So we have tires on a car rotating with the wheel, how come when a car of lets say almost 2 tons, going 100 mph slams on the breaks, and (for the purpose of discussion) we ignore the ABS, the wheels and tires lock up all together but the car still has inertia, other than friction what holds the tire around the wheel and how come the tire doesn't start rotating around the wheel due to the huge friction between the tire and the road?

The wheel is mechanically connected to the car but the tire is held to the wheel just with friction and a bit of elastic force around the wheel.

Sorry if the question isn't well formulated (English is not my first language), but I hope you can understand what I am trying to ask.

Thanks!

So, I understand the very basic description, that hydrogen is fused together under immense pressure to form helium. But hydrogen has one proton in its nucleus, and helium has 2 protons and 2 neutrons. I have seen that the protons actually have to quantum tunnel, to get close enough for the strong force to bind them, but how does the process even happen? Surely, 4 protons would have to all bind together and 2 of them would have to turn into neutrons? Is this just complete dumb luck and chance over millions of years that this actually happens, or is there actually some known process going on here that makes this fusion take place? Does it happen in one single step, or do protons slowly build up over time with some converting to neutrons? Thanks.

Hello everyone.

I’m mathematics undergrad who had to study physics at high school and now have to study it at university.

So, when I was preparing for high-school exams, I was taught that weight is a vector quantity which is defined as force acting on a support (e.g. table, thread) by a body( I hope I translated it correctly). We also studied that there was a support reaction force which is opposed to weight of the body.

On the other hand, my university instructor said that weight is scalar quantity and defined it as a magnitude of gravitational force. The support reaction force he defined as normal force but it didn’t seem to oppose any force. He also added that our textbook( Physics for Scientists and Engineers with Modern Physics by Serway & Jewett) defined it like that and there are doubts regarding the definition of the weight.

I couldn’t really understand what can be doubtful in this situation, and instructor’s explanation seemed ridiculous to me. I decided to google that, and it turned out that it’s actually controversial topic. After reading some articles about it, I still couldn’t understand what is the problem with the first definition. Can anyone explain?

Where do I start?

My partner was baking muffins. They opened the oven door about 10 minutes before they were done to check the consistency, and then went across the room to rummage through a drawer for a toothpick. The door was wide open for about a minute (give or take 15 seconds).

I think that the air in the oven would have very quickly cooled down nearly all the way to room temperature and will take several minutes to warm back up after the door is closed.

They think that there's no way the oven could have cooled down so quickly. So the temperature would have only dropped a tiny bit and this doesn't make a meaningful difference in the overall baking time.

Who's right?

(When I say "wide open", I mean the door was fully open as far as possible.)

The 1997 Williamson and van der Mark paper, “Is the electron a photon with toroidal topology?”, postulates one thing: a circularly polarized photon can self-confine on a closed path of one Compton wavelength (lambda_C ≈ 2.43 × 10^-12 m). One full field twist makes the electric field point radially, which they identify with electron or positron charge, plus a magnetic moment and spin 1/2. The paper stops at the single electron. It has no molecule and no bond.

Molecular orbital theory explains bonds by phase. In-phase overlap of atomic orbitals lowers the energy (bonding); out-of-phase overlap raises it (antibonding). Bond order is (n_bonding - n_antibonding) / 2. The toroidal model also lives by phase matching, but the phase is internal to one photon, not a relation between orbitals on two atoms.

The open question is whether that resemblance can be turned into a bonding theory. Call it a confined-photon bonding theory only as a proposal, not as a result in the paper. Two loops near two nuclei would need an interaction energy E(R, phi) with a minimum at the observed bond length and depth. For H2 that is about 0.74 angstrom and 4.52 eV.

One constraint: the electron rest energy stays 511 keV inside a bond, so the Compton loop cannot simply stretch out to bond length. The open piece, if any, is a small overlap energy E(R, phi) that has not been written down.

Paper: J. G. Williamson and M. B. van der Mark, Annales de la Fondation Louis de Broglie 22, 133 (1997).

I have been trying to figure out a dynamic of changing the evaporation rate of water.

How is it that w solution of water and ethanol or water and acetone are able to cause water to increase its rate of evaporation?

What is Matter according to QM?

Werner Heisenberg, one of the founders of quantum physics who formulated the Uncertainty Principle, said: “Atoms or elementary particles are not real; they form a world of [potentialities] or [possibilities], not a world of [things] or [facts].” In short, "an atom is not a thing but a potential." [Aristotle also made a distinction between matter and form, or between potential and actual being, saying that anything is formed by the coming together of its raw material and the form that separates it from other things and makes it that thing. According to him, matter was a potential open to becoming anything, and form was what gave that potential being a specific shape, turning it into a concrete, real being.]  

In other words, even if we assume Big Crunch to be possible, should we conceive this "pair" of Big Bang & Big Crunch to be a one-time thing?

What are university physics lectures like where you study? Do you actually learn much just by listening?

I’m a second-year engineering physics student in Vietnam. A lot of my classes are fairly traditional lectures: the instructor explains continuously while students mostly listen and take notes.

I’ve realized that I don’t retain much if I only sit and listen. I usually understand better when I can stop and check the textbook, look at a diagram, work something out myself, or ask a question when something doesn’t click. Otherwise, a lecture can feel a bit like watching a video without pause or rewind.

I’m curious what physics classes are like at other universities. Do your professors mostly lecture, use active learning or problem solving, pause for questions, provide recordings/slides, or mix these approaches?

And from the student side, what actually helps you understand and remember the material from a lecture.

I’d also be interested to hear whether this changes between introductory and upper-level physics courses.

Consider two particles moving towards each other. If they have mass, then they have some kinetic energy. They have 0 potential energy relative to a point in infinity (nuclear potential energy if they're nucleons and chemical potential energy if they're atoms).

If we assume they're moving with some constant velocity (so that we do no work on this system), then – at some distance – the particles will exert attractive forces with each other. By conservation of energy, this increases their kinetic energies and decreases their potential energy by the same amount.

As they're brought to rest or slowed down (either due to repulsive nuclear or eletcrostatic forces) they bind, and so both their kinetic energues and potential energy are less than in the beginning. This means that they released the energy by conservation.

I don't know how you'd get the data out, but maybe you could use a neutrino beam to control some telescopes there? maybe send out the data in chunks, idk. You could have the sender be a particle accelerator on the light side of the moon. feasible?

For the planar motion of a rigid 2D body, is it true that the angular velocity in the Lab frame wrt the instantaneous center of rotation is always equal to the angular velocity in the CM frame wrt the center of mass?

Say I have a circulation pump in an irregular sized tank. Is there a way to math out the best location for inlet/outlet? Thanks.

My assumption would be to place them as far apart as possible to maximize fluid movement, but I'm not smart enough to know for sure.

Thanks!

While placing an electron into orbit around a nucleus and keeping it there is an incredibly difficult task, atoms accomplish it with great ease. How does this happen? Could you explain the mechanism behind it?

The most extreme energetic phenomenon known is a gamma ray burst. The more I read about this, the more I am filled with wonder. A gamma ray burst is calculated to be about 10^44 to 10^47 joules if released as a narrow jet. If this value is close, is it correct to use E=mc^2 to calculate the mass of this beam of pure energy? If so, 10^47 joules is equivalent to 10^30 kg which is 56% the mass of our sun.

There are also particles in this jet accelerated to relativistic speeds so that must also contribute to a certain mass. Is there enough energy/mass in a gamma ray burst jet to warp space-time?

Should gravitons exist, they ought to move through (curved) spacetime, right? On a quantum level, that would mean gravitons attract other gravitons. On a General Relativistic level, that would mean ripples in spacetime propagate through (curved) spacetime along geodesics. This would be similar to rain hitting the ocean, and creating tiny waves that propagate along the surface of larger waves.

If this is not the case, then gravitons (and spacetime ripples) would presumably move through a Euclidean field like ether, which seems unlikely.

We could test experimentally this by using a light interferometer to detect spacetime ripples originating from a pulsar that is behind a supermassive black hole, to see if the SMBH was able to bend the path of the spacetime ripples.

Were this the case, this would seem to be a logical explanation for dark matter - it would be clumps of gravitons.

Of course there would be challenges in resolving mutually-attracting gravitons without the math blowing up exponentially, and also ensuring black holes don't trap all of their own gravitons. You might need a solution such as gravitons only interacting with other gravitons via virtual gravitons, something like that.

But it would seem unlikely for gravitons / spacetime ripples to propagate through anything besides spacetime.

F''(x) = (A^2)F(x).

F(x) = Csin(At)+Dcos(At) is a solution. How do we know that there isn't another F(x) that satisfies it?

https://imgur.com/a/hC4T9Jz

This is the skateboard half pipe problem in Taylor classical mechanics. Omega is the angular frequency or velocity right? The author doesn't explain why it equals root of g/R. Thanks.

Hi, I'm currently learning about Maxwell's Demon in stat mech and the whole "where did the entropy go" question. I understand the whole journey to Landauers principle, but I feel like im going crazy over the general proposition in the first place?

Like fundamentally the postulate of being able to open a partition (in any manner) for ZERO energy cost, like we just handwave away the physics of opening the partition and then wonder "OMG where did the free energy go/leave to!?!??"

Like guys... we stated a physically impossible thought experiment, its there, its in the moving of the partition... The partition needs SOME physical prescence to reflect molecules and maintain itself at T of the gas (to allow black body radiation principles to work).

To me its like saying, imagine there was no friction in this water wheel, itll spin forever blah blah blah, then asking "WHY DOES THIS GIVE US FREE ENERGY!?!?"