PHY138Y - Mechanics - Class 9 - Monday Oct. 6, 2003


An early statement on the Principle of Conservation of Energy.

"Inside a closed system, i.e. a system into which neither energy is transported nor taken out of it, in which arbitrary processes (mechanical, thermal, electrical, chemical) happen, the total energy remains conserved"

Helmholtz (1848)

Problem Set #5

In past classes we have used some Flash animations that require Version 6 or later of the Flash player. This problem set uses an animation that also requires Flash 6. If you have not yet upgraded, it is available free at If you are not sure which version of the player you are using, from any Flash animation if you right-click on it the version number will be shown.

You may access the problem set by clicking on the button to the right. It is due by 5PM Tuesday October 14 in the "Drop Boxes" in the basement of McLennan. Note the unusual date. The file is in pdf format, with a size of 86k.

Today's Class

Since many of our Jewish students could not attend today's class, I have tried to make this summary more complete than is usual. Also, we discussed a Supplemental Topic, for which I have prepared course notes; access to those notes appears below. The time we took with discussing this supplemental material means we did not introduce a lot of new Physics content in class.

We finished §6.6 - The Nonisolated System.

As part of that discussion we talked about some of the history of the idea of Conservation of Energy. Prior to the early 1800's most people believed that heat was some sort of fluid that flowed from one body to the other; the fluid was called caloric. People attempted to measure its mass by measuring the increase in mass of a body when it was heated. It was only later that it was realised the heat was just another form of energy. In those days the unit of heat was the calorie.

The laboratory has an experiment that is very similar to a classic experiment done by Joule in 1847 to measure how much energy is contained in a calorie of heat. The experiment will be available to you in the 2nd term, and is called The Mechanical Equivalent of Heat. The guide sheet for the experiment is available at:

Here is a perhaps apocryphal story about Joule. He married and went to a resort for his honeymoon. The resort had a waterfall. He was so fascinated by the identification of heat as just another form of energy that he spent a large amount of time measuring the difference in temperature of the water and top and the bottom of the waterfall. This caused his wife to leave him. So the "honeymoon" was over before the honeymoon was over.

We then talked about §6.8 - Power. The text is in error when it states that the British unit of power is the horsepower: it is actually ft-lb/sec. The horsepower is a derived unit, due to James Watt in 1775: he noticed that "a strong horse could lift 150 pounds a height of 220 feet in 1 minute." The defines one horsepower. It is equal to:

150 lb x 220 feet / 1 minute x 1 minute/60 seconds = 550 ft-lb/sec.

We then introduced a Supplemental Topic, where we discussed metabolic rates, and used dimensional analysis to guess how it depends on the mass of an organism. Notes are available on this, which may accessed via the links to the right. The html version is 26k, and the pdf version is 47k; each will appear in a separate window.

The topics of non-integer dimensionality and the Hausdorff method of measuring dimensions, which is discussed by a separate document accessed from the course notes for the Supplemental Topic, are not examinable, and are only included for interest.

We began Chapter 7 - Potential Energy with a discussion based on §7.1 - Potential Energy of a System. We have not quite finished discussing the material in this section.

You may access the "Journal" from today's class by clicking on the button to the right. Separate window, 250k.

The arrows jump to the previous/next class summary.