Monday (Sep 20):

 

 $10 Lab Fee due 30 Sep (or fine)

 

Key Concept: Temp

 

 L#6:  1-16, 20   Due Monday 20 Sep

(Temperature)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Tues 21 Sep Lab: Mass, Weight & Density

Review

 

Test L# 1-6 Thurs

 

 

 

Thurs/Fri    30Sep/1 Oct Library ( Key scientists)

 

 L#7:  1-3, 7 - 14, 17   Due Mon 27 Sep (Motion & Avg Acceleration)

 L #8: 1- 7, 12, 15, 18    Due Wed 29 Sep  (Friction)

L #10: 1- 3, 7 - 10, 14, 17    Due Mon 4 Oct (Motion Graphs)

L #11:  1- 4, 9, 11,  13, 15, 18 Due Wed 6 Oct (Newton's 2nd & 3rd)

L #14:  1- 3, 5, 10, 11, 14 Due Mon 11 Oct  (Free Body)  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Introduction to Motion:

Aristotle

384 B.C. Birthplace: Macedonia (now Greece) Died: 322 B.C.

Aristotle's writings on motion are important for at least 2 reasons:

  • They influenced scientific thought for centuries.
  • They provide a "common sense" framework that most people would agree "make sense."

Aristotle categorized motions as either "natural" motions or "violent" motions:

Natural Motion:

Any motion that an object does naturally - without being forced - was classified by Aristotle as a natural motion. Examples of natural motions include:

  • A book lying at rest on a table naturally remains at rest.
  • If you let go of a book it naturally falls toward the earth.
  • Smoke naturally rises.
  • The sun naturally rises in the east, crosses the sky, then sets in the west.

Violent Motion:

Aristotle classified any motion that required a force as a "violent motion". (He did not mean violent in the modern sense...) Examples of violent motion include:

  • Pushing a book along a table.
  • Lifting a book.

Summary:

Basically, Aristotle's view of motion is "it requires a force to make an object move in an unnatural" manner - or, more simply, "motion requires force".

After all, if you push a book, it moves. When you stop pushing, the book stops moving. (Not right away, of course, but, unless you push it, it gradually slows to a stop.) To keep a bicycle moving (on level ground) you have to keep pedaling. To keep a car moving, you have to keep the engine pushing it.

To most people, this is a very reasonable and "common sense" notion. There are only two problems with this idea:

  1. It doesn't work, and
  2. It isn't logical.

 

 

 Copernicus

The Solar System Before Copernicus:

The Greeks had many and varied ideas about the structure of the universe - after all, they were critical and original thinkers.

Aristotle's wrote that the heavenly bodies were fundamentally different from earthly bodies, both in behavior and composition. In Aristotle's view, the heavens were perfect and unchangeable.

The Alexandrian Greek scientist Ptolemy, following Aristotle, wrote that (naturally) the earth was at rest in the center of the universe, and the Sun, Moon, planets, and stars moved about the Earth in circular orbits. (A solar system with the Earth at the center is called a geocentric solar system - "geo" = Earth, etc.)

Historically, scholars of the Middle Ages also had great respect for the ideas of the Greeks, and gradually many ideas of Aristotle - and Ptolemy - became linked to Church doctrine.

Scientifically, the big problem with Ptolemy's ideas were the orbits of the planets. Viewed from the Earth over the course of several months, planets have a strange motion - sometimes they move forward, sometimes they stop, and move backward (retrograde motion). Actually, Greek astronomers had made accurate-enough observations by Ptolemy's time to know that circular orbits for the planets just didn't fit - if you predict the position of a planet using the idea of circular orbits, then look for the planet in that position - it isn't there.

Greeks tried to modify their model:

  • Every time more-accurate observations are made, more fiddling has to be done to the model (making it more complicated) in order for the model to fit the observations.
  • All of these complications didn't really fit well with Aristotle's idea of "elegant perfection".
  • There is a growing feeling among scientists that "nature must actually be simpler than this!"

Copernicus's Ideas:

 

Nicholas Copernicus (1473 - 1543)

The Polish Copernicus suggested, in the late 16th century, that the Sun was actually the center of the solar system, and that the Earth was a planet that revolved about the sun, just like any other planet. (This is a heliocentric solar system - "Helios" = sun...)

What Copernicus had done was no less than to change people's place in the universe. In the universe of the time (Ptolemy's, essentially), the Earth was the center of the Universe.  Copernicus reduced the Earth to one of several planets revolving one of billions of stars.

Scientific Objections to Copernicus:

There were serious scientific objections to Copernicus' ideas also. Copernicus knew that he couldn't answer these objections any more than he could answer the religious objections. For instance:

The heliocentric solar system requires the Earth to rotate on its axis once per day which means that you, at this moment have a speed of about 1 000 mi/hr (= 25 000 mi/24 hr). (If you calculate the speed that the Earth must have to orbit the Sun once per year, it makes 1 000 mi/hr seem pokey...)

  • Why does it feel like the Earth is standing still?
  • Why can't such an enormous velocity be detected?
  • Why isn't everything (you, your dog, the air, etc.) on the Earth flung off into space as the Earth rotates at such a high speed?
  • What keeps you moving along with the Earth at such high speed, and why can't this influence be detected?
  • When you jump up into the air, why do you land in the same spot since the Earth is moving at enormous speed underneath you?
  • When you toss a coin into the air, how does it come back to your hand if you and the Earth are moving with enormous speeds while the coin is in the air?
  • And on and on...

 

 

 

 

 

 

Galileo

Galileo and the Leaning Tower

Galileo made extensive contributions to our understanding of the laws governing the motion of objects. The famous Leaning Tower of Pisa experiment: dropping two objects of very different weights from the tower to prove that (contrary to popular expectations) they would hit the ground at the same time.  The realization that the acceleration due to gravity is independent of the weight of an object was important to the formulation of a theory of gravitation by Newton. Here is an animation (http://www.edstephan.org/Animation/galileo.falling.html) of experiments with inclined planes that Galileo did prior, to confirm these ideas.

Galileo and the Concept of Inertia

Perhaps Galileo's greatest contribution to physics was his formulation of the concept of inertia: an object in a state of motion possesses an ``inertia'' that causes it to remain in that state of motion unless an external force acts on it. In order to arrive at this conclusion, which will form the cornerstone of Newton's laws of motion (indeed, it will become Newton's First Law of Motion), Galileo had to abstract from what he, and everyone else, saw.

Most objects in a state of motion do NOT remain in that state of motion. For example, a block of wood pushed at constant speed across a table quickly comes to rest when we stop pushing. Thus, Aristotle held that objects at rest remained at rest unless a force acted on them, but that objects in motion did not remain in motion unless a force acted constantly on them. Galileo, by virtue of a series of experiments (many with objects sliding down inclined planes), realized that the analysis of Aristotle was incorrect because it failed to account properly for a hidden force: the frictional force between the surface and the object.

 

 

 

 

 

 

Q's on L#4