PHY138Y - Mechanics - Class 5 - Mon. Sept. 27, 2004


“Nature and nature's laws lay hid in night;
God said Let Newton be! and all was light.”

-- Alexander Pope,
"Epitaph intended for Sir Isaac Newton"


Next Representative Assembly

The next Representative Assembly will be for Thursday's tutorials. It will meet:

Friday, October 1
12 noon - 1 PM
MP713 (7th floor of the tower)

WebCT Access

I have discovered a web site that discusses issues in using WebCT. If you are having difficulties, please go to and carefully read it. The URL is If after following the instructions on this site you are still having difficulty, contact Dr. Savaria in MP901A.

The latest version of the page on the Pre-Class Quizzes and WebCT document at has a new section that includes the tips about quiz access we talked about in the last class, plus more, and provides another link to the "tuneup" web site.

For the first Pre-Class Quiz, due last Wednesday, 981 students completed it. Thus, most students had no insurmountable difficulties with accessing the WebCT site. If you did not complete the quiz because of technical problems with the software contact Dr. Savaria in MP901A immediately.

Curriculum Change

I have added §3.5 - Tangential and Radial Acceleration to the curriculum.

About the Labs

  1. Effective immediately, do not use ROSI to change your laboratory section. If you need to change it, see Dr. Serbanescu in MP124. (This office is right beside where you picked up your Survival Guide.)
  2. Dr. Serbanescu has a huge number of lab section changes to make. Please be patient: she will certainly have all the changes processed in good time for your first laboratory. The laboratory home page at will contain all current information.

Journal Mistake From Last Class

A small mistake was made in an equation of the Journal file from last Wednesday's class. The correction was made on Thursday morning, Sept. 23. You can access the corrected version from the summary for last Wednesday's class.

When (Not "If") I Screw Up

In Con Hall, because of the huge number of you, questions from students about content do not work very well. However, inevitably from time to time I will write something clearly wrong on the Tablet or say something that is obviously wrong. In such a case, please bring it to my attention immediately.

Written Homework #3

The button to the right gives you access to the third Written Homework assignment. It is due by 5PM Monday October 4 in the Drop Box for your tutorial group. Separate window, pdf format.


The Pre-Class Quizzes (WebCT), MP Problem Sets (MasteringPhysics) and Written Homework will help you learn Physics. We count them towards your mark in the course only to help motivate you to do them.

Your knowledge of Physics will be determined by your performance on the tests and final exam, which together count for 65% of your mark in the course.

If you do not do the assignments yourself you will learn less Physics and will not do as well on the tests and the final exam. This is a "bad thing."

Writing Is a "Good Thing"

One of the aspects of learning involves getting your hand, eye and brain coordinated. One way to accomplish this is to do lots of writing. Although the notes can be a useful study aid, the process of note taking is also important in achieving the necessary coordination. It also helps you to concentrate on the material you are reading.

For example, when we read fiction we typically go at about a page a minute. Trying to read a textbook at this speed is much too fast: you will miss most of the information. This is true for all textbooks, not just the PHY138 one. One way to force yourself to slow down and pay close attention to what you are reading is to take detailed notes on what you are reading. If there is a figure in the textbook, copy it into your notes: then you will be sure to have seen all its details. Write down all the definitions. Write down all the equations. If there is a mathematical derivation that skips some steps, fill them in. You may wish to keep your notes as a study aid, or may prefer to just throw them into the blue box. As I wrote above, it is the process of note taking that is important.

One of the reasons why I use a Tablet PC in class is because if I am writing something on it I am almost guaranteed that the pace is sufficient that you have time to write it down in your notes. I find this pacing very hard to do with PowerPoint or overhead projectors with pre-prepared slides. So I'm trying to use the Tablet in this way to help you to write down things in class.

Similarly, if you are doing an MP Problem Set resist the temptation to just scribble on a piece of scrap paper. Copy the figures, copy the equations, etc. Not only will this help you to concentrate on what the problem is really asking you, but in this case your notes may be a valuable study guide for later review. Remember that these problem sets are not timed, although students at other Universities typically take somewhat less than an hour for each.

Flash Animation

We showed a Flash animation of a classic lecture demonstration that, sadly, we couldn't figure out how to make work in Con Hall. Separate window. (The demo only works effectively in a class of less than about 50 students.)

S & J Figure 3.12

In class we discussed the fact that the textbook's Figure 3.12 on page 90 can be a bit misleading.

Say you interpret the figure as the path of a particle on a track near the Earth's surface and the Earth is at the bottom of the figure. Then if the only forces acting on the object are gravity and a normal force due to the track, at position A it will not have a tangential acceleration.

An MP Problem Set had a similar circumstance in which those assumptions about the forces were made. Some of you thought that the text's figure made those same assumptions, although it doesn't explicitly say so.

You may prefer to interpret the figure as motion in the horizontal plane and we are looking down at it from above.

In-Class Questions

We did a question on projectile motion. Initially, about 90% of the class answered 3) Both get hit at the same time. This is incorrect. After some small group discussion, almost 100% of you got the correct answer: 2) B. You can check this result using the Flash animation of projectiles that we used in the last class.
We also did a question on Newton's 3rd Law. Initially half of the class answered Yes and half answered No. After some small group discussion about 95% of the class got the correct answer: 2) No.

Master Mo

In class we discussed the fact that the "revolutionary" idea of Newton's, his 1st and 2nd Laws, was anticipated much much earlier in China. Master Mo (Mo Tsu) lived in the 3rd century BCE, and his thoughts and those of his school are collected in Mo's Book (Mo Ching or sometimes Pulse Classic). It is hard to know what parts of the Mo Ching were Mo's and which were added later. But certainly by the 3rd century of the current era it stated what we call Newton's 1st law and nearly the second one too:

"The cessation of motion is due to the opposing force ... If there is no opposing force ... the motion will never stop."


We discussed an application of Newton's 3rd Law: the ballistocardiogram. You may access a small html document on this topic by clicking on the button to the right. Separate window.

Today's Journal