# PHY138Y - Mechanics - Class 6 - Wed. Sept. 27, 2006

## Introduction

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

## Two Reminders:

1. When I make administrative and/or other announcements in class, they are accompanied by slides in the PowerPoint being displayed on the side screens. A pdf of that PowerPoint is always available via the class summary. The link typically appears towards the bottom of the summary.
2. Most students agree with the PHY138 staff that by and large the text does an excellent job of explaining the key concepts and demonstrating them with worked examples. Since you will have already read the text before class (Pre-Class Quizzes!) I don't see the point in just going over the textbook again in class. Instead, I often will approach a topic from a different perspective, ask you an In-Class Question, do a demonstration, show a Flash animation, etc. When I don't feel that I have anything to add to a topic I will clearly say so, and go on. I hope that this means that our classes are more useful for your learning than me just re-iterating the textbook.

## Today's Class

We finished Chapter 5 by doing some examples, and nearly finished Chapter 6.

### About Studying - Especially Physics

 We discussed study techniques. The discussion was perhaps particularly relevant to studying Physics. Here is a one page document summarising the discussion, in pdf format.

### In-Class Questions

 Here is a pdf of the questions that we asked in class today.

#### Who is richer: you or your astronaut friend on the moon?

Almost 80% of the class got this correct: that your astronaut friend is richer; Answer B. Well done! If you missed this question you need to review weight and mass in the textbook

#### Who is richer: you or your astronaut friend on the space shuttle?

Just over 80% of the class got this one correct too. The point is that for someone in free fall, some method of measuring the quantity of gold besides trying to measure its weight will be necessary. When we get to Chapter 9 we will discuss such a method. So the correct answer to this question is D: you cannot tell who is richer.

#### What does friction do?

 Here is the distribution of answers to this question. (Reminder: the ticks on the vertical axis are every 20% of the class, and the ticks on the horizontal axis correspond to answer A, B, C, D, and E respectively.) About 70% of the class answered incorrectly that friction always slows things down. The figure accompanying the question is a big hint: the only horizontal force acting on the upper block is friction, which causes it to accelerate. Thus the right answer is C: friction can both speed things up and slow things down.

If you are still bothered by this, consider the following situation:

You are standing motionless on a surface that is not frictionless. You then take a step.

• Have you accelerated?
• What caused that acceleration? (Hint: what would happen if you tried to take the step while standing on a frictionless surface such as super-slippery ice?)
• What is the direction of the force that caused your acceleration?

### Flash Animations

 A Flash animation similar to the text's Figure 6.17 was used in class. It is available via the button to the right. In class we did a demonstration. A Flash animation similar to that demo is available.

### Other Class Materials

Double MISTEAK! In proving that the path of a projectile is parabolic, I got it somewhat wrong both on the PowerPoint on the side screens and in the journal. The journal has been corrected in purple, and the pdf of the PowerPoint on the sidescreens has also been corrected! These updates occurred at 8 AM on Thursday September 28.

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

## Suggested Problems

Chapter 6: 10, 21, 33, 64