Tuesday (Sep 21):

 

Key Concept's) Today: Temperature

 

Journal:

1. What are the differences (mass & weight) of brick (10-kg) at PHS,

top of Mt Rainer

and on the moon (and why)?

 

 

 

 

 

 

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)  

 

 

 

 

 

 

 

Film: Absolute Zero

 

 

 

 

 

                              

 

 

 

 

 

 

 

 

  1. define force.
  2. ...find the net force acting on an object if:
    1. no forces push/pull on it.
    2. 1 force pushes/pulls on it.
    3. 2 forces push/pull on it in the same direction.
    4. 2 forces push/pull on it in opposite directions.
  3. ... state Newton's First Law.
  4. ... define inertia.
  5. ... identify the conditions necessary for a body to be in equilibrium, and describe all of the forces acting on the body.
  6. ... correctly apply Newton's First Law and the concept of inertia to explain various motions, such as the motion of a person inside an accelerating car.

Possible Preconceptions to Correct:

Do you believe that any of the following statements are true? They AREN'T! When you finish Chapter 4, you should understand that each of these statements is FALSE, and WHY it is false .

 

 

    • strings above and below hanging weigh - demonstrates both parts of the 1st Law

 

Purpose:

To demonstrate the "objects at rest" part of Newton's First Law.

Equipment:

3 pieces of chalk and a narrow strip of paper

Description:

Place the strip of paper so at least half of it hangs over the edge of a table. Stand the chalk on end on the paper. Holding the free end of the paper, deliver a sharp blow between the end of the paper and the edge of the table with your free hand.

Explanation:

This is similar to, but not as impressive as, the TableCloth Demo.

 

 

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.

 

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.

The ideas that came to be held by most medieval thinkers descended from Aristotle. 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, Ptolemy's work survived mainly due to the Arabs, who had the highest regard for Ptolemy's ideas. Ptolemy's main work became known in the West during the Middle Ages as the Almagest, which is from the Arabic for "The Greatest". The monastic 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.

Ptolemy's solution to matching the model with reality was to create a rather complex system of adjustments to the circular orbits. For some planets, the Earth's position was offset from the exact center of the planet's orbit by a distance called the eccentric. It might be necessary for the planet to be attached to a smaller circle called an epicycle, whose center followed the main orbit of the planet. This might not suffice to accurately portray the orbit of a planet - the center of the epicycle may need to be offset by an amount called the equant.

This system seemed to work pretty well for predicting the motions of the planets, except:

  • Some planets needed as many as 7 epicycles (circles inside circles inside circles...) for their orbits
  • There is no theoretical basis for adding eccentrics, epicycles, and equants - if the model doesn't fit, the astronomer just had to "play with the numbers" to find a way to fit a new epicycle to make it fit.
  • 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 cleric 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...)

Religious and Political Objections to Copernicus:

To understand the religious and political furor that Copernicus' ideas set off, you have to understand some of the history of the time.

First, the Catholic Church was not only a religious, but also a political power. However, the Church was not without its troubles - this was the time of the Reformation. A king or even local lord would convert to Protestantism (sometimes for political or financial reasons). This would trigger a war with the neighboring Catholic king or lord. These wars were always bloody and brutal, and often long (the Thirty Years War, for instance).

In this atmosphere, no one was ready for any dissent whatever - and certainly not revolutionary ideas like Copernicus'. Copernicus new this - he delayed publication of his book so that he saw the first copy as he lay on his deathbed. He wasn't going to be around for the fallout from this! People had been (and continued to be) burned at the stake (or worse) for far less that this!

Philosophically and religiously, Copernicus had posed serious problems, even in the most serene of times. 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), people were the center of God's Universe. Everything revolved around the Earth - and the people on it. Copernicus reduced the Earth to one of several planets revolving one of billions of stars, and reduced the status of the people on the Earth - to what?

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 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 may be apocryphal. It is likely that Galileo himself did not drop two objects of very different weight from the tower to prove that (contrary to popular expectations) they would hit the ground at the same time. However, it is certain that Galileo understood the principle involved, and probably did similar experiments. The realization that, as we would say in modern terms, 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 of experiments with inclined planes that Galileo probably did to confirm these ideas, and here is a page with some MPEG film clips illustrating the same ideas. Here are additional MPEG filmclips illustrating Galileo's experiments concerning the motion of projectiles in a gravitational field.

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.

Thus, as we push the block of wood across the table, there are two opposing forces that act: the force associated with the push, and a force that is associated with the friction and that acts in the opposite direction. Galileo realized that as the frictional forces were decreased (for example, by placing oil on the table) the object would move further and further before stopping. From this he abstracted a basic form of the law of inertia: if the frictional forces could be reduced to exactly zero (not possible in a realistic experiment, but it can be approximated to high precision) an object pushed at constant speed across a frictionless surface of infinite extent will continue at that speed forever after we stop pushing, unless a new force acts on it at a later time.

Galileo and the Church

Galileo's challenge of the Church's authority through his assault on the Aristotelian conception of the Universe eventually got him into deep trouble with the Inquisition. Late in his life he was forced to recant publicly his Copernican views and spent his last years essentially under house arrest. His story certainly constitutes one of the sadder examples of the conflict between the scientific method and "science" based on unquestioned authority. Unfortunately, there still are many forces in modern society that would shackle the scientific method of open enquiry in idealogical chains of one kind or another.