# PHY138Y - Mechanics - Class 11 - Wednesday October 19, 2005

## Introduction

"Here arises a puzzle that has disturbed scientists of all periods. How is it possible that mathematics, a product of human thought that is independent of experience, fits so excellently the objects of physical reality? Can human reason without experience discover by pure thinking properties of real things?"

-- Einstein

## Announcements

• The Association of Part-Time Undergraduates (APUS) represents part-time students.
• Part-time is 3.5 credits or less
• They are seeking up to 3 representatives from PHY138
• If you are interested contact me.
• Pre-Class Quiz Chapt 13 released
• Due by 10 AM Monday October 24
• MP Problem Set Chapt 13 release
• Due by 5 PM Friday October 28

## Today's Class

We began today by jumping ahead, and doing a version of the proof on §11.2 that the total work equal the change in kinetic energy. Our version didn't explicitly use integrals.

We then went back to Chapter 10 and finished it.

Next we returned to Chapter 11, and finished it except for the last section on Power. We will finish Chapter 11 next class and then begin Chapter 13.

## Section 10.6

 We discussed a two dimensional elastic collision between 2 objects of equal mass where initially one mass was moving and the other was stationary. Then, in the absence of spin, I proved that after the collision the angle between the 2 velocity vectors is /2 radians, or 90 degrees. Examples of such collisions are in playing billiards or in curling. The figure illustrates for a game of pool in which the player is trying to sink the 8 ball into the right-hand pocket. If from the point of collision the angle between the path to the right-hand pocket and the path to the left-hand pocket is 90 degrees, then if the 8 ball goes into the right-hand pocket the cue ball will go into the left-hand one. This is a "scratch" and causes the player to lose the game. Good players learn how to put spin on the cue ball to avoid the scratch. Beginning players should avoid trying to make such a shot.

## Flash Animations

 We showed this Flash animation about Hooke's Law for springs. We also showed this animation about the dot product of two vectors