标签: physics

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Engineers and scientists exist in a world defined by many metrology standards and constants. We start with time, mass, and length, and then expand to electric current, temperature, and many others. There are also fundamental physical constants such as the speed of light or Avogadro's number.   While all these constants are important, some of them are far removed from our daily lives. But one is not: the gravitational constant G. Even since Isaac Newton formulated the law of gravitational attraction F = G (mass1 × mass2)/r2, inspired by that apple falling from a tree , the value of G has been of great interest. Given how pervasive and accessible gravity is, it should be pretty easy to measure G accurately, right?   Well, yes and no. It turns out that gravity is easy to measure, but hard to measure with precision. A fascinating article in the latest issue of Physics Today, " The search for Newton’s constant ," discusses the history of measuring G. It looks at the various experimental setups that have been used over several hundred years (torsion-balance, pendulum, beam-balance, and others) and the data spread in results of each. Some of the sophisticated tests by serious researchers produce results with low uncertainty, yet they differ significantly from other tests, which also claim low uncertainty.   While researchers have certainly improved the accuracy and precision of their results, the article explains why G is still so hard to measure. It's not only an interesting, well written article, it's also a sobering and thought-provoking one as well, because you likely assumed that G's value is pretty much nailed down solid, end of story.   Yet, as most engineers and scientists know, getting consistent, accurate results in any test-and-measurement challenge to better than three or four significant figures is rarely easy. Every added significant figure means ever-more-subtle sources of error must be uncovered, understood, calibrated out, or compensated for in the fixture and equipment.   If you're lucky, the test can be structured so some of these errors actually drop out, or self-cancel, much as the value of mass m cancels out in some basic physics experiments and even carnival rides, such as the "rotor ride" or Gravitron (Figure 1) where participants "stick" to the wall via centripetal force and friction. The mass of the person doesn't matter, only the size of the rotor, the speed of rotation, and the coefficient of friction between their clothes and the wall (Figure 2). (If you can't explain why the person sticks, and why their weight is not a factor, go to a basics physics book.)   Figure 1 The Gravitron's rotor spins and pins people to the wall. A functionally similar but more sophisticated version is used in centrifuges to acclimate astronauts to high-G environments. (Source: NASA)   Or maybe there's another explanation about the elusiveness of a precise, accurate value of G, one that keeps physicists and metrologists worrying: Perhaps the "squared" exponent in the denominator of Newton's Law is not exactly 2.0 out to as many places as you care to pick. Or maybe G itself is not a true constant, but actually changes slightly over time and place. Stranger things have happened; just ask those physicists who believed in the absoluteness of time and distance, but had to change their beliefs to accommodate the curvature of time and space, as well as time dilation itself and even E = mc², as Einstein's 1905 paper on Special Relativity became accepted principle.   Figure 2 The rotor ride spins and people inside the cylinder stick to the wall, irrespective of their mass. Riders are subject to three forces: weight, normal force, and frictional force. (Source: stuegli.com )   Have you ever had a constant or fixed assumption in engineering or science that you had to abandon or at least become flexible about? Have you ever stopped and wondered what "gravity" is, as well? What are your thoughts are gravity waves and gravitational frame-dragging, as Gravity Probe B is exploring?

For engineers and scientists, the world is defined by many metrology standards and constants. We start with time, mass, and length, and then expand to electric current, temperature, and many others. There are also fundamental physical constants such as the speed of light or Avogadro's number.   While all these constants are important, some of them are far removed from our daily lives. But one is not: the gravitational constant G. Even since Isaac Newton formulated the law of gravitational attraction F = G (mass1 × mass2)/r2, inspired by that apple falling from a tree , the value of G has been of great interest. Given how pervasive and accessible gravity is, it should be pretty easy to measure G accurately, right?   Well, yes and no. It turns out that gravity is easy to measure, but hard to measure with precision. A fascinating article in the latest issue of Physics Today, " The search for Newton’s constant ," discusses the history of measuring G. It looks at the various experimental setups that have been used over several hundred years (torsion-balance, pendulum, beam-balance, and others) and the data spread in results of each. Some of the sophisticated tests by serious researchers produce results with low uncertainty, yet they differ significantly from other tests, which also claim low uncertainty.   While researchers have certainly improved the accuracy and precision of their results, the article explains why G is still so hard to measure. It's not only an interesting, well written article, it's also a sobering and thought-provoking one as well, because you likely assumed that G's value is pretty much nailed down solid, end of story.   Yet, as most engineers and scientists know, getting consistent, accurate results in any test-and-measurement challenge to better than three or four significant figures is rarely easy. Every added significant figure means ever-more-subtle sources of error must be uncovered, understood, calibrated out, or compensated for in the fixture and equipment.   If you're lucky, the test can be structured so some of these errors actually drop out, or self-cancel, much as the value of mass m cancels out in some basic physics experiments and even carnival rides, such as the "rotor ride" or Gravitron (Figure 1) where participants "stick" to the wall via centripetal force and friction. The mass of the person doesn't matter, only the size of the rotor, the speed of rotation, and the coefficient of friction between their clothes and the wall (Figure 2). (If you can't explain why the person sticks, and why their weight is not a factor, go to a basics physics book.)   Figure 1 The Gravitron's rotor spins and pins people to the wall. A functionally similar but more sophisticated version is used in centrifuges to acclimate astronauts to high-G environments. (Source: NASA)   Or maybe there's another explanation about the elusiveness of a precise, accurate value of G, one that keeps physicists and metrologists worrying: Perhaps the "squared" exponent in the denominator of Newton's Law is not exactly 2.0 out to as many places as you care to pick. Or maybe G itself is not a true constant, but actually changes slightly over time and place. Stranger things have happened; just ask those physicists who believed in the absoluteness of time and distance, but had to change their beliefs to accommodate the curvature of time and space, as well as time dilation itself and even E = mc², as Einstein's 1905 paper on Special Relativity became accepted principle.   Figure 2 The rotor ride spins and people inside the cylinder stick to the wall, irrespective of their mass. Riders are subject to three forces: weight, normal force, and frictional force. (Source: stuegli.com )   Have you ever had a constant or fixed assumption in engineering or science that you had to abandon or at least become flexible about? Have you ever stopped and wondered what "gravity" is, as well? What are your thoughts are gravity waves and gravitational frame-dragging, as Gravity Probe B is exploring?

1927. It was an interesting year as all kinds of things were happening. For example, work started on the Mount Rushmore National Memorial in South Dakota, Charles Lindbergh made the first solo transatlantic non-stop flight from New York City to Paris, and Leon Trotsky was expelled from the Communist Party allowing Josef Stalin to take control (that was not a good day, generally speaking). Oh yes ... one other thing ... the world's longest-continuously-running scientific experiment was kicked off, which means this little rascal has now been running for 85 years. Thomas Parnell (1881 – 1948) was the first Professor of Physics at the University of Queensland. Professor Parnell wanted to demonstrate to students that some substances that appear to be solid are in fact very-high-viscosity fluids. In order to do this, he took some tar pitch – a type of tar that is so brittle you can shatter it with a hammer – heated it up, poured it into a funnel, and let it cool and solidify ... for three years! After that time he broke the seal at the bottom of the funnel and waited for the tar pitch to start dripping out. And drip it did, although (thus far) no one has actually observed a drop fall. The first drip occurred in 1938 – eight years after the bottom of the funnel was opened. This was followed by drips in 1947, 1954, 1962, 1970, 1979, 1988, and 2000.   Based on these results, experimenters now calculate that the pitch has a viscosity approximately 230 billion (2.3×10 11 ) times that of water. Furthermore, based on the current drip rate, it is anticipated that the experiment will continue for another 100 years or so before all of the pitch has exited the funnel. You can watch this ongoing experiment on webcam by clicking here (the next drip could happen anytime). And speaking of long-running "happenings", have you heard about the renowned kinetic sculptor Arthur Ganson who creates mechanical art demonstrations and Rube Goldberg machines with existential themes? Some of Ganson's extremely elaborate creations have only one very simple function, such as oiling themselves or causing a chair to bounce around a toy cat, while others do nothing at all, but in a visually fascinating manner. The one that really grabbed me when I saw it is the Machine with Concrete as illustrated below.   This little beauty runs uninterrupted even though the final gear is embedded in concrete, and the gear reductions mean the final gear will make one revolution in roughly 2.3 trillion years.