# PHY138Y - Mechanics - Class 5 - Monday Sept. 22, 2003

## Introduction

“Nature and nature's laws lay hid in night;
God said Let Newton be! and all was light.”

-- Alexander Pope,
"Epitaph intended for Sir Isaac Newton"

## Problem Set #3

 You may access the problem set by clicking on the button to the right. It is due by 5PM Monday September 29 in the "Drop Boxes" in the basement of McLennan. The file is in pdf format, with a size of 118k.

## Today's Class

 A sharp-eyed PHY138 student pointed out to me that in the side-screen review of the previous class, I got the formula for maximum height for projectile motion wrong. The correct formula appears to the right.

We finished our discussion of projectile motion by solving for the range R. We then deferred discussion of the rest of Chapter 3 until next class.

We briefly discussed frogs jumping. Here is some more information on frog jumping courtesy of Nora Espinoza of Erskine University (private communication):

"For the Green treefrog, Hyla cinerea, the average take-off angle was 34 degrees, but the range was quite large. Take-off angle ranged from about 5 degrees (lazy jump) to nearly 90 degrees (startled frog). For the Gray treefrog, Hyla chrysocelis, I tested newly metamorphosed frogs weekly for six consecutive weeks. They showed a similar average of 36 degrees. Again, the range was huge 3 degrees to 77 degrees."

The data were taken by Prof. Espinoza as part of her PhD thesis from the University of Chicago. I include her communiation only for general interest; these numbers are not examinable in PHY138.

We began Chapter 4. We demonstrated an air track from the laboratory to discuss horizontal motions. Some of you will be doing the experiment using this apparatus in the first term; the experiment will also be available in the second term. The Guide Sheet for the experiment appears in the First Year Physics Laboratory Manual.

We discussed §4.1, §4,2 and then §4.4. These deal with Newton's first two laws of motion

I introduced some notation: wrt is shorthand for "with respect to." I shall be using this notation for the next few classes.

We began a discussion based on §4.3, which we will continue next time. Our approach was based on a realisation that Newton's laws of motion are only true for some observers. Roughly, they are only true for "non-acclerating" observers. The question arises: "not acclerating wrt what?" The Newtonian answer is wrt absolute space.

Here are 2 quotations by Newon on absolute space and absolute time:

"Absolute space, of its own true nature without reference to anything external, always remains homogeneous and immovable ..."

"Absolute, true, and mathematical time, in and of itself, and of its own nature, without reference to anything external, flows uniformly ..."

 You may access the "Journal" file from class by clicking on the button to the right. Separate window; 185k.