# PHY138 - Mechanics - Class 4 - Wednesday, September 21, 2005

## 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)

Some time late last week, subscribers to Rogers' internet service had access to MasteringPhysics blocked. Service was restored yesterday morning by at least 10:20 AM. Thus, for all students I have extended two deadlines:

1. Pre-Class Quiz Chapts 2 - 4:
• This was due on Monday at 10 AM.
• It is now due this Friday at 10 AM
2. MasteringPhysics Problem Set Chapts 1 - 4:
• This was due this Friday at 5 PM.
• It is now due Monday, September 26 at 5 PM

I am not extending the deadline for the Pre-Class Quiz Chapts 5 - 6: it is still due on Monday, September 26 by 10 AM.

## Today's Class

Today we finished Chapter 2, discussed Chapter 3, and got over half way through Chapter 4. So our schedule as listed in the syllabus is slipping a bit, but not too much. (Yet.)

## In-Class Questions

 Although our treatment of friction will be only qualitative, we asked a question about it. Accessible with the button to the right; separate window.

The correct answer to this question is 3: it can both slow things down and speed things up. The overwhelming majority of the class instead answered 1: friction slows things down. We will use this problem as our primary example of free body diagrams on Monday.

 I also asked this question about the pace of the classes so far.

The results were:

Too slow: about 10% of the class
Too fast: about 70% of the class
Just right: about 20% of the class

## Class Materials

 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. Pdf version of the PowerPoint on the side screens. Today's Journal.

## Suggested Problems

• Chapter 3: 1, 2, 18, 46
• Chapter 4: 12, 13, 14, 28, 50