Ask a physicist anything. (5)

[Nabobalis]

Yes. It is one of the four forces that govern our universe, along with electromagnetism, the strong nuclear force, and the weak nuclear force.

Well I've only covered about a half of my General Relativity module but the lecturer says that GR implies that gravity is not a force and I thought that GR is our best description of gravity to date.

 

[pgp_protector]

My question is: HOW ON EARTH DOES THIS GUY DO IT????????? Watch the whole video:

YouTube - 【2010央视春晚 1080HD】17 - é[bless and do not curse]”术表演 Magic Show 2/2 刘谦 Liu Qian

Not that hard, I've done that one before, and a few other fun Coin / Card Magic illusions

How, buy the book if you really want to learn (I'd suggest Bobo's Coin Magic for starters.)

 

[Wiccan_Child]

Well I've only covered about a half of my General Relativity module but the lecturer says that GR implies that gravity is not a force and I thought that GR is our best description of gravity to date.

Oh it is, it's almost uncannily accurate, and it's definitely our best gravitational theory - don't pay any attention to those silly string physicists, even we theorists shun then at parties. Your lecturer might be hinting that the other forces are mediated by other particles (e.g., the electromagnetic force is an attraction/repulsion of charged particles, but this attraction/repulsion is due to particles - photons - mediating between them), while gravity, according to general relativity, is a warping of spacetime around one mass, while slops potential gradients towards it.

But it's still a force. Ultimately, it does indeed cause a change in momentum over time. The mechanic may be different, as per GR, but it's a force regardless.

 

[Nabobalis]

Oh it is, it's almost uncannily accurate, and it's definitely our best gravitational theory - don't pay any attention to those silly string physicists, even we theorists shun then at parties. Your lecturer might be hinting that the other forces are mediated by other particles (e.g., the electromagnetic force is an attraction/repulsion of charged particles, but this attraction/repulsion is due to particles - photons - mediating between them), while gravity, according to general relativity, is a warping of spacetime around one mass, while slops potential gradients towards it.

But it's still a force. Ultimately, it does indeed cause a change in momentum over time. The mechanic may be different, as per GR, but it's a force regardless.

Ok thanks for clearing that up.

 

[Steffenfield]

I'm so lazy that I tend to pour 4 liters of hot water on my windshield whenever it's frozen with ice.

I don't have the patience to warm the car for 20 minutes and scrape the ice by hand.

My neighbors keep yelling at me thinking that the glass on my windshield is going to explode or something stupid.

They're so full of it, yes?

 

[Nabobalis]

I'm so lazy that I tend to pour 4 liters of hot water on my windshield whenever it's frozen with ice.

I don't have the patience to warm the car for 20 minutes and scrape the ice by hand.

My neighbors keep yelling at me thinking that the glass on my windshield is going to explode or something stupid.

They're so full of it, yes?

No, it can crack if you keep doing that.

 

[pgp_protector]

I'm so lazy that I tend to pour 4 liters of hot water on my windshield whenever it's frozen with ice.

I don't have the patience to warm the car for 20 minutes and scrape the ice by hand.

My neighbors keep yelling at me thinking that the glass on my windshield is going to explode or something stupid.

They're so full of it, yes?

Explode, no
Crack & shatter Yes.

If you have as much as a small nick it the glass the shock of hot watter can cause that nick to grow into a massive crack right before your eyes.

If you really wish to clean the window, use COLD water, or Water / Rubbing Alcohol Mix (50/50)


http://www.ehow.com/how_10756_remove-ice-windshield.html

 

[sandwiches]

Explode, no
Crack & shatter Yes.

If you have as much as a small nick it the glass the shock of hot watter can cause that nick to grow into a massive crack right before your eyes.

If you really wish to clean the window, use COLD water, or Water / Rubbing Alcohol Mix (50/50)


How to Remove Ice From Your Windshield | eHow.com

I might be mistaken but I thought tempered glass wasn't affected the same way by rapid temperature changes as other glass.

 

[Wiccan_Child]

I'm so lazy that I tend to pour 4 liters of hot water on my windshield whenever it's frozen with ice.

I don't have the patience to warm the car for 20 minutes and scrape the ice by hand.

My neighbors keep yelling at me thinking that the glass on my windshield is going to explode or something stupid.

They're so full of it, yes?

As others have said, repeatedly pouring hot water over your windscreen and letting it cool again will crack the glass. Big no no!

 

[mzungu]

I might be mistaken but I thought tempered glass wasn't affected the same way by rapid temperature changes as other glass.

Tempered glass is extremely durable but it (as all glass) has an Achilles heel; It is called cracks! Even a microscopic crack can cause the windshield to end up in fragments. Besides how would you feel if you were cold and were thrown into a boiling cauldron?

Be nice to Glass!

 

[TheReasoner]

Im taking a course in statistical thermodynamics, and to be frank the Maxwell Relations, Schrodinger equation etc are not very well described by the book - nor by the lecturer.
Got any resources to recommend?

 

[Nabobalis]

Im taking a course in statistical thermodynamics, and to be frank the Maxwell Relations, Schrodinger equation etc are not very well described by the book - nor by the lecturer.
Got any resources to recommend?

Either F. Mandl, Statistical Physics or M. Glazer and J. Wark, Statistical Mechanics - a survival guide. These books were recommenced for our second and third year modules.

 

[Wiccan_Child]

Im taking a course in statistical thermodynamics, and to be frank the Maxwell Relations, Schrodinger equation etc are not very well described by the book - nor by the lecturer.
Got any resources to recommend?

I used Tippler & Mosca - fantastic book that covers pretty much everything you could ever be hit with in physics. I'll see if I can dig out the exact title.

 

[TheReasoner]

I used Tippler & Mosca - fantastic book that covers pretty much everything you could ever be hit with in physics. I'll see if I can dig out the exact title.

Physics for scientist and engineers I think. I have the latest edition. Though for some reason it lacks the quantum mechanics section :-(

 

[Wiccan_Child]

Physics for scientist and engineers I think. I have the latest edition. Though for some reason it lacks the quantum mechanics section :-(

That's the one. They had a good section on semiconductors, I thought. Part VI talks about quantum mechanics, though.

 

[Maxwell511]

Oh it is, it's almost uncannily accurate, and it's definitely our best gravitational theory - don't pay any attention to those silly string physicists, even we theorists shun then at parties. Your lecturer might be hinting that the other forces are mediated by other particles (e.g., the electromagnetic force is an attraction/repulsion of charged particles, but this attraction/repulsion is due to particles - photons - mediating between them), while gravity, according to general relativity, is a warping of spacetime around one mass, while slops potential gradients towards it.

But it's still a force. Ultimately, it does indeed cause a change in momentum over time. The mechanic may be different, as per GR, but it's a force regardless.

I disagree. Gravity is not a force (at least under GR). The only way I think we can really define what a force is something that changes an object's movement away from a geodesic. In GR we are saying that gravity literally defines what the geodesic is. It does not change an objects trajectory away from it.

You understand claiming that gravity is a force sort of going against the equivalence principle. An object falling to earth in a vacuum is not experiencing any momentum change, or any force, from its point of view (save microgravity). It just looks like that from our non-inertial reference frame while we are standing on earth.

If are going to start defining forces as things that change momentum in non-inertial reference frames then there are not four fundamental forces. There is potentially an infinite number of them. I think that people should start investing in particle colliders and systems that will find the well elusive and sought after "when I take a sharp left in my car and I feel a force towards the right" particle.

 

[Chesterton]

.

 

[Steffenfield]

Do any of you own anything really cool?

Like what are some of your favorite stuff that you have?

 

[AV1611VET]

Do any of you own anything really cool?

Like what are some of your favorite stuff that you have?

I have a digitally-remastered:

 

[Wiccan_Child]

I disagree. Gravity is not a force (at least under GR). The only way I think we can really define what a force is something that changes an object's movement away from a geodesic. In GR we are saying that gravity literally defines what the geodesic is. It does not change an objects trajectory away from it.

I'm tempted to cite Newton's Laws, but Einstein kinda trumps Newton

You understand claiming that gravity is a force sort of going against the equivalence principle. An object falling to earth in a vacuum is not experiencing any momentum change, or any force, from its point of view (save microgravity). It just looks like that from our non-inertial reference frame while we are standing on earth.

Hmm, I'm not so sure I agree. Acceleration isn't relative. If it accelerates down a potential well, no matter what our inertial frame, we will still see it accelerate the same amount; we see it increase its velocity by 5ms[sup]-1[/sup] after 1s, no matter what we decide its initial velocity is. That was Einstein's whole point: the falling object can decide if it's accelerating or not. We can define where V(r) = 0 lies, but the actual topography of the potential well is the same.

If are going to start defining forces as things that change momentum in non-inertial reference frames then there are not four fundamental forces. There is potentially an infinite number of them. I think that people should start investing in particle colliders and systems that will find the well elusive and sought after "when I take a sharp left in my car and I feel a force towards the right" particle.

Non-inertial frames? No no no. Only inertial frames. As I'm sure you know, fictitious forces arise in non-inertial frames, but only the real forces exist in inertial frames. And, if we pick an inertial frame, we see four forces - including gravity.