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Physics
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Democritus (460-370 BC) was a Greek philosopher who advanced the theory that all matter consisted of tiny, indivisible and also invisible particles called atoms. These atoms are in constant motion and have properties that explain the physical characteristics of substances. They are eternal and cannot be created or destroyed. While this theory was discredited in Aristotelian thought, modern physics proved it accurate, though with many modifications: although atoms as building blocks of matter determine the characteristics of substances, they themselves are made up of even smaller subatomic particles. |
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 Archimedes (3rd century BC) was the greatest mathematician
of the ancient world, and also an engineer. He discovered the principle
of buoyancy as well as the principle of the lever. The equation governing
the latter is shown on this Nicaraguan stamp. (Detail).
Another invention attributed to him is the Screw of Archimedes, shown
on the right. A cylinder containing a screw is placed in water, which
is then transported to a higher level when the screw is turned. For more
stamps, check out Stamps
of Archimedes and also Mathematics and Computation I. |
 Otto von Guericke (1602-1686) was a German natural philosopher and writer
who also became the mayor of Magdeburg. Inspired by the work of Torricelli
and Galileo, he tried to create a vacuum and invented the air pump. He
devised the astonishing spectacle of the Magdeburg Hemispheres, which
brought him fame in his own time. Here two large metal hemispheres were
joined together with a gasket and evacuated; two teams of horses pulling
in opposite directions were unable to separate them. Once air was readmitted,
they fell apart. |
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Evangelista Torricelli (1608-1647), the Italian physicist and mathematician, is famous for his discovery of the principle of the barometer. He was Galileo's assistant until the latter's death. The unit Torr (1/760 of a standard atmosphere) bears his name. (Detail). | |
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Robert Boyle (1627-91), British physicist and chemist, formulated Boyle's Law for an ideal gas: at constant temperature, pressure and volume are inversely proportional. With Robert Hooke, he developed an improved air pump (shown on the stamp) based on von Guericke's designs. Boyle was foremost an experimentalist who recorded data so that experiments could be reproduced, in what is now called the scientific method. | |
Christiaan Huygens (1629-1695) was a key figure of 17th century physics, astronomy and mathematics. A member of the French Academy of Sciences and the Royal Society, he made many important discoveries. His telescopes had improved lenses of his own devising and surpassed those of his contemporaries. He was the first to see a moon of Saturn (Titan), and determine that Saturn had a ring. He constructed and patented the first pendulum clock to aid mariners in navigation, but the effects of a ship's motion defeated his purpose of dealing with the longitude problem. He was interested in games of chance and wrote the first book dealing with the theory of probability. He also advanced the wave theory of light. |
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Blaise Pascal (1623-62), the French philosopher remembered by many college students of that language for his Pensees, was also a mathematician and physicist of note. He defended Toricelli's assertion that a vacuum existed in the enclosed end of a barometer, and that air had weight. He took a barometer up a mountain, demonstrating that air became thinner with altitude and therefore exerted less pressure to counterbalance a column of mercury. Pascal worked out the hydraulic principle, that pressure is transferred through a fluid in an enclosed container, and he also developed a calculating machine that was capable of adding and subtracting. Like for many other inventors of calculating devices, technical difficulties encountered here outweighed sound principles. (Detail). | |
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 James Watt (1736-1819) was a Scottish engineer who did not actually invent the steam engine, but devised such improvements to the existing Newcomen steam engine as to make it commercially useful. His invention converted reciprocal engine motion to rotary motion. In 1782 he Invented the double-acting engine. To rate his engines, he devised the term horsepower, a unit describing the work done by a horse (the usual source of energy at that time) in pound-foot/minute. The SI system unit of power is called the watt in his honor. He also invented, in 1767, an attachment that adapted telescopes for distance measurement. The Mali stamp shows the double-acting engine. |
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Austrian physicist Christian Doppler (1805-1853) derived the mathematical reliationship describing the change in the frequency of sound observed when source and receptor are moving relative to each other. We are familiar with the increasing pitch of a train whistle as the train approaches, and the decreasing pitch when it has passed and is moving away. The sound waves are crowded together and then strung apart. The same principle applies to electromagnetic waves, for example in astronomical observations of distant galaxies or other objects which show spectral shifts in their emission of light. Red shifts of galactic spectra indicate that the galaxy is moving away, a sign that the universe is expanding. The motion of weather systems can likewise be detected by constantly reflecting radio waves. The weather report references to Doppler radar are now commonplace. | |
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 The Austrian
physicist and philosopher Ernst Mach (1838-1916) investigated the flight
of projectiles both theoretically and photographically and noted the dynamic
processes that occurred once the speed of sound was exceeded. His name
lives on with the Mach number, the ratio of an object's speed to the speed
of sound in the medium in which it travels. The first aircraft to achieve
supersonic speed was the American X-1 rocket plane, piloted by Chuck Yager
in 1947, built in Buffalo, NY, and shown on the stamp at right. Modern
aircraft have traveled many times that speed, as have spacecraft. The
sonic boom, which can sometimes be heard, is due to the compressed sound
waves ahead of the plane, intersecting with the ground below. |
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Leon Foucault (1819-68), the French physicist, is remembered for his pendulum experiment which proved the rotation of the earth. A very long pendulum is suspended and left to swing in the vertical plane containing the pendulum bob and the point of suspension. As the earth rotates beneath the bob, the plane of oscillation appears to rotate uniformly 2pi cos theta radians a day, where theta is the colatitude of that point on earth. Visitors to the United Nations in New York may observe just such a pendulum in the lobby. Foucault also successfully measured the speed of light by means of a system of rotating mirrors, and observed a bright yellow spectral line in an electric arc light at the exact position of the yellow lines observed by Fraunhofer.(Detail). |
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 Heike
Kamerlingh Onnes (1853-1926) discovered superconductivity when studying
the properties of substances at very low temperatures. He succeeded in
liquifying helium by first cooling it with liquid air. In 1913 he received
the Nobel prize in physics for his investigations. This work was continued
by, among others, the Russian Piotr Kapitsa (1894-1984), who discovered
a way to produce liquid helium by cooling it by periodic expansion. At
2.3 K, this liquid changes into a new form, He II, with unusual properties:
a superfluid with vanishing viscosity, a quantum fluid with zero entropy.
Kapitsa shared the 1978 Nobel prize in physics for his remarkable experimental
and technical achievements in the study of superconductivity. See Penzias
and Wilson. |
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 K.
E. Tsiolkovsky (1857-1935) was a self-trained Russian scientist who
was a pioneer of space flight and rocketry. His writings, about liquid
fuel and multi-stage rockets and space travel, predated the work of
the American rocket engineer Robert Goddard and laid the foundation
for the Russian space program launched with Sputnik in 1957. His "rocket formula" of
1903 on the stamp describes the increase in a rocket's velocity: (u-u0)/v=ln(M0/M),
where u,u0 and M,M0 are initial and final velocity and mass of the
rocket, respectively, and v is the velocity of the exhaust gases. On
the stamp, inside the outline of a space capsule suspended from two
parachutes, we see a blazing multi-stage rocket over the curved earth
and a much enlarged outline of central America featuring Nicaragua.
(Detail). |
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