PHY138Y - Mechanics - Class 4 - Wed. Sept. 22, 2004

Introduction

“One of the biggest problems is how to state the problem. It's an old saying that the minute you can state a problem correctly you understand 90 percent of the problem.”

-- John A. Wheeler (physicist)

About WebCT

Some, but not all, students are having difficulties accessing the Pre-Class Quizzes delivered via WebCT.

Make sure the JavaScript and Cookies are enabled for your browser.

The following things have also been observed or reported:

• The Quiz itself appears in a separate window. Make sure that window is not hidden under other windows.
• One student reported that the Norton Firewall makes the quizzes not appear: turning off the firewall fixed the problem.
  • This is clearly a defect in the software, which we have reported.
• For some browsers, you may need to turn off the blocker of popup windows.
  • This too is clearly a defect in the software, which we have reported.

We are not certain but suspect that this is not a complete list of the problems.

Your mark in PHY138 will not be effected by these problems. although at the moment we have not decided quite how we will deal with these difficulties. Make sure the course coordinator, Dr. Savaria, knows about any difficulties you are experiencing.

Reminders

Tutorials begin this week.

You can determine your tutorial group and location from:

• PHY138 Home Page
• PHY138 Notice Board (McLennan, 1st floor, North Wing, West wall
• STORM

Each tutorial group should choose a Representative, using any method that you wish.

The first Representative Assembly is for Representatives from Wednesday tutorials:

• Fri, Sept. 24
• 12 noon - 1 PM
• MP 713 (McLennan, 7th floor of the tower)

Written Homework #1 is due this week in tutorial.

The first credit MP Problem Set is due by 5PM Fri. Sept. 24.

Written Homework #2 is due on by 5PM Mon. Sept. 27 in the "Drop Box" for your tutorial group.

Announcements

The second credit Pre-Class Quiz (WebCT) is now released:

• Covers material for the classes of Wed. Sept 29 and Mon. Oct 4: Chapters 5 - 6.
• Due by 10 AM Wed. Sept 29

The second credit MP Problem Set (MasteringPhysics) is now released:

• Covers material for the classes of Mon. Sept 27 and Wed. Sept 29: Sect. 3.4, Chapters 4 - 5.
• Due by 5 PM Fri. Oct. 1.

Experiments From the Lab

We mentioned a couple of experiments from the laboratory.

First we talked about the Free Fall experiment from the laboratory: many of you will be doing this experiment in the Fall term. In case you're interested, the Guide Sheet for the experiment is at: http://faraday.physics.utoronto.ca/IYearLab/Intros/FreeFall/FreeFall.html.

I also briefly mentioned the Air Table experiment, which will be available in the second term. Just for interest, the Guide Sheet for that experiment is at: http://faraday.physics.utoronto.ca/IYearLab/airtable.pdf .

Motion Diagrams

As we discussed last class, whenever I feel that I don't have anything to add to a discussion in the textbook, I will not waste your time by just repeating Serway & Jewett's presentation in our class. An example is §1.5 Order-of-Magnitude Calculations. Also, I only briefly mentioned the topic of Modeling discussed in §1.11. Note that both of these sections are listed in the Curriculum for the course, and are examinable.

Another example of a section which I didn't talk about is §2.5 - Motion Diagrams. However, I have prepared a Flash animation, similar to Figure 2.11 of the text, which you might find useful. You may access it with the button on the right. Separate window, of course.

Flash Animations

Throughout Chapter 2, S & J talk about slopes and areas under curves to find positions, velocities and accelerations. The animation accessed with the button to the right shows these relations: separate window.

I have a brief comment on the above animation, based in part on some questions I received after class. As the text discusses in §2.1, the area under a velocity-time graph is the displacement. In class we stressed that the mathematical relationship betwen velocity, displacement and time is the same as the relationship between acceleration, velocity and time. Thus, if the area under the v-t graph is displacement, the area under an a-t graph will be the velocity. This is used in the the first part of the second scene of the animation.

We used a Flash animation to illustrate the text's Fig. 2.14 and an unlabelled figure on the top of page 82.
We also used a Flash animation for the general case of projectile motion.

In-Class Question

The runner and the bird: Answer 2) 15 km

The class did very well on this question: almost everybody got it right. Well done!

Today's Journal

I discovered a small mistake was made in one equation of the above journal file. I have corrected the mistake in purple. The correction was made on Thursday morning, Sept. 23.