PHY138 - Mechanics - Class 2 - Wednesday, September 14, 2005

Announcements

• The printed schedule of when labs start is incorrect. The correct dates are here.
• If you have not already done so, pick up a "Survival Guide" for the lab and an "Information Sheet" for PHY138 outside MP124.
  • This is on the first floor of the North Wing of McLennan. Just keep walking North and you will see them.
• Copies of MasteringPhysics for Knight and the Student Workbook, separate from the shrink-wrapped package, will be in the textbook store very soon.
• My office hour for this Friday, September 16, is cancelled.
• We had a small glitch in enabling MasteringPhysics (MP) software for you. It was fixed by late Monday afternoon.
• You should set up your MP account right away.
  • The "Pre-Course Assessment Quiz" is due this Friday by 5 PM.
  • The "Pre-Class Quiz Chapts 2 - 4" is due by Monday, September 19 by 10 AM.
  • The "Problem Set Chapts 2 - 4" is now available. It is due next week, Friday September 23 by 5 PM.
• Tutorials begin next week on Wednesday, Thursday and Friday.
• Effective immediately, do not use ROSI to change your tutorial section; see Dr. Savaria in MP 129E instead.
• Labs begin the week of October 10.
• You may use ROSI to choose or change your lab section until we announce otherwise.

Today's Class

We did not quite finish our discussion of Chapter 1; we will finish this on Monday.

As part of our discussion of the mechanistic worldview of classical physics, I showed a beautiful image. Here it is:

The render was done by cazzanova and is used by permission. I have reduced the size of the original so it fits in a browser window. The original is at: http://www.deviantart.com/view/211768/

In-Class Questions

We asked the text's Stop To Think 1.1. Virtually all students who participated got this question correct.

We asked the text's Stop To Think 1.2. Virtually all students who participated got this question correct.

You may see a pdf version of another question we asked by clicking on the button to the right. It will appear in a separate window.

The class answered this as follows:

1. The form without a vector sign over the 0: 5%
2. The form with a vector sign over the 0: 85%
3. The two forms are equally correct: 10%

We discussed how for a mathematician answer 2 is correct. However I then argued that a vector has both a magnitude and direction by definition, but a vector of zero length has no direction; this argument indicates that answer 1 is correct. Since in physics we use mathematics as a language to communicate, not as some analytical logical structure, personally I prefer answer 3.

I will wish to re-cycle questions such as this in subsequent years. Thus I want to keep the question confidential between you and me, and have disabled printing. The question will remain available to you via the above link until after the Final Exam in the course.

Other Class Materials

We used a simple Flash animation about the difference between distance and displacement. You may access it with the button to the right; it will appear in a separate window.
A pdf version of the PowerPoint on the side screens is available with the button to the right; separate window.
See the pdf version of the Journal via this button; separate window.

After Today's Class

After class today many of you asked about the text's Stop To Think 1.5 on page 30. In particular the question was how many significant figures are in the number:

8200

The answer is two. Consider writing this number in scientific notation

8.2 x 10³

This shows that it really is two significant figures.

Consider this number:

8200.

The trailing decimal point is a notation that indicates that this number as four significant figures. In scientific notation this would be written as:

8.200 x 10³

The arrows let you jump to the previous/next class summaries.