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  1. Counting in Babylon

    Babylon had in all probability the earliest written language. At the same time, an elegant system of weights and measures kept the peace in the marketplace. Their method of counting was in some ways better than our present one! We look at some ancient math tables, and ideas about Pythagoras' theorem 1,000 years before Pythagoras.

  2. Early Greek Science: Thales to Plato

    In the ancient port city of Miletus, there took place a "discovery of nature": philosophers tried to understand natural phenomena without invoking the supernatural. The Greeks imported basic geometric ideas from Egypt, and developed them further. Members of the Pythagorean cult announced the famous theorem, and the (to them) alarming discovery of irrational numbers! The Greeks had some ideas about elements and atoms. Hippocrates looked for non-supernatural causes of disease. Plato formulated a rationale for higher education, and thought about atoms.

  3. Motion in the Heavens: Stars, Sun, Moon, Planets

    A brief review for moderns of facts familiar to almost everybody in the ancient world: how the sun, moon and planets move through the sky over the course of time.

  4. Aristotle

    A brief look at the beginnings of science and philosophy in Athens: Plato's Academy and Aristotle's Lyceum. On to Aristotle's science: "causes" in living things and inanimate matter, Aristotle's elements, and laws of motion.

  5. Measuring the Solar System

    We look at some startlingly good measurements by the Greeks of the size of the earth and the distance to the moon, and a less successful (but correct in principle) attempt to find the distance to the sun.

  6. Greek Science after Aristotle

    Strato understood that falling bodies pick up speed (contrary to Aristotle's assertions). Aristarchus gave a completely correct view of the solar system, anticipating Copernicus by 2,000 years or so. Science flourished for centuries in Alexandria, Egypt: Euclid, Apollonius, Hypatia and others lived there, Archimedes studied there. Archimedes understood leverage and buoyancy, developed military applications, approximated Pi very closely, and almost invented calculus! (See also the next lecture.)

  7. Basic Ideas in Greek Mathematics

    Nailing down the square root of 2. Zeno's paradoxes: Achilles and the tortoise. Proving an arrow can never move - analyzing motion, the beginning of calculus. How Archimedes calculated Pi to impressive accuracy, squared the circle, and did an integral to find the area of a sphere.

  8. How the Greeks used Geometry to Understand the Stars

    The universe is like an onion of crystal spheres: Plato, Eudoxus, Aristotle. More earthly ideas: Eudoxus and Aristarchus. Understanding planetary motion in terms of cycles and epicycles: Hipparchus and Ptolemy. These methods were refined to the point where they gave accurate predictions of planetary positions for centuries (even though Ptolemy believed the earth was at rest at the center of the universe).

  9. Galileo and the Telescope

    Copernicus challenged Ptolemy's worldview. Evolution of the telescope. Galileo saw mountains on the moon, and estimated their height - the first indication that the moon was earthlike, not a perfect ethereal sphere at all.

  10. Life of Galileo

    A few facts and anecdotes to try to give something of the flavor of Galileo's life and times, plus references to books for those who would like a more complete picture.

  11. Scaling: why giants don't exist

    One of Galileo's most important contributions to science (and engineering): the realization that since areas and volumes scale differently when the size of an object is increased keeping all proportions the same, physical properties of large objects may be dramatically different from similar small objects, not just scaled up versions of the same thing. We explore some of the consequences.

  12. Galileo's Acceleration Experiment

    Galileo argued against Aristotle's assertions that falling bodies fall at steady speeds, with heavier objects falling proportionately faster. Galileo argued that falling bodies pick up speed at a steady rate (until they move so fast that air resistance becomes important). He constructed an experiment to prove his point (and we reproduced it).

  13. Naturally Accelerated Motion

    This lecture presents the core of Galileo's analysis of motion in free fall, which he referred to as "naturally accelerated motion". This is challenging material if you're new to it, but crucial in progressing from an Aristotelian or medieval worldview to that of Galileo and Newton, the basis of our modern understanding of nature. Galileo used his new-found understanding of falling motion to prove that a projectile follows a parabolic path, if air resistance can be ignored.

  14. Describing Motion

    A simple introduction to the modern way of describing motion using arrows - "vectors" - to indicate speed and direction. Galileo (and, later, Newton) made heavy use of Greek geometry in analyzing motion. It's much easier, and just as valid, to use vectors.

  15. Tycho Brahe and Johannes Kepler

    These two colorful characters made crucial contributions to our understanding of the universe: Tycho's observations were accurate enough for Kepler to discover that the planets moved in elliptic orbits, and find some simple rules about how fast they moved. These became known as Kepler's Laws, and gave Newton the clues he needed to establish universal inverse-square gravitation.

    This lecture links to more detailed lectures I gave previously, and to a neat NASA simulation of Kepler's laws.

  16. Isaac Newton

    A brief account of Newton's life, followed by a discussion of perhaps his most important insight: that a cannonball shot horizontally, and fast enough, from an imagined mountaintop above the atmosphere might orbit the earth. This tied together Galileo's understanding of projectiles with the motion of the moon, and was the first direct understanding (as opposed to description) of motion in the heavens.

  17. How Newton Built on Galileo's Ideas

    Newton's famous Laws of Motion generalized and extended Galileo's discussion of falling objects and projectiles. Putting these laws together with his Law of Universal Gravitation, Newton was able to account for the observed motions of all the planets. This lecture gives a careful development of the basic concepts underlying Newton's Laws, in particular the tricky concept of acceleration in a moving body that is changing direction - essential to really understanding planetary motion.

  18. The Speed of Light

    Aristotle thought it was infinite, Galileo tried unsuccessfully to measure it with lanterns on hilltops, a Danish astronomer found it first by observing Jupiter's moons. Rival Frenchmen found it quite accurately about 1850, but a far more precise experiment was carried out in 1879 in Annapolis, Maryland by Albert Abraham Michelson.

  19. The Michelson-Morley Experiment

    By the late 1800's, it had been established that light was wavelike, and in fact consisted of waving electric and magnetic fields. These fields were thought somehow to be oscillations in a material aether, a transparent, light yet hard substance that filled the universe (since we see light from far away). Michelson devised an experiment to detect the earth's motion through this aether, and the result contributed to the development of special relativity.

  20. Special Relativity

    Galileo had long ago observed that in a closed windowless room below decks in a smoothly moving ship, it was impossible to do an experiment to tell if the ship really was moving. Physicists call this "Galilean relativity" - the laws of motion are the same in a smoothly moving room (that is to say, one that isn't accelerating)as in a room "at rest". Einstein generalized the notion to include the more recently discovered laws concerning electric and magnetic fields, and hence light. He deduced some surprising consequences, recounted below.

  21. Special Relativity: What Time is it?

    The first amazing consequence of Einstein's seemingly innocuous generalization of Galileo's observation is that time must pass differently for observers moving relative to one another - moving clocks run slow. We show how this comes about, and review the experimental evidence that it really happens. We also show that if times pass differently for different observers, lengths must look different too.

  22. Special relativity: Synchronizing Clocks

    Another essential ingredient in the relativistic brew is that if I synchronize two clocks at opposite ends of a train I'm on, say, they will not appear to be synchronized to someone on the ground watching the train go by. (Of course, the discrepancy is tiny at ordinary speeds, but becomes important for speeds comparable to that of light).

  23. Time Dilation: A Worked Example

    At first sight, it seems impossible that each of two observers can claim the other one's clock runs slow. Surely one of them must be wrong? We give a detailed analysis to demonstrate that this is a perfectly logically consistent situation, when one remembers also to include effects of length contraction and of lack of synchronization - special relativity makes perfect sense!

  24. More Relativity: the Train and the Twins

    Some famous paradoxes raised in attempts to show that special relativity was self-contradictory. We show how they were resolved.

  25. Momentum, Work and Energy

    An elementary review of these basic concepts in physics, placed here for the convenience of nonscience majors who may be a little rusty on these things, and will need them to appreciate something of what relativity has to say about dynamics - the science of motion.

  26. Adding Velocities: A Walk on the Train

    A straight forward application of the new relativistic concepts of time dilation, length contraction etc., reveals that if you walk at exactly 3 m.p.h. towards the front of a train that's going exactly 60 m.p.h., your speed relative to the ground is not 63 m.p.h. but a very tiny bit less! Again, this difference from common sense is only detectable if one of the speeds is comparable with that of light, but then it becomes very important.

  27. Conserving Momentum: the Relativistic Mass Increase

    How the very general physical principle of momentum conservation in collisions, when put together with special relativity, predicts that an object's mass increases with its speed, and how this startling prediction has been verified experimentally many times over. The increase in mass is related to the increase in kinetic energy by E = mc2. This formula turns out to be more general: any kind of energy, not just kinetic energy, is associated with a mass increase in this way. In particular, the tight binding energies of nuclei, corresponding to the energy released in nuclear weapons, can be measured simply by weighing nuclei of the elements involved.

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