am accelerated the opposite at F=ma.

Thank-you both and that link looks interesting.

Can comeone explain the difference between spin 1/2, spin 1 and spin 2 in such a way that the numbers make sense. I realize they mean quantum spin and relate to classic spin by analogy only but in what sense is spin 2 "twice" spin 1, or spin 1 "twice" spin 1/2. I realize also that spin 1 and 2 are in a different "family" than spin 1/2.

They assumed that the light waves needed a medium in which to propagate. The trouble arises when you discover that we are neither stationary nor moving with respect to the proposed medium.

 

So the conclusion is that EM waves do not need a medium through which to propagate.

Is that your conclusion/you agree fully with it?

I push on you plus the bar with force F.

 

If you didnt push back' date=' then there is a force on me of F.

 

But you also pushed me with force F, so its 2F.[/quote']

 

Lets say you're standing still and your weight is 200 lb.

 

The floor is pushing you upwards with a force of 200 lb.

 

Now lets say you push back with a force of 200 lb.

 

Now is the force 400 lb or 200 lb?

I push on you plus the bar with force F.

 

If you didnt push back' date=' then there is a force on me of F.

 

But you also pushed me with force F, so its 2F.[/quote']

 

Nope

 

Now' date=' move me to the edge of the bar, and we both push simultaneously, [b']equally hard[/b]. You say you exert a greater force. Explain this to me, i expect you to say something about torque.

If equally hard means equal force...

I said if we were equally powerful.

Can you do this with numbers?

 

Suppose we push on the bar in opposite directions' date=' both of us through the center of mass of the bar.

 

We push equally hard, because we are equally strong.

 

Now focus on things in the center of mass frame of the bar.

 

The bar remains at rest in this frame.

 

You move away from me, and I move away from you, and the bar remains at rest in the CM frame.

 

The force upon me is 2F, and same on you.

 

I exterted a force on you of F, and an equal force was reflected back upon me, and you exerted a force on me of F, and an equal force was exerted back upon you, so the total force upon me was 2F, and letting M denote my mass, my acceleration in the CM frame (bar at rest) is found through:

 

2F = M a

 

Now, move me to the edge of the bar, and we both push simultaneously, equally hard. You say you exert a greater force. Explain this to me, i expect you to say something about torque.[/quote']

 

2F? It's only F.

All [/b'] of it will go into translation.

Maybe this will help:

Assuming the force does not act through the centre of mass.

All of the force will also go into rotation. The moment will depend on the magnitude of the force and the "lever" due to the alignment of the force wrt the centre of mass.

Yes' date=' it will move away, I know that. My center of mass, and it's center of mass will separate, in the center of mass frame (me+it) I know this (assuming Newton's third law true). But, how can one applied force F, at the end of a rigid body, be equivalent to the same force acting in the same direction at the center of mass, plus a rotational moment? <--- thats what i don't get.

 

[b']Doesnt that violate conservation of energy[/b]?

It takes more energy to maintain the force.

If you and I were in outer space with bars and we were equally powerful and I pushed with all my might through the centre of mass and you pushed with all your might on the end as described on an equivalent bar then I would exert more force.

See I don't think this is right. Won't some of the applied force go into translation, and some go into rotation?

All of it will go into translation.

I don't follow. I am picturing a bar in space. I am floating next to it, and am at rest with it. Then I use my finger to push the leftside of the bar, in a direction perpendicular to the bar. That bar is going to start spinning isn't it? If not, then yes I am doing something wrong. Why won't the bar start spinning?

It will spin and translate (wave good-bye to it)

It will move away in the direction of the force and you will move in the opposite direction. Depending on the alignment of the force with respect to your centre of mass you may spin also.

The force on the leftmost particle (upwards) is equivalent to having the force on the middle particle (upwards) plus half of that force on the leftmost particle (upwards) plus half of that force on the rightmost particle (downwards) which resolves to a translational F=ma plus a rotational moment.

 

As the "bar" accelerates the point of contact of a constant upward force on the original particle will shift to the right (with the particle) reducing the rotational moment. It will eventually oscillate left and right (constant frequency) while continually accelerating upwards at greater velocity.

What I was trying to show here is that any force acting on a rigid body that is not acting through the centre of mass is equivalent to an equal force in the same direction acting through the centre of mass plus a rotational moment.

Ok' date=' just to make sure I followed you, let the net applied force to the system have a magnitude F[b']. Suppose that X is the portion of F that goes into translating the center of mass[/b]. Let the remainder be Y. Therefore, X+Y=F. And you are also saying that X=Y.

 

.

I think this is where you are going wrong. ALL of the force acts to translate the centre of mass.

F=X, if X+Y=F then Y=0

Picture 100 particles. Any configuration. Some glued together rigidly. Some totally detached. Some attached by elastics.

Now apply a force upwards on any particle or particle group. Call that force F.

F will equal ma, where a will be the acceleration upwards and m will be the mass of the system.(elastics and all)

Redo the math for the same thing adding a mass the size of Jupiter to the system where Jupiter is 100 miles away and is totally unaffected.

F will equal ma, where a will be the acceleration upwards and m will be the mass of the system including Jupiter.

Redo the math for the same thing adding a mass the size of Jupiter to the system where Jupiter is 100 miles away and is the only body affected by the force (upwards again whatever that means in the reference frame you are using)

F will equal ma, where a will be the acceleration upwards and m will be the mass of the system including Jupiter.

If F accelerates the mass in translation and in rotation it doesn't matter.

F will equal ma.

(if you sum the "rotational accelerations in the upward, or any direction they will cancel out)

If this seems odd it is probably because you intuitively feel that since F may be doing work to stretch elastics or cause rotations etc that maybe some of F is "used up" in this regard.

It is not.

What is different and does need to be divided up is the energy or work done by the force F.

The work done by F is very different in each of the above cases.

Does it? In the classical sense, surely, it cannot. Space does not have mass.

I'm thinking in terms of the masses themselves. Their receding velocities (regardless of why) would require the same change in momentum to slow down or speed up??

']Well' date=' the universe did not inflate to its current size in a few nano seconds. In a brief time after the big bang, it did inflate extremely quickly, however this soon slowed down to the speed of inflation we have today. The universe has had at least 13 billions years to expand, and has been expanding continuously for all of that time.

 

I'm not sure how valid your analogy is about the christmas ornament, at least as it pertains to the shape of the universe. That would be a good analogy for explaining how it is the universe expands, all points getting further away from eachother, none getting closer to eachother. For the curvature of the universe I think you'd have to look at it differently, althought im not sure about this.

 

http://en.wikipedia.org/wiki/Universe#Shape_of_the_Universe

 

It mentions a sphere like yours in that paragraph, so perhaps your analogy is correct for this point.

 

You other people need to read my original post in this topic, there is a lot of confusion on what this is. Remember- calling the universe flat, or like the surface of a sphere, or whatever are results of thinking of the universe in 4 dimensions, which we cannot properly visualize. These should not be taken too literally.[/quote']

 

All of these analogies have their limits and can be useful if we are careful about where they break down. The bowling ball on the matress works well but "requires" a gravitational force (the very thing that we are trying to model) to make it work. A "closer" analogy would have the matress shrink in the area of the bowling ball instead of stretch but the mental image is not as familiar.

.

 

The christmas ball could just as well be a large soap bubble that you blew thru a hoop when you were little. The ball is round' date=' but if you look at the surface you can see "designs" and swirls on the outside. Nothing on the inside. This is what our universe is. I wish I could explain myself better.

 

Bettina[/quote']

 

I think these are good analogies. Your "surface" (2D) is our space (3D).

I don't know you seem to know a lot more than I about this subject, but my intervention was a reaction to thing like a pancake. Saying the universe is flat doesn't mean that the universe is two dimentional

I think that's right (the second part, not so sure about my armchair cosmology ).

Yeah I thought so' date=' so let me ask you this. Suppose I am a source which is going to "push" the thing in the direction which is tangent to the elliptical orbit this thing finally takes on. I would have to "push" perfectly to get the orbit to be elliptical. Not enough push would mean the things spiral together, and too much would mean they sail away from each other. So how is it that nature managed to get things perfect? Do you know?

 

Regards[/quote']

 

Isn't it the inverse squared laws which lead to stable orbits?

Flat in the cosmologist language doesn't mean flat like a pancake.

Flat means that two parallel lines will never meet and remain at the same distance from each other. A positive curvature mean that two parrallel line will meet and a negative curvature mean parallel will never meet and the distance between them will increase.

The curvature of the universe' date=' like was said is defined by the quantity of matter in the universe.[/quote']

 

I'm not sure but isn't your definition for flat space (Euclidian), which is different from both a "flat" universe (slowly slowing to infinite) or "flat" space-time (an SR rest frame or GR "no tidal forces" frame)?

The phrase 'an explosion of [/b']space' would have been a better choice. Space and time did not exist prior to the big bang. They were created at that 'instant'. (Alledgedly.)

Why does the expansion OF space have MOMENTUM? Is it assumed to hold to the same equations as an explosion IN space?

I cannot imagine a flat universe' date=' nor can I imagine one like a Christmas ornament, (although that would be easier than flat)

 

I have always wondered why it all has to be [b']one[/b] big bang?

 

Look, we think we know that there are such things as black holes. And we think we know that there are many of them. Why couldn't it be the case that once a black hole becomes dense enough--once it has attracted enough matter into it--that it just explodes and creates a galaxy?

 

Does it have to be the whole damned universe at once?

 

Why not a black hole exploding somewhere in the universe every billion years or so? Let's call it the "popcorn theory" of galaxy creation.

I like it already!

I would like to see a few more competing theories but you have to adapt them when you have a contradiction or you can add/remove assumptions until they collapse under their own "weight".

I read a book by British astronomer Martin Reese "Just 6 numbers" where he says he was originally against the Big Bang Theory, then converted by the evidence to being 90% certain 10 or 15 years ago, and now is 99% certain.

seems rather high considering all the basic things we don't know.

What are your odds?

I'm a Steady State Theorist at heart but I'll go with 50% for some form of the Big Bang, 25% for some form of the Steady State Theory and 25% for "something else".

Be interesting to see if people who are really knowledgable about these things would be committed in the 90+ percent range.

Edit: I think a universe based on the 3Dsurface of a 4D hypersphere that slowed in expansion forever but never stopped and reversed would be considered "FLAT" (mathematically) no pancake necessary.

If the reverse-big-bang theory is 'wrong' then this universe MIGHT be in deep trouble. The reason being: if the universe kept expanding' date=' then eventually the stars that populate the universe will decrease (because 1 star dying doesnt mean 1 star being born) and eventually most of the stars will burn out and the universe will be nothing but an huge, cold, peice of nothingness. No species would ever be able to survive.

 

does any1 have a response to this theory???[/quote']

 

Welcome to entropy.