Intro to waves

So - Waves.....  

We spoke about energy.  Energy can, as it turns out, travel in waves.  In fact, you can think of a wave as a traveling disturbance, capable of carrying energy.

There are several wave characteristics (applicable to most conventional waves) that are useful to know:

amplitude - the "height" of the wave, from equilibrium (or direction axis of travel) to maximum position above or below

crest - peak (or highest point) of a wave

trough - valley (or lowest point) of a wave

wavelength (l) - the length of a complete wave, measured from crest to crest or trough to trough (or distance between any two points that are in phase - see picture 2 above).  Measured in meters (or any units of length).

frequency (f) - literally, the number of complete waves per second.  The unit is the cycle per second, usually called:  hertz (Hz)

wave speed (v) -  the rate at which the wave travels.  Same as regular speed/velocity, and measured in units of m/s (or any unit of velocity).  It can be calculated using a simple expression:

There are 2 primary categories of waves:

Mechanical – these require a medium (e.g., sound, guitar strings, water, etc.)

Electromagnetic – these do NOT require a medium and, in fact, travel fastest where is there is nothing in the way (a vacuum). All e/m waves travel at the same speed in a vacuum (c, the speed of light):

c = 3 x 10^8 m/s

First, the electromagnetic (e/m) waves:

General breakdown of e/m waves from low frequency (and long wavelength) to high frequency (and short wavelength):

Radio

Microwave

IR (infrared)

Visible (ROYGBV)

UV (ultraviolet)

X-rays

Gamma rays

In detail, particularly the last image:

http://hyperphysics.phy-astr.gsu.edu/hbase/ems1.html

http://csep10.phys.utk.edu/astr162/lect/light/spectrum.html

http://www.unihedron.com/projects/spectrum/downloads/full_spectrum.jpg

Mechanical waves include:  sound, water, earthquakes, strings (guitar, piano, etc.)....

Again, don't forget that the primary wave variables are related by the expression:

v = f l

speed = frequency x wavelength

For e/m waves, the speed is the speed of light, so the expression becomes:

c = f l

Note that for a given medium (constant speed), as the frequency increases, the wavelength decreases.

practice for Wednesday's quiz

Be sure to review notes from the entire year so far, as the quiz will cover:  pendulums, simple harmonic motion, the basics of waves.  You will be given any needed equations.

Also:

1.  Determine the frequency of a 120 pm x-ray.  (pm means picometers, or 10^-12 m)

2.  Give the differences between mechanical and electromagnetic waves, citing examples of each.  (This is a leftover from the last homework.)

3.  If you were designing a grandfather clock to work on Venus, where the gravity is roughly 90% of Earth's, how long should the pendulum be so that the period is 1 second?

4.  Waves can be represented with mathematical curves (usually sine graphs).  Consider amplitude and frequency for a moment.  What real wave characteristics corresponds to these things in a sound wave?  How about in a light wave?

Don't forget to review all of the earlier material and homework.

HW for Thursday

For next class:

1.  Find an equation that relates wavelength, wave speed and frequency.  Use this for the next 2 problems.  (By the way, if you don't quite know what these things mean, look them up first.)

2.  Calculate the wavelength of a 440-Hz sound wave (which is concert A), if the speed of sound is 340 m/s.

3.  The speed of light is approximately 3 x 10^8 m/s.  What is the frequency of a 0.03-m microwave?

4.  Find definitions for mechanical wave and electromagnetic wave.  

HW for Monday

By Monday, please arrive to class with a definition of "wave" - ideally, a definition that is in your own words, and not a wikipedia quote.  Also, using a picture if helpful, define the following parts:

amplitude
frequency
wavelength
wave speed

And if possible, find an equation for wave speed that relates measurable variables.

As always, physicsclassroom.com may prove useful.

Thanks!

Pendulum Homework for Thursday

Pendulum homework

1.  When we say the “period of the pendulum,” what exactly do we mean?

2.  What does the period of a simple pendulum most depend on?

3.  What is the basic shape of the graph for the period of a pendulum versus its length?  Please draw this.

4.  What is the basic shape of the graph for the period of a pendulum versus the mass of the bob?

5.  Imagine that you have a “motion detector” set up close to the pendulum – something that could track the pendulum bob as it oscillates.  What might a graph of location/position versus time resemble for several oscillations?

6.  Where in the pendulum’s path does it come to a complete stop?

7.  Where in the pendulum’s path does it have maximum speed?

8.  Given what you know about energy, how might you describe the energy of the pendulum

9.  Calculate the period of a 3-m long pendulum.

10.  How long should a pendulum be so that it has a 0.5-s long period?

11.  How does the period of a pendulum depend on gravitational acceleration? 

12.  How does the period of a 1-m long pendulum vary on the Moon (versus the Earth)?  The acceleration due to gravity on the Moon is 1/6 that of Earth.  So, how does the period of oscillation compare (exactly) to that on Earth.  Try to come up with some type of comparative equation.

Graphing homework reminder

Today you collected some data for period and length for a pendulum.  Ideally, you have at least 10 pairs of data points.

Create a graph, using Logger Pro please.  This graph should have length on the x-axis and period for one oscillation on the y-axis.  Don't forget units.

If you can, play around with the curve fit and see if you can make a curve fit the data.  I suggest using "Power" under "Curve fit".

Using the data (A and B) from the theoretical curve, write a theoretical equation for the period of your pendulum.

Now look up the actual equation for the period of a simple pendulum.  Are there similarities?  Discuss.

(FYI:  Useful site for physics learning:  http://www.physicsclassroom.com/)


If you can't get Logger Pro to download successfully - and some folks have not been able to - download the demo version (but make sure you don't input any credit card data to do so).  If you are still not able to make Logger Pro work for you, either use Excel or use a computer at school with Logger Pro.  You can also try a different browser.  If it appears to download, but you can't find it on your machine, search for "Logger" (not loggerpro).  There should be a folder with the program inside.

Finally, if you do not have a computer at home, maybe we can find one for you.  Chat with me and I will connect you with our very own organization, Computers for Kids.

Thanks!  Happy weekend, physics phriends!

sean

HW for Thursday

homework to bring Thursday 

Hey everybody! 

You've collected some data today.  For homework, take a look at the data.  What variable(s) seem to affect the time of the pendulum?  What affects it a lot?  A little?  Nothing at all?  Can you tell?

Pick the set(s) of data that matter most and graph them.  Time (the dependent variable) will be on the vertical (y) axis.  Mass, initial height/angle, or length will be on the horizontal (x) axis - these are all independent variables.  In general, the independent variable goes on the x-axis and the dependent variable is on the y-axis.

Make one or more graphs and see if any relationship pops out at you.

Either hand-draw the graphs on graph paper or use a computer (Excel or some online graphing program).  Bring the print-out of the graph(s) or the handwritten copies to class.

Notes on notes

Some thoughts on how to take notes in physics.  This is all personal preference, but it has served me well.

1.  Use a bound notebook, ideally with quad-paper.  Spiral-bound notebooks fall apart and binders are cumbersome (and the pages fall out before long).  Bound notebooks keep you "honest."

2.  Use a new page for each new class and/or topic.

3.  Write on only side of the page - it makes it easier to read, particularly if you use pen (which can bleed through to the other side).

4.  Give each new page a heading and date.

5.  Use color to highlight central concepts and/or put boxes around important ideas or equations.

6.  Draw pictures that represent the problem - label relevant things in the diagram.

7.  Don't use a laptop to take notes.  You spend too much time getting down trivial details and will inevitably miss something critical.  It is also way too tedious to include equations, pictures, graphs, calculations and anything that really demands visual representation.  The evidence is also pretty clear on this - taking notes on a laptop is not as helpful (toward understanding or remembering) as taking notes by hand.  

http://www.scientificamerican.com/article/a-learning-secret-don-t-take-notes-with-a-laptop/

8.  Keep a running page of the important equations or ideas - maybe on the inside cover of the notebook.  Label formulas.

9.  If things are moving too quickly for you, leave space for omitted notes – with a heading of what that topic is.  And then touch base with me or a classmate to fill in the gaps.

10.  Leave space in your notes – don’t cram them all together.  Again, writing on one side of the page is helpful.

11.  If you have questions, but don’t want to ask them in class (or see that we have moved beyond that topic already and you don’t want to revisit it at the moment) – write down the questions in the margin of your notes, and circle it.  And then seek out the answer during a break, after class, or by email.  But get your question answered!

12.  If taking notes on a laptop is really important for your learning, take images of the board and incorporate them into your notes.  Some students like OneNote or EverNote for note-taking, but other programs exist and may be worth considering. 

13.  By and large, if it is on the board it should probably be written down.  For me personally - if I think it is important for the students, I will write it on the board.

14.  Never be shy about asking for extra help – even to have me look at your notes.  Sometimes I can tell where you went wrong in your thinking by seeing errors in your notes.

15.  I AM ON YOUR SIDE.

Welcome to Physics!

Please watch this video.  I'm not sure exactly why I like it so much, but I do.  Richard Feynman (1918 - 1988) was an American physicist - probably the most famous one of the 20th century.  I often like how he thinks about thinking.

https://www.youtube.com/watch?v=3D2RaDVkylY

And also this, because it is just so cool:

https://www.youtube.com/watch?v=oSCX78-8-q0

Lastly, please take this survey for the science department:

https://www.surveymonkey.com/r/2015summerscience