/r/AskPhysics

/r/AskPhysics exists to answer questions about physics.

Questions should be relevant, and answers should be on-topic and correct.

We don't condone cheating on school work, and homework questions should be handled according to these guidelines.

Incivility will not be tolerated.

If your question isn't answered in a day, you can post it in the Tuesday thread in /r/Physics (unless it's homework-related).

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/r/AskPhysics

0

I have a question about the Big bang theory and the second law of thermodynamics. The second law of thermodynamics is basically entropy. Everything goes from order to disorder. Entropy is a scientific concept that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields. that's a law of the universe. Uncertainty randomness and disorder. The Big bang theory is the ultimate in entropy. A supermassive explosion. Billions of light years in every direction in seconds. That's not order that is disorder. But look at the Earth. Seems pretty ordered to me. night follows day every single day. Summer winter summer winter summer winter repeat forever. The sun will rise tomorrow. Or will it? Is it going to be summer next July or winter? Who knows? It's so random and chaotic. I mean it's daylight now but in 10 hours from now it will probably be noon how long will the sun shine? How long will the day be tomorrow? Will it be winter in July next year? Who knows? It's so chaotic and random. Entropy at work. I wonder how many days there will be in this year? 700 or 150? Have you ever seen a Galaxy? It's so random. I wonder what's going to be like next July? Freezing cold or hot as hell? Thermodynamics at work. So the universe was created by supermassive explosion that sent everything in all directions billions of light years in seconds. Disorder. But look at it now it seems pretty ordered to me. Seems to me that the earth and the solar system and galaxies disobey entropy and second law of thermodynamics. You're going from disorder to order. "Entropy is a scientific concept that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields." Doesn't The Big bang theory disobeying the second law of thermodynamics? My theory is that the Big bang is not true. According to entropy it can be. I believe that the universe has always existed. Or that it was created.

8 Comments

2023/12/08

13:17 UTC

13:17 UTC

1

Let us suppose there is a rod of length L that is moving towards the right with a velocity V1.

There is also a ball (Mass = m) that is moving towards the right with velocity V2. Moreover, the ball is L/2 below the centre of mass of the rod. Let us also consider that V2 > V1(just not to overcomplicate things)

If I write the angular momentum of that ball about the COM, would it be (m)(V2)(L/2) or (m)(V2-V1)(L/2)?

My initial logic was that it should be V2 - V1, but it's actually V2 only. Basically, I was told that there is a difference between "taking something in respect of" and "sitting in frame of" in the context of Rotational Motion. I just wanted some more clarity on this.

Thanks!

1 Comment

2023/12/08

13:03 UTC

13:03 UTC

8

I know Astrophysics has a broad range (from data analysis, to theoretical cosmology etc), but I was wondering what are the alternative options to Academia (other than NASA which ofcourse everyone wants to aim for)?

I’d love to hear about your opinion and experiences on:

1.) Pros and Cons of that industry in comparison to Academia

2.) Salary differences

3.) Research interest

2 Comments

2023/12/08

12:33 UTC

12:33 UTC

2

I was recently asked this question and I can't seem to figure it out.

So assume there are two plane waves of the same frequency travelling in opposing directions. They meet in a way such that their phase difference is pi/2, so they interfere destructively and annihilate. Where is the energy gone?

Normally I would say into the constructive interference, but I don't see where there would be any.

(I know a perfect plane wave is not real and would have infinite energy, but I think the problem still holds up for a wave packet?)

3 Comments

2023/12/08

12:29 UTC

12:29 UTC

2

I love physics (astrophysics), I truly do and would love to ideally continue publishing papers until the day I die.

But I also need the financial stability to support my family, give them the best life possible and also like the idea of financial freedom.

I understand the typical circumstances for physicist, where they’re not obviously poor by any sense of the margin, but certainly are living “grant to grant” beyond just the salary they may earn from their respective university.

I was wondering if there are ‘flexible’ circumstances, where you can continue publishing papers via working for a university (I’m assuming that’s how respected physicists usually publish) and still maintain your primary income coming from e.g. finance etc?

Or is it simply impossible and a person may HAVE to choose one or the other?

Either you’re a respected physicist OR no one is really going to take your work seriously but you still get to fill your pocket?

3 Comments

2023/12/08

11:33 UTC

11:33 UTC

0

Can someone explain to me: If we are certain the universe is expanding, wouldn’t this be clear evidence the Big Bang did happen?

Also are we certain the stars are slowly burning up all the gases, which would also indicate the universe had a starting point?

6 Comments

2023/12/08

11:01 UTC

11:01 UTC

1

I was listening to some science youtube video a long time ago where it was stated there that civilizations that can produce a dyson swarm still needs to maintain some separation between each object in the swarm as packing them too dense would create a black hole. I did not see the math behind it as I was just listening to the audio and now I forgot the title of the video. But the reasoning was that the schwarzschild radius scale with mass but mass scale with the cube of radius.

I then watched one of the new vids of kurzgesagt where they stated that what holds white dwarfs are the electrons and quantum mechanics and then in the neutron star with degeneracy pressure. After that, no force could hold the collapse so it leads to black hole.

I know my question is stupid because it's basically the same with what special relativity predicts if you move faster than the speed of light. So for the question, suppose that we do have some force of very large strength that it can still hold the collapse of the neutron star and we are able to shrink the star to a radius less than its schwarzschild radius. From the outside I would think that it would still be a black hole since the escape velocity is still larger than c. But for the inside, does general relativity force its collapse into a black hole or does the math allows some rigid structure inside the event horizon given that somehow there is a very strong force that can support it?

2 Comments

2023/12/08

10:53 UTC

10:53 UTC

1

Consider this Thin Film Interference diagram here. The Optical Path Difference (OPD ) between the reflected light rays from C and A is

OPD=n2(AB+BC)−n1AD.

I can understand this as the path travelled by the light rays after C and D is same. So the extra path travelled can be calculated.

But what if I want to find out the OPD between the reflected light from A and BC ? Will it be just n2(AB) , or am I missing something? The ray BC is not parallel with AD , so we do not have a point after which the path travelled by both rays is going to be equal (if we do not allow the ray to refract). In that case, we will have a point of intersection after which the path difference is going to be zero. So, we can calculate the extra path from there.

I do not understand which of my logic is correct or both are wrong, so please help me.

0 Comments

2023/12/08

10:21 UTC

10:21 UTC

0

13 Comments

2023/12/08

10:03 UTC

10:03 UTC

1

So all of us must be aware of the matchstick party trick . I was trying to make a free body diagram to understand how the torque is being balanced . My initial intuition was that the bottom matchstick must be providing the necessary force to the matchstick which comments the bottom matchstick and the top matchstick such that the torque generated balanced out.

But when i solved for the net force on bottom matchstick, this is what i get Which didnt make sense to me , if the bottom matchstick doesn't provide any force in the upwards direction , then how would the torque balance out right.

So i set up the same experiment and scaled the size up, but i used a wooden ruler which can bend , so that i could visually see the forces . And these are the results for 3 cases :

- when ∅1 < ∅2 : https://ibb.co/yfg6z1z
- when ∅1 > ∅2 : https://ibb.co/NVs5Kp3
- wheb ∅1 = ∅2 : https://ibb.co/x2Rpq79

If we look at case 1 , we can see the ruler experience a net upwards force , which makes it bend . Similarly , when ∅2<∅1 , we get a net downward force which us opposite of what i found on paper .

So my question is , What did I do wrong ? my brain cant seem to figure out anything.

3 Comments

2023/12/08

09:25 UTC

09:25 UTC

1

Working on a youth novel. I have this question for a scene.

Can you actually levitate using magnets ? I know magnetic levitators are a thing, tho how powerful would it have to be to actually lift a human being of the ground a few inches ?

I'm more interested about how large would he have to be to actually work. I'm working on a fun scenario where a child fakes superpowers using just basic physics and magnets, but i want to know how realistic it could be.

If you stuffed with your shoes with north pole magnets, being yourself on top of a powerful south pole oriented electromagnet, how powerful and big would the magnet have to be to support he's weight ? Can it fit in a box and could it be done by amateur engeeners ?

Anyway, don't want to flood you with questions. Thanks already for repleying if you did !

3 Comments

2023/12/08

09:25 UTC

09:25 UTC

1

Just curious.

Assuming the snow stays intact, how long of a "runway" would be needed for a snowball the size of a baseball (9 inch diameter) to reach the size of a house (25 feet diameter), how much would the snowball weigh, and what speed would it be going (45 degree slope)?

If I'm missing any variables feel free to make some up

1 Comment

2023/12/08

09:11 UTC

09:11 UTC

1

Firstly I'm wondering if this can happen without any extra energy applied to the system, such that after a certain point a larger bubble will subdivide on its own. Also I am wondering by which type of mechanism any type of energy, such as from agitation, can subdivide a larger bubble. Most of what I have read talks about smaller bubbles diffusing or coalescing into larger ones, but I can't find any reading material on the inverse process.

1 Comment

2023/12/08

08:18 UTC

08:18 UTC

1

I have been using a reference by Willam H. Hayt, Jr. John A. Buck 8th edition called Engineering Electromagnetics

Chapter Seven introduces Biot Savart's law and Ampere's law.

In this chapter, I feel like explanations are missing an aspect or are contradictory to previous statements

This is Problem 7.16 part c:

Show that Hz = 0 for ρ > a: Suppose the rectangular loop was drawn such that the

outside z-directed segment is moved further and further away from the cylinder. We

would expect Hz outside to decrease (as the Biot-Savart law would imply) but the same

amount of current is always enclosed no matter how far away the outer segment is. We

therefore must conclude that the field outside is zero.

The problem with such a statement is during the proof for magnetic field of infinite plane, the authors used this exact way to get |H| = Ks/2.

so I feel contradicted.

6 Comments

2023/12/08

07:53 UTC

07:53 UTC

9

We here at the undergrad team believe so, but the answer says the object will land on the same spot no matter if its in motion or not. We dont believe this so want this clarified.

Edit: Sorry for the typo in title. The main thing we're asking here is if a object is released at one point it will not land on the same spot if its in static or in motion correct?

24 Comments

2023/12/08

07:26 UTC

07:26 UTC

5

I’ve seen lasers be able to engrave metals and cut some materials but what makes a mirror surface immune to the lasers destruction?

7 Comments

2023/12/08

07:00 UTC

07:00 UTC

2

Parton distribution functions are distributions of quarks and gluons inside the nucleon, but besides the three valence quarks, they are virtual particles right? So if virtual particles are just a mathematical tool to calculate amplitudes, what exactly are these distributions? Is the quark-gluon sea inside the nucleon actually physical?

1 Comment

2023/12/08

05:36 UTC

05:36 UTC

1

I'm trying not to fail my final as it's worth a lot which will determine if I pass the class or not there. My issue is because i can't understand in depth the concept which leads me to getting bad grades and I wish to know what I'm I suppose to do in order to understand the concept in depth in order to not fail specifically because it's hard for me to understand what to do when there's only letter and no number in the question.

3 Comments

2023/12/08

05:27 UTC

05:27 UTC

1

I’m told the gravity of a black hole is so strong that even light cannot escape. I’m also told that light has no mass. My question is if light has no mass how is it subject to the force of gravity?

Another When I shine a flashlight as soon as I turn off the switch the beam of light disappears. We often hear of stars exploding billions of years ago and the light of those explosions are just now being seen on Earth. How is that light able to be seen for so long after source of the light is long gone?

What is the life of a photon?

Scott Phillips Franktown Colorado

3 Comments

2023/12/08

05:11 UTC

05:11 UTC

0

I have this feeling that because 4d objects cast 3 dimensional shadows... (thats basically how they draw 4d objects in their 3d form) ...that we are possibly the shadows of a 4 dimensional universe right now.

Idk if this is a physics question or a philosophy question or where this belongs exactly but I'd rather ask people who have actually thought of this, so I felt ask physics to be the best thread

13 Comments

2023/12/08

04:34 UTC

04:34 UTC

0

2023 resistors, of which 1011 have a resistance of R and 1012 a resistance of 2R, were connected in series into a polygon, to the two vertices of which a voltage source was connected. What should be the resistance of two sections of the polygon located between these vertices so that the power released in the circuit is minimal?

Tried google search and chatgpt promting, bad luck(

2 Comments

2023/12/08

04:32 UTC

04:32 UTC

0

Hi

I watch a lot of hockey and have seen players shooting the puck 100-105 mph during skills compeititions.

My question is, how fast does a puck need to be travelling to penetrate through skin? I understand this varies on places in the body with skin thickness, but im more curious if a puck was slap shot at a players stomach/abdominal region (where there is no protective gear) how hard would it need to be going to actually penetrate skin.

I understand this is impossible probably for a player to generate that power, but nontheless, I am curious!!

Thanks

0 Comments

2023/12/08

04:22 UTC

04:22 UTC

1

I've recently been analysing further the famous stability valley graph, and i have a question. Upon inspection, i realized there's a specific isotope (Which according to me counting the squares is Xe-119) is marked as having an "Unknown" form of decay, but i couldn't find anything on it...

Here's the image: https://upload.wikimedia.org/wikipedia/commons/c/c4/Table\_isotopes\_en.svg

1 Comment

2023/12/08

04:21 UTC

04:21 UTC

3

Hi there everyone. I'm a mathematics and biology student (might change out of biology and switch to a minor perhaps), and am admittedly curious, what is the reputation amongst physicists for people who popularize physics, such as Brian Keating, Sabine Hossenfelder, and Neil DeGrasse Tyson. I was not really aware of what they said, except for Hossenfelder when her videos were brought to my attention. Generally I try to look at research papers, read textbooks, and talk to professors to try to understand what's going on as opposed to getting my information from pop science sources, as I have often noticed (especially in biology and chemistry) that the information there can be either misleading, superficial, or lead people to thinking they have a higher level of proficiency in the material than they really do. So what do you all recommend? Looking at popular science videos in physics but then trying to look at appropriate papers and find textbooks needed to understand the material, even if you can't get as deep as you'd like, as there's an ever increasing mountain of data and knowledge related to well, nearly every area of physics.

8 Comments

2023/12/08

03:56 UTC

03:56 UTC

1

A big "T" sign is held by a cord and a hinge attached to the bottom of the left side of the top bar as seen in the diagram. Considering that the sign has a mass of 20.0 kg, and the length, L, is 12 cm, determine the tension in the rope and the horizontal and vertical components of the force that the hinge exerts on the sign. [10 pts]

There is also some additional information on the rest of the page. There is an image, which show us the big T. The top of the T is 5L long. the small sides of the T are 1L, the big sides are 4L and the bottom is 1L.

The T is attached to two walls, one on the left (attached to the bottom left corner of the top of the T) and, most importantly, one on the right (cord is attached to the top right part of the T). The cord has an angle of 40 degrees.

I also know that the final answer is 124.6N.

I tried finding the overall tension of the cord, as well as Ty and Tx, to no avail.

I'm aware that this would be much easier to picture with an image, but i wasn't able to link one to this post! Thank you for your help!!!

1 Comment

2023/12/08

03:00 UTC

03:00 UTC

9

After the Big Bang, things would stick together and interact with other for a little while because of the other forces (or would they?)

But eventually there’d just be floating molecules and atoms that just dissipate in a gaseous form until the end of time, right?

Is that the end of the show?

15 Comments

2023/12/08

01:43 UTC

01:43 UTC

0

Hello, anyone who can handle this:

Consider the Boolean expression F (ABCD) = ∑m(0, 3, 5, 6, 8, 9, 14, 15)

i. using K-Map minimization method implement its CLC

ii. implement using appropriate Decoder

You can do it on a paper and attach. The correct answer will recieve a reward. Thanks

3 Comments

2023/12/08

00:24 UTC

00:24 UTC

1

Hi! Thanks for taking the time to read this. English isn't my first language, so if it sounds weird, that's why.

So, I'm a film student and have a physics final exam next week where I have to talk about sinewave interference in general and in particular in relation to sound waves. I remembered this video of a dude standing in between 4 tornado sirens and hearing them harmonize (I know there's been much debate about whether this is real or not. I don't mind if it is, I just want to use it as an example to explain the theory).

I feel like I sort of understand it, but I would love it if someone could explain it to me clearly, so I'd know for sure it's a valid example of how waves interact with one another. I've heard on other subreddits that it has to do with a doppler effect and different frequencies and amplitudes interacting, but I'd love to be sure.

Video reference: https://www.youtube.com/watch?v=pap4GsOIrpQ&ab_channel=DustyGalline

Thanks!

5 Comments

2023/12/07

22:29 UTC

22:29 UTC

2

In this question, there is a constant magnetic field perpendicular to the plane of the disc. The question then goes on to say that the disc rotates 25 times a second and asks you to calculate the induced emf. Apparently what you were supposed to do was:

ε = ΔΦ/Δt = (0.17π×0.36²)/(1/25) = 1.7V.

I don't understand how the disc rotating induces an emf; the flux through the disc at any instant is constant (field is constant and disc remaines perpendicular to field throughout). I also don't understand how the time period of the rotation of the disc has anything to do with the time derivative of the magnetic flux; the latter describes how much the magnetic flux changes in an infinitesimal interval of time, the former describes the frequency at which the disc rotates and has nothing to do with changing the flux through the disc.

I've found this Wiki article, and it states the following:

"An often overlooked fact is that Faraday's law is based on the total derivative, not the partial derivative, of the magnetic flux. This means that an EMF may be generated even if total flux through the surface is constant."

How does the magnetic flux being equal to the total derivative instead of the partial of magnetic flux mean that a constant flux can generate an emf? If the flux is constant, then the (total) time derivative of the magnetic flux is zero, meaning zero induced voltage, no?

7 Comments

2023/12/07

22:12 UTC

22:12 UTC

1

So I have a question about the angular momentum operator L, and a function F(r), where r^2 = x^2+y^2+z^2 . The question asks to show that "L F(r) = 0".

If I treat F(r) to be a function (ie. not acting on anything) then the answer is easy - but when I treat F(r) to be an operator instead, I get all these extra x,p terms that don't cancel out.

So how am I meant to tell when some object F(r) is to be treated as a function instead of as an operator?

More specifically if F is an operator I get (eg. take coordinate x):

-ih y d/dz( F(r) * func) + ih z d/dy( F(r) * func)

= -ih yz/rF'(r) * func - ihyF(r) d/dz(func) + ih zy/rF'(r) * func +ihzF(r) d/dy(func)

= -ihF(r) (y d/dz - z d/dy)

which is very much not zero...

5 Comments

2023/12/07

20:50 UTC

20:50 UTC