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Top new questions this week:

Error concerning projectile motion in respected textbook?

In the textbook Fundamentals of Physics by R. Shankar of Yale Open Courses, appears the following assertion pertaining to a car driving off a cliff, which seems correct: This is exactly how long it ...

newtonian-mechanics newtonian-gravity projectile textbook-erratum  
user avatar asked by Trever Thompson Score of 30
user avatar answered by R Shankar Score of 81

How to turn a sum into an integral?

In, An Introduction to Thermal Physics, page 235, Schroder wants to evaluate the partition function $$Z_{tot}=\sum_0^\infty (2j+1)e^{-j(j+1)\epsilon/kT}$$ in the limit that $kT\gg\epsilon$, thus he ...

thermodynamics statistical-mechanics integration partition-function  
user avatar asked by GedankenExperimentalist Score of 16
user avatar answered by CR Drost Score of 17

Can a big mass defect make the mass negative?

Can two particles with small masses and a strong attractive interaction have a total negative mass when brought together? Let $m_1, m_2$ be the (rest) masses of two particles when infinitely distant. ...

special-relativity particle-physics potential-energy mass-energy binding-energy  
user avatar asked by HomoVafer Score of 11
user avatar answered by Jos Bergervoet Score of 10

Continued calibration of atomic clocks

First off I am not well versed in physics, but as I understand things the second is defined by the ceasium fountain clock which is calibrated as follows How was the first atomic clock calibrated?. Now ...

experimental-physics atomic-physics time measurements experimental-technique  
user avatar asked by Emma Harris Score of 9
user avatar answered by BowlOfRed Score of 15

Quantum: why linear combination of vectors (superposition) is described as "both at the same time"?

I want to get a better understanding of quantum phenomena and out world in general. Before long I've thought of Schrödinger cat as being both alive and dead (or spin both up and down). Now after some ...

quantum-mechanics hilbert-space superposition  
user avatar asked by Martian2020 Score of 9
user avatar answered by ZeroTheHero Score of 10

Does the cosmological constant entail a mass for the graviton?

If I consider the Einstein equations into the form $$ R_{\mu\nu}=\kappa \left(T_{\mu\nu}-\frac{1}{2}g_{\mu\nu}T\right)+\Lambda g_{\mu\nu} $$ and then linearize them, we should get by moving to ...

general-relativity quantum-gravity cosmological-constant de-sitter-spacetime  
user avatar asked by Jon Score of 9
user avatar answered by Andrew Score of 18

Why does the pet's water bowl overflow?

So when i give the pet fresh water in a stainless steel bowl that i place on a mat according to the attached picture, from $t=0$ the bowl is at rest, the water normally oscillates in the bowl like a ...

fluid-dynamics waves everyday-life oscillators  
user avatar asked by user721108 Score of 8
user avatar answered by mmesser314 Score of 11

Greatest hits from previous weeks:

Why do diamonds shine?

I have always wondered why diamonds shine. Can anyone tell me why?

visible-light reflection refraction  
user avatar asked by Pranit Bauva Score of 10
user avatar answered by Waffle's Crazy Peanut Score of 19

Why change in internal energy is zero in isothermal process?

In isothermal process $\Delta U =0$. But I am having trouble understanding it. Say we have an ideal gas, and say my temperature is constant but I move the pressure, volume from $(P, V) \to (P-dP, V+...

thermodynamics energy temperature  
user avatar asked by user3001408 Score of 17
user avatar answered by DavePhD Score of 19

Electric potential vs potential difference

What is the difference between electric potential and potential difference? In our course book, they are given as separate topics but their definition is given the same.

electrostatics potential voltage  
user avatar asked by Rafique Score of 3
user avatar answered by Nikolaj-K Score of 6

Why does the air flow faster over the top of an airfoil?

I understand the common explanation of lift, which describes the airflow over the top of the wing as moving faster than the air below the wing. However, I don't quite understand why the air moves ...

aerodynamics  
user avatar asked by Michael Score of 29
user avatar answered by Alan Rominger Score of 13

What is the difference between "kinematics" and "dynamics"?

I have noticed that authors in the literature sometimes divide characteristics of some phenomenon into "kinematics" and "dynamics". I first encountered this in Jackson's E&M book, where, in ...

kinematics terminology  
user avatar asked by nibot Score of 89
user avatar answered by Chad Orzel Score of 61

In an alternating current, do electrons flow from the source to the device?

If electrons in an alternating current periodically reverse their direction, do they really flow? Won't they always come back to the same position?

electricity electric-circuits electric-current  
user avatar asked by noob Score of 22

Would you die if you put your hands on a powerline?

You know how birds perch on powerlines without getting electrocuted? What if by some chance that I find myself falling and I grab on one of them? Let's say both of my hands are on the same line, would ...

electricity potential  
user avatar asked by Lemon Score of 16
user avatar answered by hdhondt Score of 17

Can you answer these questions?

Material that Increases Angle of Incoming Light

Snell's law states that light refracts when entering a substance and refracts back to its original angle upon exit. This is shown in the diagram below from this webpage. I was wondering if there are ...

optics optical-materials  
user avatar asked by user109474 Score of 1
user avatar answered by paulina Score of 0

Does the coherence of light sources make a difference in exciting fluorescence?

Using LED vs laser as light sources in wide-field epi-fluorescence microscopy are both popular choices. It seems that high intensity single color LEDs (such as this one) emits high radiometric power ...

laser microscopy light-emitting-diodes  
user avatar asked by Jason M Score of 1

Directionality of EM Waves in Double Slit Experiment

In the experiment of any wave through the double slit we see and model the interference pattern as shown. In this depiction that means the wave propagates in/out of the page. ie we are seeing a sin ...

electromagnetism waves  
user avatar asked by cav3 Score of 1
user avatar answered by Farcher Score of 0
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