# All Questions

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### Is this webcomic accurate?

I was considering this xkcd comic from 5/10/14, with the alt-text "Trains rotate the Earth around various axes while elevators shift its position in space." I'm wondering about its accuracy. I ...
1k views

### Using Ampere's Law for a Solenoid

To calculate the magnetic field, a rectangle amperian loop was drawn, and since the sides of the rectangle are perpendicular to the magnetic field, and the top is too far away to have any field ...
308 views

### speed of the light emitted from a fast moving object [duplicate]

Would not speed of the light emitted from the front of the fast moving object be the speed of light + the speed of the fast moving object?
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### Understanding emergent phenomena in the block universe. (Reworded question)

Each person exists as an unchanging 4D worldtube in the block universe. At each slice of the worldtube there is a present, past and future. However, there is a black box* which appears to exist in ...
396 views

### Leonard Susskind's videos. Which order? [closed]

As a mathematics student with almost no modern physics background (just an introduction to relativity when I was in secondary school) I find Leonard Susskind's lectures videos (freely available in ...
1k views

### Why does a water drop on a hot plate at 150°C evaporate faster than on a plate at 200°C?

I recently read that: A drop of water landing on a hot plate at 150°C (300°F) evaporates in a few seconds. A drop of water landing on a hot plate at 200°C (400°F) survives a whole minute. How ...
56 views

### Significant error conversion

So here is my question: Say we have measured something to be 15,67 mm and the significant error is $\pm 0,01$mm. then we convert the measurement to meter to be 0,01567m would the significant error ...
57 views

### Energy in nuclear decays

After a nuclear decay is it a necessity that the total energy of the products is more than the energy of the original particle before decaying? (NB: by 'energy' I don't intend to include mass-energy ...
108 views

### Ohms law and multiple resistors

In my physics class, we did an experiment in which we supported Ohm's law. We used a simple circuit that includes one resistor. The relationship of I = VR, and variants thereof, became apparent. We ...
802 views

### Balloon, lighter than air and vacuum?

As I understand, in a balloon/air balloon, a gas burner is used to heat air or using some lighter atom like helium. Since helium or hot air is lighter and less dense than the cool air around the ...
273 views

### Confused about the concept of time and time dilation [duplicate]

I am having a hard time understanding what is time. If scientists define time as a multiple of caesium frequency, then time itself is dependent on motion, so what if I have a number of particles that ...
1k views

### What are hot electrons?

What are they? How are they created? And what do they have to do with plasmons? I searched the web, but I would like more reliable and straightforward sources.
2k views

### A ball is thrown up, and a second ball is dropped- what is the maximum initial velocity of the first ball so that [closed]

This question only exists in one dimension, the y axis. A ball is thrown upwards with an initial velocity $V_o$ from a roof with height $h$. One second later, a second ball is dropped from the same ...
203 views

### Help w/ speed of sound experiment report question

Experiment was done by using an oscilloscope and a piezoelectric transducer to generate ultrasonic sound waves. We had to move the transmitter whilst receiver remains constant on a angled 1 meter ...
67 views

### Super-String and Anti-Particle

I'm afraid this is quite basic question . What I learned from the String theory is that the theory unify the Path-integral, Superposition Principle, and commutation relation . In that context , I ...
75 views

### Unit determinant for relevant symmetry groups in QFT

When treating QFT we want our theory to be invariant under different symmetry groups, for example, the Standard Model is a non-abelian gauge theory with the symmetry group $U(1)×SU(2)×SU(3)$. ...
92 views

### moment of inertia of a ring about an axis at 45° to the normal [closed]

I wanted to calculate the moment of inertia of a ring about an axis at 45° to its normal outside the plane of the ring . How do i calculate without using integration? I was thinking about using ...
2k views

### What is the exact cause of flow separation in a viscous fluid?

I recently got into a lengthy debate about the exact nature of boundary layer separation. In common parlance, we have a tendency to talk about certain geometries as being too "sharp" for a viscous ...
379 views

### Intuition Behind Conservation of Angular Momentum

I'm having a fairly hard time understanding the intuition behind Noether's derivation of the conservation of angular momentum from the rotational invariance of the Lagrangian, though I do understand ...
373 views

### Minkowski metric — why does it follow from the constancy of the speed of light [duplicate]

In all the sources I’ve been able to find, the Minkowski metric appears ad hoc, or is defined analogously to the euclidean metric. I’d love to see an argument why this metric (time coordinates ...
183 views

### Uniform Circular Motion with Banked Road and Car [closed]

In Uniform Circular Motion, if a car is rounding a curve at a certain speed, and the angle of the road allows the car to drive around at that speed, that speed is called the "design speed." If the ...
403 views

### When can we add a total time derivative of $f(q, \dot{q}, t)$ to a Lagrangian?

The other day, I was listening to this lecture on the Lagrangian for a charged particle in an electromagnetic field, and at one point in the video, the lecturer mentions that we can add any total time ...
56 views

### Can someone explain to me the Rocksar-Kivelson Hamiltonian?

The following paper shows the hamiltonian of the 2D quantum dimer gas (page 2) http://www-thphys.physics.ox.ac.uk/people/ClaudioCastelnovo/Talks/050209_MIT.pdf Here are some questions I have. Why ...
206 views

### First law of thermodynamics applied to atmospheric fluid

I was reading an article that talks on a climate prediction model. This article proposes the following equation (energy balance): \rho_0 c_v\left( \frac{\partial T}{\partial t} + \sum^{3}_{j=1} ...
1k views

### Where's the missing helium in the Universe?

I'm confused: Big Bang nucleosynthesis is adamant about the 1 neutron to 7 proton ratio which yields 75% hydrogen to 25% helium (with a nominal amount of partially-reacted deuterium and heavier ...
117 views

### Toroid moments tensor decomposition

I am currently working on my bachelor's thesis on the anapole / toroidal moment and it seems that I am stuck with a tensor decomposition problem. I have actually never had a course about tensors, so ...
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### Subnuclear physics vs wave function

This question is more a philosophical question than a physics one. When we appreciate particle physics we study that in order to explain some experimental results we have to introduce a new particle ...
228 views

### Why are popcorn seeds soft after popping?

When a seed of popcorn is heated up in oil, it pops like this: You can take one of these popped pieces and eat it with little to no problem. However, if you get an un-popped seed and sink your ...
355 views

### Nonzero ground state energy of the quantum harmonic oscillator [duplicate]

Since $\frac{1}{2}\hbar \omega$ is the zero point energy of the ground state of the harmonic oscillator, then there is no way to extract this energy. Therefore, in what way is this value different ...
103 views

### Size of an image formed by a concave mirror

A short linear object of length $b$ lies along the axis of a concave mirror of focal length $f$ at a distance $u$ from the mirror. What is the size of the image? what i have done so far: since ...