ARISTOTELIAN PHYSICS

Aristoteles (Aristotle) (384-322 BC)
had very strong influence on European philosophy and science

• "four elements":
  everything on Earth made of (mixture of) four elements:
  earth, water, air, fire
• every element has a "natural plac": earth at center of Earth, water above earth, air above water, fire above air;
• celestial bodies (stars, planets, Moon) made from fifth element, "ether", which also fills space between them;
  ether is perfect, incorruptible, weightless
• two kinds of motion of things on Earth:
  ``natural'' and "violent" motion
• natural motion:
  things tend to move towards their natural place - natural motion, happens by itself, needs no push/pull (e.g. stone falls).
• violent motion:
  = motion contrary to natural motion; needs effort (external push or pull)
• celestial motion:
  = natural motion of ether; natural motion of bodies made from ether is circular motion, regular and perpetual

PROBLEMS WITH ARISTOTELIAN PHYSICS:

(Galileo Galilei's thought experiments and real experiments:)

• falling bodies:
  • according to Aristoteles, heavy bodies (contain more earth element) fall faster than lighter bodies
  • observation: fall equally fast if they have same shape and size
  • Galilei: difference in speed of differently shaped falling bodies due to air resistance
• thought experiment about two falling bodies -
  "reductio ad absurdum":
  • consider two bodies, one light (L), one heavy (H)
  • Aristoteles: L falls more slowly than H
  • L put under H should slow down fall of H
    with L under it should fall more slowly than H alone;
  • but (L + H) heavier than H alone should fall faster than H alone
    contradiction
• pendulum: ball suspended on string reaches same height as that to which it was lifted to set it in motion (not quite; - due to friction) height independent of path (pendulum with shortened string)
• ball rolling on inclined plane:
  • ball rolling down inclined plane speeds up; ball rolling up slows down
  • rate of slowing down depends on steepness of incline: less steep longer distance travelled
  • extrapolation to zero slope of incline: ball will go on forever

GALILEI'S NEW SCIENCE

Galileo Galilei (1564 - 1642)
founder of modern science;

• new methods introduced by Galilei include:
  • controlled experiments designed to test specific hypotheses
  • idealizations to eliminate any side effects that might obscure main effects
  • limiting the scope of inquiry - consider only one question at a time;
  • quantitative methods - did careful measurements of the motion of falling bodies
• from observations and thought experiments,

• LAW OF INERTIA:
  without external influence (force) acting on it, a body will not change its speed or direction of motion; it will stay at rest if it was at rest to begin with.
• inertia = property of bodies that makes them obey this law, their ability to maintain their speed (or stay at rest)

• LAW OF FALLING:
  If air resistance is negligible, any two objects that are dropped together will fall together; speed of falling independent of weight and material.

POSITION AND COORDINATES

• to specify a position, need:
  • reference point (``origin'') O,
  • distance from origin
  • direction from origin
    (to define direction, need reference direction(s)
• position along a line:
  position specified by one (signed) number
• position in a plane:
  position of point P specified by
  • length of ``vector'' (distance)
    and angle of wrt reference direction,
  • or by two numbers x,y
• position in 3-dimensional space:
  need a third number (e.g. height above the x-y plane