Roman Concrete

The concrete dome of the Pantheon in Rome. Public domain image by MariamS on Pixabay.

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 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

An iconic image of Wernher von Braun standing in front of a Saturn V rocket first stage, showing the sheer size of the F-1 engines and bell-shaped nozzles. The Saturn V was the spacecraft that carried astronauts to the Moon in 1969. Von Braun was a German rocket scientist, involved in the design of V-2 rockets which targeted London during the Blitz and later "recruited" by the Americans, eventually ending up as chief architect of the Saturn V launch vehicle.

 

Image public domain by NASA.

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?

NIST-F2 cesium fountain atomic clock. National Institute of Standards and Technology - Physics Laboratory: 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.

Related reading, the Twin Paradox

Flight Controls

A nice little animated GIF from Wikipedia showing how the flight controls in an aircraft work. The rudder pedals and control column (also known as the control stick, control yoke or control wheel) or joystick on more modern aircraft, move the control surfaces to make the aircraft roll, pitch or yaw. A): aileron, B): control stick, C): elevator, D): rudder. In early aircraft, the linkages would have been steel cables like bicycle brake cables or car handbrake cables. This system is still used on some light aircraft. As aircraft became larger, it would have been infeasible to move heavy control surfaces by hand simply because of their weight and aerodynamic forces on the surfaces (like when you try to control an umbrella in high winds) so hydraulics was used to move the surfaces (similar to the way a digger works.) Nowadays most aircraft are fly-by-wire, meaning when the pilot moves the stick, a control signal is sent to electrical servos which move the control surfaces. The Airbus A320 was the first commercial aircraft to use this system in 1987 although fighter aircraft previously used a version of the technology. Fly-by-optics is another development, allowing higher data transfer rates and immunity to electromagnetic interference (EMI). 

There's a discussion about fly-by-wire on Aviation StackExchange here.

Image attribution: Piotr Jaworski, CC BY-SA 3.0 via Wikimedia Commons. Original animated GIF converted to this MP4 video.

The Beginning of Radio Communication

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.

Photo © Eugene Brennan

 

Sharpen Your Garden Tools!

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

Image generated by AI using Bing Image Creator

Laces that keep opening do my head in. Even with double or triple knots, they seem to find a way to come undone. The reason is probably because they're made of polypropylene or nylon, two materials with a low coefficient of static friction, meaning the material is slippery. So as an experiment, I might try using one of these types of products which are used in industry to stop belt slip on machinery. The stuff, available in an aerosol can like WD40, is effectively the opposite of oil and improves friction.

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

Lightning, probably striking the River Liffey or trees in New Abbey woods. Photo © Eugene Brennan

July 25th, 1985. Probably one of the worst thunderstorms in Ireland that night. There's even a multi-page thread about it on Boards.ie. I managed to capture this stroke from my bedroom window by leaving the camera shutter open, probably hitting the trees in woodland at New Abbey. Unfortunately the photo wasn't great and it was overexposed (I've reduced the brightness, increased contrast and fiddled with the histogram), so I've been waiting for opportunities for better photos ever since. 

Here's the Met Éireann "Exceptional Weather Event" report. I didn't realise hundreds of farm animals were killed:

This is Des Cahill's RTE News report from the day after.

The Right to Repair

Image courtesy iFixit

I know one of the rules on this group is no pushing agendas, but admin does have a bit of leeway and I feel strongly about this 🙂. I think it's important for all technology so we don't get locked out of the appliances we buy and are forced to use "qualified service personnel only for servicing". Sure, you can make mistakes and have catastrophes, but you learn by doing so.

Big Picture Science Podcast — Nuts and Bolts

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SETI Astronomer Seth Shostak and producer Molly Bentley talk to Roma Agrawal about her book Nuts and Bolts: Seven Small Inventions That Changed the World in a Big Way in which she explores fasteners, pumps and some of the other inventions which shaped our world.

You may remember Seth Shostak from the epic 80s documentary series about space, Cosmos, written and presented by astronomer Carl Sagan. Seth occasionally featured in discussions about life on other planets and the SETI program (Search for Extraterrestrial Intelligence)

Graphing Calculator Tools

Image generated by AI using Bing Image Creator

I've been evaluating free online graphing tools for creating graphics on maths tutorials. Many allow very little user control over plot colour, plot thickness, axis tick frequency, grid steps, grid colour, grid line thicknesses, navigating (panning and zooming) etc. Some are good at one thing but don't allow control over other attributes of a graph or output resolution of saved images is low. Eventually I found this website with lots of graphing tools. The Geogebra tools include online 2D & 3D graph calculators, tools for drawing geometric figures and a scientific calculator. There's also downloadable applications for Windows and an Android app.

Parabolas in Nature

 

The parabola, a geometric shape that can be defined as the locus of points equidistant from an axis called the directrix and a point called the focus.

Parabolas crop up everywhere in nature and in things we make. If you kick a ball or throw a stone up at an angle into the air, the trajectory or path traced out is in the shape of a parabola. Similarly for a jet of water or projectile from a gun. Satellite dishes, radio and optical telescopes, flashlights and car headlamps all have parabolic reflectors because of a useful property of the shape: Rays from the focus of the parabola hitting the reflector are "bounced" outwards in a parallel beam (in the case of a light, heat, microwave or sound source) and vice versa for incoming parallel rays so they're focused on a detector (for a satellite dish or telescope).

Graphic created with Geogebra (GIF export resolution is a bit low and limited to 400 pixels wide, hence the fuzziness, so I have to figure out if it's possible to increase this)

Carving the Riverbed

Public domain image by Ala Beara via Wikimedia Commons

The Kilcullen Diary's article about water safety made me think about the "Sally Hole" adjacent to St Brigid's Well being one of the danger spots where the river narrows a bit. I'm thinking there's a venturi effect and speed increases (like the way a carburetor works). Possibly this scours the bend, deepening the river bed, hence the Sally Hole? 

Image courtesy OSI (Taillte Éireann)

 

 

"Nearby" Earth-Like Planet Found

Image created with Bing Image Creator

 

In our backyard, only 40 light years away. Considering a leisurely bike ride to the Sun would take 600 years, getting to the planet named Gliese 12B anytime soon would be out of the question though. 40 light years is 240 trillion miles (240,000,000,000,000 miles)

You can read the BBC article here

JFDI!

THE LONGEST HEAVY-DUTY RAILWAY BRIDGE IN THE WORLD built by the Chinese. Meanwhile the Dublin Metrolink was thought about in the 70s, proposed in 2000 and won't be completed until 2035. The Chinese system of bulldozing ahead with new projects is at the other extreme and 1.3 million people were displaced for instance to complete the Three Gorges Dam project. Still, you'd be a bit envious of how things like this get done, whereas here we have projects stalled because of rare snails, nimbyism and other ridiculous issues.

The Most Dense Metals

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Did you know that the liquid metal mercury has a density of 13.55 grams per cubic centimetre (g/cm³), more dense than lead which has a density of 11.34 g/cm³ ? Gold is even more dense at 19.28 g/cm³. The most dense metal however is osmium, with a density of 22.59 g/cm³ and is the densest naturally occurring element. Most elements don't occur naturally, and have to be refined and extracted from ore which is often a compound of the element of interest and other elements such as oxygen and sulphur. Osmium is too brittle to be used on its own, and alloyed with other metals for applications that require durability such as instrument pivots and electrical contacts. It's also used as an abrasive for polishing optical lenses.

There used to be a bottle of mercury (which was quite weighty) in the BNS in Kilcullen when it was situated in the current library building. Hopefully the mercury was disposed of safely or is now stored properly. There was also a collection of scientific apparatus, glassware, and a balance from a time when science was taught in primary schools. (Before revised curriculums in more recent decades when it was reintroduced, although we did have "Nature Studies" in the 70s). I wonder what happened all that equipment? According to this Irish Times article, science was dropped as a compulsory subject following independence in 1922 (I've run out of my free article quota for this month, so I can't read the full article yet)

Explosions and Condensation of Air

Interesting the shockwave here caused by a defence forces Javelin missile striking an armoured car and condensing the air.
Usually increases in pressure cause moisture to condense out (as in the case of an air compressor which has to be drained regularly). But that's when there's an isothermal compression and time for air to cool. In an explosion, the air compresses rapidly which makes it hot (so it should hold more moisture before condensing). But this competes with the moisture tending to condense out due to the air being compressed. Higher temperature wins out and moisture doesn't condense. It's actually the negative phase of the explosion that causes underpressure and partial vacuum that's responsible for causing air cooling and condensation (the second of the "booms" of the double boom heard during an explosion, when there's a rebound and pressure drops ). This phenomenon can also be observed when gas from an aerosol, e.g. deodorant, is released and can be at near freezing point or even produce frost, again due to the drop in temperature because of gas expansion.

Arcs and Sparks

There's something mesmerising about electric arcs and lightning. I've always found them fascinating. The dielectric strength of air is 3000 volts per mm, meaning it takes 3kV for a spark to jump a distance of one mm between rounded electrodes, less if there are sharp edges. Between the electrodes, a plasma is formed, the fourth state of matter (the other three being solid, liquid and gas). The temperature of an electric arc can be 5000 degrees C or more, hotter than the surface of the Sun, the arc generating intense short wavelength UV radiation, light and heat (and can cause sunburn when arc welding, if PPE isn't used). Arcs also generate radio waves and spark transmitters were used in the early part of the 20th century for transatlantic and ship to shore communication. Information was sent by Morse Code, and although telephones had been in use since the end of the 19th century, it would be several more years before the transmission of voice using radio waves became possible and commonplace.

Why Are the Northern Lights Different Colours?

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Usually green and pink, but sometimes blue. The display is effectively like a giant fluorescent tube (which is a type of discharge lighting, along with sodium and mercury vapour lamps, used for street lighting.) Just like the way a neon lamp gives out pink light as electrons collide with neon molecules in the lamp (an electric current is simply a flow of electrons), high energy charged particles from the Sun hit the atmosphere, causing various gases to fluoresce and emit light of different colours. Green is produced by oxygen in the atmosphere and red and pink are produced by oxygen at higher altitudes when the electrons in oxygen atoms are excited to higher levels. More info here:
https://www.space.com/aurora-colors-explained

Inertial Guidance Systems and Velocity Measurement

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The Solar System and the Earth within it is moving around the Milky Way galaxy at a velocity of 826,000 km/h. The human body can't sense constant speed, only acceleration when we're speeding up or slowing down, so we're not aware of the motion. (In physics we tend to call it velocity rather than speed. Velocity is a vector quantity having both magnitude and direction. The magnitude of the vector is the speed). Instrumentation can't directly detect velocity, except from relative motion to something else. So for instance in a car, speed was traditionally detected mechanically by having a flexible steel cable connected between the mechanical speedometer and transmission, but more likely electronically nowadays using a Hall Effect sensor or similar to count rotations of a component in the transmission such as the front axle. (Like the sensor and magnet on your bicycle wheel to measure speed). Aircraft airspeed can be measured using tubes called pitot tubes connected to the fuselage, which measure drop in air pressure as air flows over the openings. (The same pressure drop principle used in carburetors of petrol engines and paint sprayers). The variation in air pressure then gets processed and ultimately results in a display of airspeed on a gauge on the instrument panel.

What Happens if There's Nothing to Reference Against?

Navigation is possible using the Sun, stars, a compass and a good timepiece. However another way without resorting to astronomical observations or knowing longitude is to use dead reckoning. So if you know the direction you're travelling from a compass and measure the length of time for which you were travelling and the average speed you were travelling at, you can simply multiply speed by time to find the distance and angle from the start point. This can be done repeatedly at intervals, using compass readings and speed to calculate position and plot a trajectory of a vehicle or other object. If speed isn't constant, an integration process can be used to add up all the instantaneous speed x time values. For a spacecraft or missile that doesn't have wheels or air around it to measure velocity from, accelerometers have to be used. These measure acceleration, and from acceleration, velocity and also distance travelled can be calculated. (Mathematically, velocity is the integral of acceleration and distance travelled is the integral of velocity). Direction of travel is also measured in three dimensions by determining the relative angle of a craft (attitude) with respect to references provided by three gyroscopes. The complete system with gyroscopes, accelerometers and associated components and electronics is known as an inertial guidance system.