Reflection FAIL

Submitted by Nicholas C. Darnton (inactive) on Wednesday, 1/27/2010, at 12:38 PM
Physics for future presidents

Examine the book cover to the right closely; you can click on it for a larger version.  Notice the problem?

Let's start by stipulating that it was unreasonable to actually photograph a real reflection of the Presidential Seal, so the publisher was justified in photoshopping it.  There are three schools of thought about the actual resulting book cover:

  1. Most charitable: this is all the publisher's fault.  The author would have spotted this, but no one on the editorial side noticed it.  If only the illustrator had read the book (or taken the course it's based on) this would never have happened.
  2. Less charitable:  at some point, some involved in this mishap realized that the reflection should have been inverted, but it was too much trouble to fix.
  3. Least charitable: the publisher knew the reflection job was botched but figured that the general public was
    1. too stupid to notice, and/or
    2. too stupid to comprehend reversed letters if the cover were fixed.

I understand the rationale for the reversed lettering on the front of ambulances: you want to avoid the split-second of incomprehension when a driver sees the ambulance in the rearview mirror.  But is the publishing industry really so cut-throat that the fraction of a second necessary for cognitive processing of a reversed Presidential Seal would have materially affect the sales of this book? 

This one is even harder to explain.


Submitted by Nicholas C. Darnton (inactive) on Tuesday, 1/26/2010, at 10:47 PM

A tsunami is an unusually large wave triggered by a major force (usually an earthquake) applied to the ocean.  In the open ocean a tsunami does not have a particularly large amplitude, but its wavelength is enormous.   As it approaches land, the amplitude of a tsunami can increase dramatically, leading to catastrophic flooding. 

Several sites, some more media-rich than others, present conclusion about the physics of tsunamis.  The origin of tsunamis is well understood (though, because most are caused by earthquakes, impossible to predict).  Any short, sharp excitation of sufficient size can cause a tsunami; as in many wave-bearing media, the specific details of the excitation are not as important as its total size. 

My reconstruction of the major practical difficulties in tsunami warning is

  1. Since the tsunami has a small amplitude in the open ocean, it is difficult to detect before it nears shore, and
  2. Whether a small amplitude open-ocean tsunami turns into a large amplitude wave near shore (called "shoaling") depends critically on the details of the shoreline and the direction of approach of the wave.  During the time between the triggering of a wave and its arrival on land, there is often not enough information to precisely calculate the threat to populated areas and then to communicate an appropriate warning.  After the fact it is possible to reconstruct the path of the wave, but this is small consolation to the survivors. 


Submitted by Nicholas C. Darnton (inactive) on Tuesday, 1/26/2010, at 10:45 PM

We only address linear waves in Physics 16, and really only focus on infinite or semi-infinite sine-wave-like disturbances.  In the real world waves have a finite duration; this type of wave is often called a wave packet.  If the wave is large enough, it can be affected by the nonlinear properties of the medium.  The most dramatic example of this is the soliton, a disturbance that propagates with no loss in speed, size or shape over large distances. 

Though first observed more than 150 years ago, solitons weren't understood until the last few decades and are still actively investigated in mathematics, physics and engineering.  This site gives an overview of current research as well as a historical account of the first description of a soliton on a canal in England. 

Phononic crystals

Submitted by Nicholas C. Darnton (inactive) on Friday, 11/21/2008, at 12:33 PM
Phononic sculpture

Sound waves propagate through air pretty much independently of frequency: if you listen to a sound from a distance, its amplitude is smaller (the sound is softer) but its frequency spectrum is unchanged (the tone is the same).  Everyone who has lived in a dorm knows that certain materials do selectively transmit some frequencies better than others.  That's why you are woken up by the throbbing bass from your neighbor's stereo.  This frequency selectivity of normal building materials is pretty crude, however.

Recently, people have built (or actually, discovered) phononic crystals: structures that selectively reflect certain frequencies of sound waves.  Perhaps one day objects will be designed with a particular "acoustic color" as well as particular optical colors.

You can choose a somewhat dry introduction here or a more friendly and colorful summary here; the latter site is labeled 'vulgarisation', which I can only guess reveals the author's ambivalence at making such lofty material accessible to you and me.

If you want to make your own phononic crystal, follow the directions here.

Galloping Gertie

Submitted by Nicholas C. Darnton (inactive) on Saturday, 10/4/2008, at 9:14 PM
Galloping Gertie collapse

The most famous resonance of all time. 

If you've never seen the 1940 collapse of the Tacoma Narrows suspension bridge, check out one of the many videos of the event, such as this one

The torsional waves are similar to the wave demo we have in class, though of course about 100 times larger. 

A related phenomenon afflicted the Millenium Bridge in London, though the driving force was footsteps rather than wind and it didn't lead to catastrophic failure.