SI Units

Some comments on standards. We generally use SI units in physics. To inform you:

Mass is measured based on a kilogram (kg) standard.
Length (or displacement or position) is based on a meter (m) standard.
Time is based on a second (s) standard.

How do we get these standards?

Length - meter (m)

- originally 1 ten-millionth the distance from north pole (of Earth) to equator
- then a distance between two fine lines engraved on a platinum-iridium bar
- (1960): 1,650,763.73 wavelengths of a particular orange-red light emitted by atoms of Kr-86 in a gas discharge tube
- (1983, current standard): the length of path traveled by light during a time interval of 1/299,792,458 seconds

That is, the speed of light is 299,792,458 m/s. This is the fastest speed that exists. Why this is is quite a subtle thing. Short answer: the only things that can travel that fast aren't "things" at all, but rather massless electromagnetic radiation. Low-mass things (particles) can travel in excess of 99% the speed of light.

Long answer: See relativity.

Time - second (s)

- Originally, the time for a pendulum (1-m long) to swing from one side of path to other
- Later, a fraction of mean solar day
- (1967): the time taken by 9,192,631,770 vibrations of a specific wavelength of light emitted by a cesium-133 atom

Mass - kilogram (kg)

- originally based on the mass of a cubic decimeter of water
- standard of mass is now the platinum-iridium cylinder kept at the International Bureau of Weights and Measures near Paris
- secondary standards are based on this
- 1 u (atomic mass unit, or AMU) = 1.6605402 x 10^-27 kg
- so, the Carbon-12 atom is 12 u in mass

Volume - liter (l)

- volume occupied by a mass of 1 kg of pure water at certain conditions
- 1.000028 decimeters cubed
- ml is approximately 1 cc

Temperature - kelvin (K)

- 1/273.16 of the thermodynamic temperature of the triple point of water (1 K = 1 degree C)
- degrees C + 273.15
- 0 K = absolute zero

For further reading:

http://en.wikipedia.org/wiki/SI_units

http://en.wikipedia.org/wiki/Metric_system#History

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In addition, we spoke about the spherocity of the Earth and how we know its size. I've written about this previously. Please see the blog entries below:

http://howdoweknowthat.blogspot.com/2009/07/how-do-we-know-that-earth-is-spherical.html

http://howdoweknowthat.blogspot.com/2009/07/so-how-big-is-earth.html

Welcome Physics Phriends!

How Things Work

Physics 103
Spring 2017
Sean Lally
seanplally@gmail.com
412.965.0805
M/W - 326 Smith - 7 - 8:15 PM
Office Hours:  30 minutes before class and immediately after class (as late as desired)
htwspring2016.blogspot.com
How Things Work,  Louis A. Bloomfield

Good evening physics phriends!  Welcome to a new semester and your new favorite class, "How Things Work," with your humble host, Sean Lally.

I am thrilled to be sharing some of the big ideas of physics with you this term.

There will be 3 non-cumulative exams, equally weighted.  Exams are generally multiple choice, though it is possible that there *may* be short answer questions, mathematical problems or 1-2 paragraph essays asked as well.

I will assign problem and question sets regularly, but these will not be collected.  Primarily, they are used to help you study material and see what concepts I find to be most valuable (on exams, and in general).  I will give some text references as well, but in general, if it is important I will cover it in class, with related notes on the blog.

I use this blog for my personal notes and to get important information to you.  I tend to use the document projector to work out problems or write essential things.  If it's important stuff, I'll take an image and post it on the blog.  However, don't view this as a substitute for good note-taking.  The blog will have some detail, but also at times, only a skeleton outline of notes.  Sometimes these notes will be up on the blog well in advance of class, and sometimes, well, they won't.

Welcome to "How Things Work"!

https://www.youtube.com/watch?v=wMFPe-DwULM
Just because Richard Feynman was so cool.

1.30  Introduction; course overview; SI units
2.1  How to measure things

2.6  How things move, part 1: velocity, acceleration and the math of motion (1.1, 1.2)

2.8  How things move, part 2: velocity and the math of motion (1.1, 1.2)

2.13  How things move, part 3: acceleration, gravity, and the math of motion (1.1, 1.2)

2.15  How things move, part 4:  Newton, pre-history and his first law (1.3)

2.20   How things move, part 5: Newton's laws 2 and 3 (1.3) 

2.22  How things move part 6:  gravitation again; Earth and Moon (4.2) 

2.27   How things move part 7:  center of gravity

3.1  How things move in air:  kites and planes, Bernoulli’s principle (6.2, 6.3)  / test review

3.6  Exam 1

3.8  How things sound, part 1: waves and music (9.1, 9.2)

3.13  How things sound, part 2: harmonics (9.2)

3.15  How things sound, part 3: musical instruments (9.2)

3.27  How things sound, part 4: Music; Doppler effect (9.2)

3.29  How things look, part 1: light, lasers, polarization (13.1, 14.1, 14.3)

4.3  How things look, part 2: lenses, mirrors (15.1)

4.5  How things look, part 3: optical instruments (15.1)

4.10  How things look, part 4: 3-d, optical misc(15.1)

4.12  Exam 2

4.17  How things tick, part 1: particles, static electricity (10.1, 10.2)

4.19  How things tick, part 2: current and voltage

4.24  How things tick, part 3: circuits 1(10.3)

4.26  How things tick, part 4: circuits 2 

5.1  How things tick, part 5:  magnetism, electromagnetism (speakers, etc.) (11.1) 

5.3  How things tick, part 6:  induction (microphones, pickups) (11.2) 
5.8  How things tick, part 7:  EM devices (11.2)

5.10  How things tick, part 8:  weirdo electrical stuff (11.2)
5.15  Tying up loose ends; exam prep

5.17  Final exam