# Hello new world!

This new blog is the beginning of a new relationship.

You’ll find some old blogs that I transferred from blogspot.com. They were for Physics 140 and I was just seeing if it was possible to export and reimport into my own WordPress site. It worked except for some equations.

# Exam Details

# Rotational Terminology

Our textbook *Understanding Physics* is derived from *Fundamentals of Physics* by Halliday and Resnick and therefore uses the somewhat nonstandard terminology such as “rotational velocity” instead of “angular velocity” etc. In particular the term “rotational inertia” is used instead of “moment of inertia”. I cannot teach this without slipping up occasionally. For this reason and just so the you’ll be able to understand the more common terms it’s important to make a table comparing H&R; terms with the conventional terms.

I also notice that the word “translational” is often used where “linear” may be more common. For example, “translation momentum” is used instead of “linear momentum”.

I do think the H&R; terminology is more consistent and logical, but it’s almost never used anywhere else. Too bad.

Terminology for Rotational Motion |

# Signs

There was some confusion in the definition of gravitational potential energy used in Unit 11. This is understandable because of the way that the sign of the fall distance was used.

In this course we’ll always take the value of *g* to be +9.8 m/s^{2}. Therefore when something falls its acceleration is −*g.* However, when something falls there’s always the option of using the positive sign for the downward direction. That seems to be what was done in Unit 11 where *y* is used for the fall distance. The result was that gravitational potential energy comes out as

*E*_{pot} = −*mgy*

That’s not not exactly wrong, but is probably confusing because almost everywhere else the sign is positive; so you probably remember this:

*E*_{pot}= +*mgh*

I’ve tried to repair this confusion by defining the distance of fall as Δ*y *and

as being negative. That way all is more conventional.

Other issues were using the notation *E*_{pot} for gravitational potential energy whereas *U*_{g} is more usual.

*U*_{g} = *mg*Δ*y*

The textbook uses *U*^{grav} which is close. Similarly *K* is more convenient and usual for Kinetic Energy than *E*_{kin}.

I’ve rewritten the unit’s Activity Guide with these changes and I hope it won’t be so confusing now. Have a look and let me know what’s not clear:

# Homework 10 Deadline

The deadline for Unit 10 has been extended to Nov 9, after the exam. That way you can keep Unit 10 to study and turn in the Activity Guide and Homework after the exam is finished.

# Midterm 2

Believe it or not it’s time to think about the next midterm on Nov. 9

Here are the room numbers:

D100 12:30—13:50 SUR 5380

D200 15:30—16:50 SUR 5280

The format will be the same as before with 10 multiple choice questions and 4 written problems. The total will be worth 50 points for 15% of the final mark. One difference is that we may use Exam Booklets and Bubblesheets so that our printing load will be less. This is due to a possible staff shortage in the Surrey “Document Solutions” service.

All units up through Unit 10 are fair game for this exam. I suggest studying the calculations you did in the activity guides and be prepared for variations of those types of calculations. Make sure you understand the reasons they were done the way they were. Also it would be good to study the problem examples in the textbook and textbook problems similar to those assigned in homework.

As usual bring a pencil, pen, simple scientific calculator, ruler and a protractor for vectors. You can bringyour own activity guides and homework.

**Update**

Please turn in your Unit 10 homework and Activity Guide after the exam.

# Friction

Tomorrow’s session will deal with friction. It’s not my favourite topic, it’s not really fundamental physics and the treatment at this level is a little fictitious.

As a challenge you can review your results of the inertial mass measurement in Unit 5 Session 3. (You measured the accelerations of a fan-cart with and without a bar of known mass on it.) The mass you calculated was systematically wrong because friction was neglected.

Try to figure out how to take friction into account…

- How would you experimentally measure the coefficient of kinetic friction in the movement of the fan cart?
- How can you correct the mass calculations using the measured coefficient of kinetic friction?
- How could you have modified the expriment of Unit 5 so that the systematic error due to friction would have been reduced?

# Decimal Time, Republican Calendar and other Lost Causes

A decimal clock face made shortly after the French revolution. (From Wikipedia) |

The decimal time system that was the subject of the first problem on the midterm was actually used for about 2 years after the French Revolution. It’s mandatory period of use was less than a year: 22 September 1794 to 7 April 1795. Here is a clock face from the wikipedia article on decimal time.

Republican Date on the door. (From Wikipedia) |

The Republican Calendar lasted a little longer: 12 years. This calendar system had weeks that were 10 days long. One month was three republican weeks. Thus 12 Republican months made for 360 days and there were several holidays at the end of the year to fill out the rest of the 365 or 366 days. Evidence of its use can still be seen on public buildings in France. For example, the door of the famous École Normale Supérieure displays the date 9 Brumaire III, the date of its establishment decree. (The building was built later.) Despite the best of intentions, the system was not popular, probably because a 9-day work week replaced one of 6 days.

Can you image the confusion that we would be experiencing if only part of the world had actually adopted–and stuck with–the decimal time and calendar systems? For example the date of the midterm exam, Oct. 8, 2010, would have been called 17 Vendémiaire CCXIX.

# Midterm Exam Grades

**Update**

The midterm grades are posted in the webct gradebook. There are four parts: labelled “Midterm 1 MC”, “Midterm 2 probs” and “Midterm 1 response”. These report the following:

- “Midterm 1 MC”: The Multiple-choice score out of 20,
- “Midterm 2 probs”: The Problem score out of 30,
- “Midterm 1 total”: The total out of 50. Y
- “Midterm 1 response”: Your multiple choice responses and the correct answers.
- The format is [NNNN]xxxxxxxxxx{yyyyyyyyyy}, where NNNN is the version of the exam, yyy… are the correct answers and xx…x is either a dot if you got it right, or your response if it is wrong.

Class average is 34.2/50 with at standard deviation of 7.65.

**Earlier post**

I’m going to grade these exams myself so it may take a day or so. I’ll try to get them done by Tuesday.