# PHY138Y - Mechanics - Class 10 - Mon. Oct. 18, 2004

## Introduction

"Science is facts. Just as houses are made of stones, so is science made of facts. But a pile of stones is not a house and a collection of facts is not science."

-- Poincaré

## Announcements

### MasteringPhysics

#### Miscellaneous Information

• There was a server crash early last Thursday morning. It was fixed by mid-morning on Thursday. This effected only about 1 in 8 accesses.
• The server was down from early Saturday morning until about 7AM Saturday.
• The bug involving the browsers Mozilla 1.7.* and FireFox 0.9* under Windoze has been fixed.
• There is now no need to use Internet Explorer under Windoze.
• Ever

#### Hint Bonuses or Penalties

There is an aspect of the user-interface to MasteringPhysics which I use in setting up an MP Problem Set which is a bit clumsy. I can set a problem set to award a 3% bonus if you solve a question without using hints, or set it to deduct a 3% penalty if you do use hints. Each MP Problem Set this term has had one or the other scheme applied, but sometimes it was one and sometimes the other. This is because the scheme used depends on how I happened to leave a state of a pull-down menu in the interface.

In the future we will consistently use the 3% bonus scheme.

#### MP Problem Set #4

Here are the usual statistics for the problem set due last Friday.

What Number PHY138 Students Answering PHY138 Results The Rest of the World
Potential Energy Graphs and Motion
998
99% correct
98%
Energy Required to LIft a Heavy Box
995
91% correct
94%
Circling Ball
992
76% correct
83%
Dancing Balls
991
89% correct
87%
Time to complete
NA
48 minutes
36 minutes

### Pre-Class Quiz #4

This quiz was due last Wednesday, and covered Sections 7.4 - 7.7 and Chapter 8 of the text. 980 students completed the quiz. The mean was 90.5%.

### Next Representative Assembly

• Representatives from Thursday tutorials
• Friday, October 22, 12 noon - 1 PM
• MP 713

### Assocation of Part-time Undergraduate Students (APUS)

APUS tries to insure that part-time undergraduate students have access to the full range of programs, services and resources at U of T. They are seeking up to three Class Representatives for PHY138. If you are a part-time student (3.5 credits or less) and wish to volunteer contact me.

### Written Homework #5

I got the chapter wrong for the problems from the textbook. You may just ignore those problems.

## Written Homework #6

 This assignment is the last of this unit of PHY138. It is due by 5 PM on Monday October 25 in the Drop Box for your tutorial group.

## Test #1

### What Is Examinable?

All sections of the text and the Supplementary Material that are listed in the Curriculum for this quarter of PHY138 are "fair game" for the test. In general, we are more interested in testing your understanding of the concepts and how to apply them then in your ability to take some formulae and using them to "plug and chug" to a solution.

We have used In-Class Questions, MP Problem Sets, and Written Homework including Supplementary Problems and problems from textbook. I have chosen the questions and problems that we used because in some sense I "like" them. Thus, you can expect to see some questions on the test that are at least similar to them.

I'm not sure if I have ever met an "average" student, but in the context of the final mark in PHY138 such an average student will receive a mark of 70. This is the marking standard used by U of T. The marking standard also says that about 15% or so of the students in a large class like PHY138 end up getting marks of 80 or better.

The final mark in the course has many components.

15% of the mark is from the various assignments:

• MP Problem Sets (6%). The software is designed so that almost all students end up with the correct answer, and in fact if a lot of students do not finally get the answer the designers view it as a flaw in the program. Thus the marks on these problem sets are very high.
• Pre-Class Quizzes (3%). These too are designed so that the marks are very high if you have read the textbook.
• Written Homework (6%). Since these are marked pass/fail the marks for these will be very high also.

So, we expect an average student to get about 14 marks out of 15 for these.

20% of the final mark in the course is from the laboratory. An average student will get about 70% in the laboratory, or 14 marks out of 20.

The Tests and the Final Exam count for 65% of the final mark in the course. In order for an average student to end up with a final mark in the course of 70, this student's score on the tests and Final Exam must be 42 out of 65.

The table summarises:

Performance By an Average Student
What Mark Out Of
Pre-Class Quizzes, MP Problem Sets, Written Homework
14
15
Laboratory
14
20
Tests and Final Exam
42
65
Final Mark
70
100

But, for the Tests and Final Exam 42/65 is just under 65%. Thus the average student should end up with an average of about 65% on these.

Since only the best 3 out of 4 Tests are counted towards the final mark in the course, to end up with a total of 42 out of 65 requires that the average of each individual test is somewhat less than 65%.

We know that virtually all PHY138 students got very high marks in High School Physics, and are used to getting almost all of the questions correct on tests and exams. Thus it can be a bit of a shock when you confront your first University-level test.

If it is any consolation, you may wish to know that Test Theory says that the test that is most fair to you has an overall average of 50%, although we intend for this test to have a higher average than this. Nonetheless, a "hard" test really is the most fair one. You can learn more about this in a little document I wrote a few years ago; it is on the web here.

### What If We Screw Up?

More often than we would like, we set a test and are very surprised by how the class performed on it: sometimes it is much too easy and sometimes it is much too hard. So although we will try very hard to get a class average on the test to be a bit below 65% we (not you) may fail.

• If the average is higher than 65%:
• We will congratulate the class for knowing more than we thought.
• We will live with the marks and will not adjust them.
• If the average is lower than 60% or so:
• We will adjust the marks.

### Finally ...

If you are not doing as well on the test as you anticipated, don't panic. Chances are that you are doing better than you think. Staying calm and confident increases your ability to do as well as possible.

## Math Analogies

In today's class we used a formal analogy between the force for the gravitational interaction and the electrostatic one to write down immediately the equation for the electrostatic potential energy. The general technique of using formal similarities of mathematical structures to save ourselves lots of work will be one of the "keys" to our discussion of rotational motion in Chapter 10. Thus, here I will repeat some comments that I made in class. You may wish to know that this topic is called analysis models in the textbook.

In the last class we said that if the gravitational force is:

(1)

then if we choose the zero of potential energy to be at infinity, the potential energy is:

(2)

The force exerted by electric charges on each other is given by Coulomb's Law:

(3)

Notice that in terms of the dependence of the forces on the distance between the objects, Eqns. 1 and 3 are identical. The only differences are that in one we write -G and in the other k, and that in place of the masses m we write the electric charges q. So we can immediately write the electric potential energy by just substituting into Eqn. 2:

(4)

## In-Class Question

 We asked an In-Class Question. Most of the class correctly answered 4. The 2 people are moving away from each other.

## Movie Clips

We discussed collisions and why modern cars have crumple zones. We illustrated with 2 movie clips.

 The first clip is of an older car, without crumple zones. You may access it by clicking on the button to the right. The file size is 118k, and will appear in a separate window. To view the clip requires that you have the QuickTime player installed on your computer. The player is available free from http://www.apple.com/quicktime/. The second clip is of a more modern car. You may access the clip by clicking on the button to the right. The file size is 441k. To view it requires that you have the Real Media player installed on your computer. The player is available free from: http://www.realnetworks.com/.

The clips are by the U.S. National Highway Traffic Safety Administration, and are used with permission. The original site of the clips is: http://www.nhtsa.dot.gov/cars/testing/ncap/Videos.html.