# PHY138 - Mechanics - Class 4 - Wednesday, September 20, 2006

## 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)

## Today's Class

The syllabus as given in the Mechanics home page has slipped a bit. I will try to get it back "on track" over the next couple of classes. Today we finished Chapters 2 and 3.

### In-Class Questions

 We asked some questions in class. A pdf version is available via the button to the right; as usual it will appear in a separate window.

### Stop To Think 2.2

We discussed how the "correct" answer to the text's Stop To Think 2.2 on page 49 is misleading. The answer (c) has two discontinuous breaks in the graph of speed versus time. This is not correct: classical Physics assumes that the world is continous.

Putting the same thing is more mathematical language, classical Physics assumes the the speed as a function of time must be differentiable for all values of the time.

Either way, the "breaks" in the plot should be drawn more smoothly. I have brought this to the author's attention, and he intends to correct this for the next edition of the book.

### All Flash All The Time

Here are links to the Flash animations we used in class today:

 Visualisations similar to Figure 2.29 of the text Introducing the integral sign. We only used the first "scene" of this animation. Flash animation of adding two vectors. Flash animation of adding three vectors. Flash animation of subtracting two vectors. Flash animation of vector addition by adding components. Flash animation about unit vectors.

I stressed this in class, but it is perhaps worth stressing it again:

 We will be using integrals as part of our language in describing the world. You will not need to know the mathematics of integration until MAT135 gets to this topic in mid-January.

### Other Class Materials

 Here is a pdf of the PowerPoint we used on the side screens today. Here is a pdf of today's Journal.

## Suggested Problems

• Chapter 3: 1, 2, 18, 46