PHY138Y - Mechanics - Class 6 - Wed. Sept. 29, 2004


The painting is of Newton, by William Blake (1805). The original is in the Tate Museum in London, England.


About Workload, Assignments & Time Management

The general rule for any University course is that you are expected to study 2 hours every week for each hour of class: this works out to 4 - 6 hours per week for each class. This number is certainly higher than a typical high school course.

You may recall that in our first PHY138 class I talked about how the content of this course, perhaps more than other courses you may be taking, requires some time to be assimilated. This means that it is important that you keep up with the material. Five hours of study every week is far better than 20 hours of study every fourth week.

We have designed our assignments to aid you in keeping up. We are trying to set the Written Homework, Pre-Class Quizzes and MP Problem Sets so that together they should take you somewhat less than 2 hours per week: if this is not correct we want to know about it so we can make adjustments. This leaves 2 - 4 hours per week for reading the textbook, talking about Physics in a study group, etc. So, if we have "dialed in" our assignments correctly I think the workload in PHY138 is appropriate and consistent with the requirements of other courses.

When an assignment is released you have at least a week to complete it. The fact that the different parts are released on different days (Written Homework on Mondays, Pre-Class Quizzes and MP Problem Sets on Wednesdays) is just because of the logistics of getting the assignments out at least a week before the due dates. In addition, some students like to get started early and so we make sure the release the assignments as soon as possible.

Finally, just because there are 3 different due dates doesn't mean you have to do them at different times. Say you decide to make Thursday your Physics Homework day. Then on that day you could:

Doing it this way means that essentially there is one homework assignment with three parts: two parts are on that week's material and the third part is on coming material.


Drop Boxes

On Monday the Written Homework #2 was due in the Drop Box for your tutorial group. But the labels on the boxes had not been updated from last year! This shouldn't have happened, and we apologise. The labels are now updated and (we hope) are correct.

Next Pre-Class Quiz

Since there is no class on Monday, October 11, the next Pre-Class Quiz (WebCT) covers the textbook material for that class of Wednesday, October 6 only: these are Sections 7.1 - 7.3 of the textbook. It is worth 50 marks instead of the usual 100, and has two questions instead of the usual four. The quiz is now released and is due by 10 AM on Wednesday, October 6, one week today.

Next MP Problem Set

The next MP Problem Set has been released. It covers Chapter 6 of the textbook, which we will be talking about in class next week, Monday, October 4 and Wednesday October 6. The problem set is due by 5 PM on Friday, October 8, which is next week Friday.

First Pre-Class Quiz

The first Pre-Class Quiz, delivered via WebCT, was completed by 981 students. Three of the questions were easy for people who had read the relevant sections of the text; this is the way they are supposed to be. However, the following question was answered correctly by only 43% of the class.

The text uses a simplification model in the analysis of projectile motion of an object. Choose all of the answers below that are the assumptions used in forming this model.

  1. The origin of the coordinate system is at the initial position of the object.
  2. The freefall acceleration g is constant over the range of motion and is directed downward.
  3. The trajectory is a parabola.
  4. The effect of air resistance is negligible.

Here are three sentences from the first paragraph of §3.3 - Projectile Motion:

"This very common form of motion is surprisingly simple to analyze if the following two assumptions are made in building a model for these types of problems: (1) the freefall acceleration g is constant over the range of motion and is directed downward, and (2) the effect of air resistance is negligible. With these assumptions, the path of a projectile, called its trajectory, is always a parabola. We shall use a simplification model based on these assumptions throughout this chapter."

Unless I am missing something, then, the correct answer is b and d, and only these two. The fact that the trajectory is a parabola is not an assumption, it is a result of those assumptions.

This question pulled down the overall average on this quiz: it was 70%.

Reminder: if you had technical problems in accessing the quiz:

In-Class Questions

We did a question on circular motion and Newton's Laws of motion. We also did a demonstration of the question. Before doing the demo the overwhelming majority of the class chose the right answer: B.
We did a question on tension based on the logo of Levi jeans. For this question most students got the right answer too: 1) The same as when two horses are pulling on the pants.
Finally, we did 2 related questions using Newton's 3rd Law and the buoyant force. This too had a demo. For Part A, before doing the demo, the class was essentially split between two of the answers: 1) It stays stationary and 3) It moves down. The demo showed that answer 3 is correct. For Part B of the question, the class did much better: most chose 3) It moves down, which we confirmed by doing it.

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