Measures of Motion

• Gravity causes objects with mass to move. Examples include an apple falling toward the ground and the Earth going around the Sun. So, we need to understand a bit about motion.
• Velocity or speed is the rate of change of position of an object with time. The units of velocity are:
  

or meters per second (m/sec). When you are cruising down I-25 at the speed limit, your velocity is 65 miles/hr or 29.1 m/sec. This means that in 1 hr, you travel 65 miles or in 1 second, you travel 29.1 meters.

• Acceleration is the rate of change of velocity with time. The units of acceleration are:
  

or meters per second per second (m/sec²). If you go from zero to 65 miles/hr (29.1 m/sec) in 10 seconds, your acceleration is 2.91 m/sec². Gravity causes objects to accelerate toward the center of mass of planets or the Sun or other stars.

 

Ancient Astronomy

• The Greeks, led by Aristotle & Ptolemy, envisioned an idealized universe with the Earth at the center and all planets, the Sun, & the Moon revolving around us in grand circles.
• The Greek astronomers preferred an Earth-centered or geocentric universe because of the lack of observed parallax of the stars. Parallax is produced when a nearby object appears to move relative to a distant background due to the motion of the observer.
• Interestingly, the planets were observed to undergo retrograde motion on occasion. Planets basically move west to east relative to the stars, but occasionally will reverse direction and travel east to west for weeks or even months. To explain this, the Greeks resorted to the use of epicycles and deferents. This made for complex motions and did not do a good job of predicting future positions of planets.

About early history of science:

• Plato: 427 - 350 B.C.
  • Socrates's pupil and close friend. Socrates was executed by Athens (was forced to drink a fatal cup of hemlock. Reacting to shifting moral values of his time, Plato searched for UNCHANGING standards. He turned to the world of ideas (``Platonism''). The cause of the world coming into being is the idea. The world is generated to fulfill the idea.
  • Do not trust your feelings. Allegory of the Cave.
  • Saving the phenomena

• Aristotle: 384-322 B.C
  • Poetics: poetry - comedy and tragedy. Tragedy: six factors - scenic presentation, lyrical song, diction, plot, character, thought.
  • Look for the cause of things. ``Why'' is the main question.
  • Final cause - purpose of an action.
  • Motion: not just changing places, but more general as fulfillment of Potential. (Growth of a plant, growth of a society ...).
  • Every motion has its INTERNAL cause and purpose.
  • Logic: reductio ad absurdum.
  • Earth is a sphere in the center of the Universe.
  • The universe does not change: everything is on circular motion.
• Ptolemy: Second century A.D., Alexandria. Author of Almagest
• Pythagoras: 585 - 495 B.C. The theorem. NUMBER. Beginning of Math.
• Ionian Cosmology (now Turkey and Greek Islands): search for fundamental elements. Beginning of chemistry.
  • What was difficult? ``To know thyself''.
  • True philosopher pursues scientific inquiry for its own sake; others demand that science pay in practical results.
  • Ended in 494 B.C. : Persian invasion.