Physics Weekly Newsletter
Physics Weekly Newsletter

Top new questions this week:

"Falling upward" - how far you have to be from Earth to start falling to the Moon?

Talking about gravity with my 9 y/o she asked when do we start "falling upward" to the Moon. What is the distance at which the Moon's gravitational attraction is higher than that of the Earth and thus ...

homework-and-exercises newtonian-mechanics newtonian-gravity solar-system celestial-mechanics  
asked by rafb3 32 votes
answered by Rob Jeffries 20 votes

How much air needs to be displaced to generate an audible sound?

I'm reading a book where in one scene a wizard/alchemist teleports a scroll after reading. He folded the parchment carefully and muttered a single cantrip. The note vanished with a small plop of ...

acoustics air displacement  
asked by npst 26 votes
answered by Floris 35 votes

What do we see while watching light? Waves or particles?

I'm trying to understand quantum physics. I'm pretty familiar with it but I can't decide what counts as observing to cause particle behave (at least when it's about lights). So the question is what do ...

quantum-mechanics particle-physics visible-light waves wave-particle-duality  
asked by martincpt 26 votes
answered by John Rennie 53 votes

Why aren't there spherical galaxies?

According to the Wikipedia page on Galaxy Types, there are four main kinds of galaxies: Spirals - as the name implies, these look like huge spinning spirals with curved "arms" branching out ...

gravity astrophysics angular-momentum galaxies  
asked by haneefmubarak 24 votes
answered by Simha 15 votes

Is it possible to create a parachute large enough to stop all velocity?

This idea came to me while playing Kerbal Space Program. I noticed that the larger my parachute was, the slower my rocket would fall back down to Kerbin. I would like to know if it is possible to ...

gravity air free-fall  
asked by Mitch Talmadge 21 votes
answered by paul 42 votes

Where on Earth does the mass of 1 kg actually produce a 1 kg reading on a digital scale?

Gravity on Earth varies by about 0.1% between poles and equator. If someone was buying/selling something mass critical e.g. gold, where is the standard place on Earth where a 1 kg mass produces a 1 kg ...

mass weight metrology  
asked by Dirk Bruere 20 votes
answered by steveverrill 30 votes

Melting diamond and cool down as diamond

Is it possible to melt diamond? And if possible while let it cool will it became diamond again?

thermodynamics material-science diamond  
asked by sugunan 17 votes
answered by Floris 38 votes

Greatest hits from previous weeks:

Cooling a cup of coffee with help of a spoon

During the breakfast with my colleagues, a question popped into my head: What is the fastest method to cool a cup of coffee, if your only available instrument is a spoon? A qualitative answer would ...

thermodynamics experimental-physics home-experiment popular-science  
asked by fortran 375 votes
answered by drhodes 420 votes

Why does wavelength change as light enters a different medium?

When light waves enter a medium of higher refractive index than the previous, why is it that: Its wavelength decreases? The frequency of it has to stay the same?

electromagnetic-radiation speed-of-light frequency refraction wavelength  
asked by Olly Price 10 votes
answered by Manishearth 10 votes

Can you answer these?

Magnetic Multipole Tensor

When the electric scalar potential is expanded into spherical coordinates, one gets \begin{align} \phi (\vec r) = \frac{1}{4\pi\varepsilon_0} \sum_{l=0}^{\infty} \sum_{m=-l}^l ...

classical-electrodynamics tensors multipole-expansion  
asked by Tornado 2 votes

Question about energy moving through one object to another

I think this question is part physics, and maybe part chemistry. I'm making electronic drums. On the top of the drum there's a flat surface (the drum head) you hit with a stick anywhere on the head. ...

asked by lrubin28 3 votes

Does positron-electron annihilation preserve enough info to reverse exactly

An electron-positron annihilation can produce a pair of gamma rays. In the reverse process, known as pair production, can the gamma rays carry enough information to determine the resulting ...

photons electrons radiation reversibility pair-production  
asked by Cheers and hth. - Alf 1 vote
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