Physics Weekly Newsletter
Physics Weekly Newsletter

Top new questions this week:

Freefall into snow

In the movie Frozen, the following dialogue takes place: Anna: "It's a hundred-foot drop." Kristoff: "It's two hundred." Anna: "Okay, what if we fall?" Kristoff: "There's 20 feet of ...

homework-and-exercises newtonian-mechanics free-fall applied-physics estimation  
asked by user3932000 33 votes
answered by Señor O 29 votes

Intuition as to why the orientation (of a 3D object) is not a conserved quantity?

Say you start off floating in space, in a fixed position and orientation, with zero linear and angular velocity, with no external forces. So you are a closed mechanical system. By twisting your body ...

newtonian-mechanics rotational-dynamics conservation-laws  
asked by Don Hatch 31 votes
answered by John Rennie 28 votes

Is there a limit as to how fast a black hole can grow?

Astronomers find ancient black hole 12 billion times the size of the Sun. According to the article above, we observe this supermassive black hole as it was 900 million years after the formation of ...

general-relativity black-holes astrophysics  
asked by Hritik Narayan 25 votes
answered by Rob Jeffries 18 votes

Why does my fireplace shoot flames out the front until I open my backdoor?

I have a natural gas fireplace with fake logs in it. I open the flue, turn on the gas for a couple seconds, and throw a match in. The flames kind of go up and out of the flue, but most of the flames ...

thermodynamics pressure  
asked by Josh Beam 13 votes
answered by tpg2114 14 votes

Why doesn't a cigarette lighter generate thrust?

The way thrust is created by a rocket is discussed here: How does fire create thrust in rocket? If you look at a typical rocket, say V2, it has two primary tanks — one which stores the fuel and the ...

rocket-science combustion  
asked by Shashank Sawant 11 votes
answered by lemon 24 votes

Why aren't all black holes the same "size"?

The center of a black hole is a singularity. By definition, a singularity has infinite density. So how can a black hole with a different mass or density be described?

black-holes mass density event-horizon singularities  
asked by Jiminion 8 votes
answered by innisfree 11 votes

Will the upcoming solar eclipse stress the European electrical grid?

From an article in the Financial Times today, about a solar eclipse on March 20th, 2015: An eclipse of the sun next month could disrupt Europe’s power supplies because so many countries now use ...

energy renewable-energy meteorology photovoltaics  
asked by Nicolas B 8 votes
answered by EnergyNumbers 10 votes

Greatest hits from previous weeks:

How exactly does time slow down near a black hole?

How exactly does time slow down near a black hole? I have heard this as a possible way of time traveling, and I do understand that it is due in some way to the massive gravity around a black hole, but ...

general-relativity black-holes time reference-frames  
asked by Annika Peterson 18 votes
answered by Larian LeQuella 21 votes

Would time freeze if you could travel at the speed of light?

I read with interest about Einstein's Theory of Relativity and his proposition about the speed of light being the universal speed limit. So, if I were to travel in a spacecraft at (practically) the ...

special-relativity speed-of-light reference-frames observers  
asked by Question Overflow 14 votes
answered by Kyle 9 votes

Can you answer these?

Decay of some particle involved quarks vs mesons as outgoing states

Let's have decay width of some mother particle into the state which involves hadrons. For simplicity, let's assume that creation of hadrons (on diagram) is possible only through electroweak vertices ...

particle-physics quantum-chromodynamics perturbation-theory non-perturbative  
asked by Carlo Beenakker 2 votes

Laser travelling through a radially graded index of refraction

A laser beam propagates through a region whose refractive index varies as $\mu=\mu_0(\frac{r}{r_0})$. At a distance of R the beam makes an angle of 30° with the normal. Find the minimum seperation ...

homework-and-exercises optics laser refraction  
asked by Akshay Bodhare 1 vote

How to calculate phonon decay rate?

Here, the authors calculate the quality factor for a resonator. They take it as one over phonon decay rate, given by (24). Simplified, (24) looks like this: \begin{equation} \Gamma = \gamma_{N-1,N} - ...

condensed-matter phonons  
asked by Kirill Moskovtsev 1 vote
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