The
concrete forming the dome of the Pantheon in Rome didn't need to be
reinforced, presumably because none of the components are under tension
and the structure is effectively a 3D arch (A dome can be thought of as
an arch rotated through 360 degrees around its keystone). Arches are
strong and can withstand large loads pushing down from above with the
stones being squeezed together and put under compressive forces,
resisting breaking. They also direct forces out to the side of the arch.
Concrete or stone is strong in compression, so it can be "squashed" and
loaded by stacking pieces one on top of another as in a stone or block
wall, or pillars. However it's relatively weak in tension. So if you put
a plain concrete beam over a wide opening and load it sufficiently, the
concrete will snap on the underside as it's stretched or put under
tension. Steel is strong in tension and reinforced concrete makes use of this property by employing embedded steel bars called rebar to form a composite material. This is strong under both compressive and tensile loads.
The concrete dome of the Pantheon in Rome. Public domain image by MariamS on Pixabay.
Thursday, May 30, 2024
Roman Concrete
The Checker Shadow Illusion
Our brains are constantly making assumptions as they perceive the world around us, interpreting and interpolating or "joining the dots" when parts of a scene, sound or other input to our senses are missing. Sometimes the brain gets things wrong. Here's an example, the checker shadow illusion. Squares marked A and B are the same shade of grey, but influenced by the variation of shades around them.
Image attribution: Edward H. Adelson, CC BY-SA 4.0 via Wikimedia Commons. https://creativecommons.org/licenses/by-sa/4.0/
If you don't believe me, cut out the squares in your favourite photo editor and paste the pieces onto a white background. Alternatively, use the eyedropper tool to sample the RGB components.
This, along with visual hallucinations and pareidolia (seeing faces in places) are probably responsible for many ghost sightings.
Saturn V First Stage
Recommended Scientific Calculator App — HiPER Calc
I don't use a scientific calculator anymore because app versions are more convenient. HiPER Calc has all the basic scientific functions that you'll find on a Sharp or Casio model such as trigonometric functions, square root, exponential operations (squares and higher powers of values), log functions, base conversion etc. It also has a multiline display so you can see the formulas/operands that you're typing, plus "backspace" and cursor controls for editing entered data. It can graph functions of single and double variables (surfaces) and work out definite and indefinite integrals. I'm using it here to calculate the log of a value.
One Second in 40 Billion Years?
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| NIST-F2 cesium fountain atomic clock. Public domain image |
No,
it won't be used to make sure The Angelus starts at dead on 6 pm or even
for timing sports events. The accuracy of atomic clocks has been used
to prove the validity of Einstein's Special and General Theories of
Relativity, time dilation occurring when something moves very fast or
when subjected to lower gravity. To prove the theory, clocks were flown
on jets travelling in different directions as part of the Hafele–Keating
experiment in 1971. When the clock on the eastbound jet was compared to
a clock remaining on the ground, it was found that it was was slower
than the ground-based one by 40 nano seconds. It ran slower because of
the speed of the jet, but faster because of the reduced gravity at
cruising altitude, the net loss being 40 nS. If we could travel at close
to the speed of light in a spacecraft, and could see what's happening
on Earth, everything would seem to be happening at super fast-forward
speed, while time would progress at a normal speed for us. From the
point of view of an observer viewing us from Earth, time would appear to
stand still in our spacecraft. Atomic clocks have practical uses on GPS
satellites, nanosecond accuracy giving good positional accuracy when we
use GPS to find our position on Earth. This latest clock will have uses
presumably for studying the fundamental behaviour of the Universe.
You can read about the new clock on the Popular Mechanics site here.
Wednesday, May 29, 2024
The Beginning of Radio Communication
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| Photo © Eugene Brennan |
From one of the old books that belonged to my grandfather. A note in the appendix of a volume of the ICS reference library, 1905. If only they knew the developments that were to come in the future! In modern parlance, "the high-frequency transmission wave" is known as a carrier and the modification of the carrier by the superimposed wave is known as modulation. Sound waves having frequencies in the audible spectrum can't just be converted to an equivalent radio signal and transmitted. The frequency would be too low and for various technical reasons, including providing sufficient bandwidth for other channels broadcasting on and sharing the radio spectrum, and minimizing the size of antenna required, a technique called modulation is used. This is commonly either either amplitude modulation (AM) or frequency modulation (FM). The carrier is varied by a modulating signal derived from e.g. an amplified signal from a microphone, which changes the carrier's "size" (amplitude) or frequency. Originally, carriers were low, in the hundreds of kHz or somewhat lower. Carrier frequencies nowadays are vastly higher, typically 5 gigahertz (GHz) for WIFI and up to 108 MHz for normal radio broadcast programs on the FM band.
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| Photo © Eugene Brennan |
Sharpen Your Garden Tools!
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| Photo © Eugene Brennan |
Common
sense physics. Reduce the area of an edge and you get higher pressures.
It's high pressure that cuts because of the tiny area of a sharp edge, not the force. Pressure = force divided
by area, so reducing area by sharpening a blunt edge increases pressure for the same force and tools cut better.
Annoying Laces
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| Image generated by AI using Bing Image Creator |
https://www.amazon.co.uk/Hycote-XUK301.../dp/B003MLUU5O
In machines where the components aren't subjected to high loads which would cause a lot of wear, nylon is often used instead of more costly steel or brass. So for instance gears and pulleys are made of nylon, the advantage being its low coefficient of friction and no requirement to use grease or oil for lubrication. Most analogue wall clocks have nylon gears in the mechanism.
© Eugene Brennan
Severe Thunderstorm of 1985
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| Lightning, probably striking the River Liffey or trees in New Abbey woods. Photo © Eugene Brennan |
and Des Cahill's report on the event, courtesy the RTÉ Archives site: