# Common Misconceptions in Physics

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#### topsquark

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Only virtually the same ??? What experiments show a difference??? In Physics experiment is everything ...I don't see what debate there can be unless there is experimental evidence of a discrepancy .
Just because we measure it to be the same does not mean that the two are directly related. Inertial mass and gravitational mass are apples and oranges at this point because no one has any idea of how to derive one from the other to show they are the same. This is a very deep question in the Philosophy of Physics and anyone that can show they are the same from a theoretical viewpoint would probably get a Nobel Prize.

Really ?? I didn't know that !! Are you saying mass doesn't increase with speed???

I was taught that the kilo of sugar in my kitchen reacted to other mass by the well known gravity equation , and accelerated from force by F=Ma ..

And this kilo mass increased by the well known formulae as it's speed increased , becoming infinite at the speed of light .

As far as I know all this still stands.

The reason my kilo of sugar can be more than a kilo is because I've increased its energy and energy is equivalent to mass ...E=mc2
To a degree the difference in viewpoints does not affect the theory. It's just that it is somewhat easier to work with momentum because it is easier to write out the QM equations. Note that $$\displaystyle E = mc^2$$ is only true for an object at rest. The actual equation is $$\displaystyle E^2 = (mc^2)^2 + (pc)^2$$. The momentum term is already in there so it's more natural to use.

-Dan

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#### topsquark

Forum Staff
Just looked into it ....as usual it's idiot "physicists" trying to complicate and confuse things ... make things more mysterious than they really are ... they have to make a noise so people think they're doing something ....
Wow. Um..... Have you seen an Analysis textbook in Math? Mathematicians are usually the ones that make things extra "complicated." But it's a useful complication because Physicists tend to need someone to plug the Mathematical holes in our theories. Picky details can be very important. Besides, who ever stated that the Laws of Physics predict that the Universe is simple?

And it's more like OCD than idiocy.

-Dan

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#### studiot

For those interested
Here is an extract from Connes

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#### oz93666

Alain Connes writing in 2008 for Cambridge University "On the fine structure of Spacetime" carried it further.
Carried what furthur , where ??? Theory ,pie in the sky , nothing that fine tunes our calculations and can be proved by experiment . My understanding is einstien was the last person who did that ... just a slight adjustment to newton which we have verified by experiment.

Just because we measure it to be the same does not mean that the two are directly related. Inertial mass and gravitational mass are apples and oranges at this point because no one has any idea of how to derive one from the other to show they are the same. This is a very deep question in the Philosophy of Physics and anyone that can show they are the same from a theoretical viewpoint would probably get a Nobel Prize.
Yes figuring that riddle will be the key to understanding reality !!

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#### Pmb

PHF Hall of Fame
gravitational mass vs. inertial mass: The debate goes on. They are known to be virtually the same by measurement but there is no theoretical reason why they should be, so far as I know.
All we are able to determine by experiment is that they are proportional, not equal.

rest mass vs. "this mass will increase as velocity increases": Typically we don't consider mass to depend on the speed any more. The speed dependence is taken to be a part of the momentum 4-vector, meaning that the mass of the particle (object, whatever) is always the same. (Warning: Some actually do keep the speed dependence with the mass. But I believe that the statement I made is valid for most theorists anyway)
Not really. There are a lot of physicists who don't hold that to be true. Particle physicists usually only refer to rest mass because its an intrinsic property of a particle and that's what they study. Not to mention that it makes notation look simpler. On the other hand there's no valid reason why relativists would use it other than to succumb to pressure by particle physicists. In GR and cosmology its quite common to see people refer to mass as relativistic mass.

Here's a perfect example of why its not good to think of mass as rest mass: when people use mass in that way it implies that a directed beam of radiation has zero mass. That confusing people since they conclude that since a photon "has no mass" that it shouldn't be affected by a gravitational field, but we know it does. And it can even generate a gravitational field.

Alan Guth explained this in a video he sat and did for me. See
http://www.newenglandphysics.org/common_misconceptions/Alan_Guth_01.mp4

Notice when he says "light has energy which means that it effectively has mass." And keep in mind that Alan is one of the leading physicists in the US.

Here's proof that a moving body weighs more than a body at rest which means that its passive gravitational mass increases with speed.
http://www.newenglandphysics.org/physics_world/gr/weight_moving_body.htm

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#### oz93666

... passive gravitational mass
Here we go again ... more adjectives put in front of "mass" ...

and "active gravitational mass" ...I've briefly searched these terms and they seem like more hot air ... unnecessary , confusing to layman and physicist alike ..

So are terms like "gravitational flux" ...

Two masses create an attractive force between themselves end of story

Neither is active or passive.

#### topsquark

Forum Staff
Here we go again ... more adjectives put in front of "mass" ...
Are the integers and rational numbers the same? After all, they are both numbers...

-Dan

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#### studiot

Are the integers and rational numbers the same? After all, they are both numbers...

-Dan
Of course they are not.

The rationals satisfy the Field axioms in mathematics.

The integers do not.

Equally the integer (say) 3 is the same number as the rational 3/1 or the real number 3 or the complex number (3 + 0i)

This last assertion sounds simple, but is actually not so easy to prove.

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#### Pmb

PHF Hall of Fame
Are the integers and rational numbers the same? After all, they are both numbers...

-Dan
Dan - Why do you bother arguing with Oz? Don't you recall all the past attempts to reason with him, all of which failed miserably? He is simply ill-equipped to understand the reasons we do what we do and has demonstrated a strong refusal to learn. For instance - Einstein explicitly stated in his 1916 review article on GR that mass is properly described by a tensor, i.e. the stress-energy-momentum tensor. That's what it takes to properly define a distribution of matter. However for a object in free fall in a gravitational field its mass is what causes it to respond to the field and can be described by a scalar. In this case the active gravitational mass requires a tensor to properly describe it whereas a scalar is sufficient to describe the passive gravitational mass. And what Oz thinks makes no difference to anybody except Oz.

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#### studiot

I see I misunderstood topsquark's post.

Thanks to PMB for showing me that.

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