55 votes

Why does bunched up aluminum foil become so extremely hard to compress?

"If you, the reader, were to rip a page from this journal and crumple it, squeezing it with your hands into a ball as hard as you can, the resulting object is still more than 75% air. What gives ...
Chemomechanics's user avatar
34 votes

Why does a plastic bag rip when it is raised to a higher position when being held but not when it is stationary?

This is like holding a weight $mg$ with a string which can break with a tension of $1.1 mg$. When you are not moving the bag, or moving it at a uniform speed, the string's tension is constant at $mg$,...
AlphaLife's user avatar
  • 11.6k
26 votes

Does glass undergo internal damage in a similar way to wood?

No, wood and glass fail in completely different ways. Wood is a composite structure consisting of cellulose fibers held together with lignin glue. In wood, an impact force breaks the lignin loose from ...
niels nielsen's user avatar
14 votes
Accepted

How does quantum mechanics explain the color of macroscopic objects?

There are two important details. One is that in a solid, the atoms interact with each other, which broadens the lines into a continuum.* The second is that when an atom/material absorbs a photon and ...
user34722's user avatar
  • 1,741
8 votes

Why do good materials operate in non-equilibirium conditions?

For example this could be the heating of a metal to extreme temperature and abruptly putting it in cold environment to anneal it, thus typically resulting in good mechanic resistance. The main ...
Roger V.'s user avatar
  • 58.2k
7 votes

Vinyl Records and timbre

A little less handwavy: Hold a vinyl record up to your ear in a quiet room. With your free hand, tap the center of the record and listen carefully for any "bong" noise. If the disc has any ...
niels nielsen's user avatar
7 votes
Accepted

How can things around us have different colours if they have specific emission spectra?

At room temperature, hydrogen gas exists as diatomic molecules that do not significantly absorb light in the visible spectrum, i.e., 400 nm to 700 nm. So hydrogen gas at room temperature is colorless. ...
Ed V's user avatar
  • 646
6 votes

How does quantum mechanics explain the color of macroscopic objects?

A macroscopic substance is composed of countless fundamental particles in continuous mutual interaction with every other fundamental particle nearby. Every interaction has associated, in-principle-...
g s's user avatar
  • 13.1k
6 votes
Accepted

How hot can the focal point of a convex lens get under the full sun?

The theoretical maximum is exactly the surface temperature of the Sun, by thermodynamic arguments that the 2nd Law cannot be violated. We will, of course, fail to reach this limit by far. Long before ...
naturallyInconsistent's user avatar
6 votes

Prove that an isotropic substance has only two elastic constants

In the context you are describing the isotropy implies that the tensor $\gamma_{ijkl}$ must be strictly invariant under arbitrary rotations of the three dimensional Euclidean space $M^i{}_j \in SO(3)$....
Gravitino's user avatar
  • 567
6 votes

Rope strength necessary to stop a fall

your problem is the bungee jumping problem you jump from a clip with a rope of a length $~L~$ and stiffens $~k~$ . what is the maximum of the rope tension ? you have two cases I) free falling case ...
Eli's user avatar
  • 11.8k
5 votes

Why does bunched up aluminum foil become so extremely hard to compress?

Aluminum (and most solids) are nearly impossible to compress (i.e. very high bulk modulus). What you are doing when crumpling or flexing a loose sheet of metal is causing it to move and buckle, not ...
RC_23's user avatar
  • 8,928
5 votes
Accepted

Why are less dense materials better thermal conductors?

The premise isn't really correct as stated. It's materials with a higher thermal conductivity that are better thermal conductors; that is, a given temperature gradient produces a higher conductive ...
Chemomechanics's user avatar
5 votes

Prove that an isotropic substance has only two elastic constants

If you did some quantum mechanics, you've probably seen how to combine independent spins in terms of total spin with the associated Clebsch-Gordan coefficients. Without looking at the details, you can ...
LPZ's user avatar
  • 11k
5 votes
Accepted

Why is red the brightest in the emission spectra of Hydrogen Gas?

Brightness does not correspond to the energy, it corresponds to the transition that happens most often (producing the most photons). Which will often be the ones at lowest energy levels. In fact as ...
RC_23's user avatar
  • 8,928
5 votes

Does glass slowly (invisibly) degrade until it breaks?

I think the chance to break is random. Incidentally, in the case of the art of glass blowing there is a phenomenon that can ruin a piece that is in the making. That phenomenon is called ...
Cleonis's user avatar
  • 20.5k
4 votes

How can things around us have different colours if they have specific emission spectra?

Atomic hydrogen gas does glow with a particular colour, as long as it is at sufficient temperature to excite some of its atoms into excited levels. Here is an example: First an RGB image taken with ...
ProfRob's user avatar
  • 129k
4 votes

How can things around us have different colours if they have specific emission spectra?

H absorbs very specific wavelengths, leaving almost all wavelengths undisturbed. Also the excited atoms then decay, producing the same wavelengths that were just absorbed. Those wavelengths are the ...
mmesser314's user avatar
  • 37.9k
4 votes

Vinyl Records and timbre

Handwaving: The vinyl is, on the relevant scales, very stiff and light, so the force of the needle doesn't excite it much, and the resonances are beyond human hearing.
John Doty's user avatar
  • 20.3k
4 votes

Why does a plastic bag rip when it is raised to a higher position when being held but not when it is stationary?

(An attempt to explain this with non-technical language). There're two forces acting on the bag - the force from your hand, and the Earth's gravitational force (the weight). When you lift the bag, you ...
Allure's user avatar
  • 20.1k
4 votes

What is a general definition of bulk modulus?

If $V_f \approx V_i$, then $$\ln\left(\frac{V_f}{V_i}\right) = \ln \left( 1 + \frac{V_f - V_i}{V_i} \right) \approx \frac{V_f - V_i}{V_i}.$$So the two formulas are functionally equivalent so long as ...
Michael Seifert's user avatar
3 votes

Why does bunched up aluminum foil become so extremely hard to compress?

Why does bunched up aluminum foil become so extremely hard to compress? It is hard to compress if we compare with the first steps of forming the Al foil ball. But pure Al has a compressive strength ...
Claudio Saspinski's user avatar
3 votes
Accepted

Layman explanation of the Piola-Kirchoff stress tensor?

Stress is a vector quantity, with the dimensions of force per unit area. It is useful to think of it in the same way as pressure (which is a special kind of stress that only acts in the normal ...
Tofi's user avatar
  • 2,671
3 votes
Accepted

Why do good materials operate in non-equilibirium conditions?

The induced stresses experienced during processing of engineering alloys at high temperatures and strain rates is intended to improve the mechanical properties of the alloy by diffusing beneficial ...
niels nielsen's user avatar
3 votes
Accepted

Materials known to have higher density than Osmium at at high pressure and/or low temperature

Iridium appears to become denser than osmium under high pressure (2.98 GPa) and normal temperatures. The reason is that osmium is less compressible than iridium, so at high pressures iridium densities ...
Anders Sandberg's user avatar
3 votes
Accepted

Mechanical wave traveling time in a medium

In any real material, there is dispersion, which means that the speed of the wave depends on its frequency. In that case, there are multiple speeds of sound (one for each frequency). In addition, the ...
march's user avatar
  • 7,409
3 votes
Accepted

“Emergent magnetic monopole” discovered (5th December 2023)

To put it simply, an emergent property is a property that stems from many parts of a system all working in some collaboration that produces a whole that is greater than the sum of its parts as it were....
Albertus Magnus's user avatar
3 votes

“Emergent magnetic monopole” discovered (5th December 2023)

The word $\textit{emergent}$ is used in the condensed matter community to emphasize that it comes about from many-body physics. It's not a fundamental property of the elementary particles or atoms. I ...
Dr. Nate's user avatar
  • 191
3 votes
Accepted

What is the resistance of this structure?

It's true for very thin plates. If the block dimensions are of comparable lengths, the outer half plate has a linear potential along its vertical boundary to its inner half. It is generating a current ...
Roland F's user avatar

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