Worth watching

https://www.youtube.com/watch?v=Lr_bQs4oXgU


IF you have time - it is pretty long.

A block HW

In addition to working on the test prep stuff below, look into these questions:

- What is Galileo's "Siderius Nuncius" and what does it contain?  Hint:  look for images.

- What is so controversial about Galileo's "Dialogue"?

test practice

Test is next week.  Here are some practice problems.

1.  Consider a ball dropped from rest.  It reaches a top speed of 36 m/s.

a.  From what height was it dropped?
b.  How much time did it spend in air?
c.  Draw 3 related graphs:  d vs. t, v vs. t, a vs. t.

2.  Consider a ping pong ball gun that shoots a ping pong ball horizontally at 10 m/s.  It is mounted on a desktop, 1.2-m above the floor.  Where (horizontally) should you place a cup to catch the ball?

3.  A soccer ball is kicked at a 20-degree angle, with an initial speed of 15 m/s.

a.  How far will it travel (horizontally)?
b.  How long will it be in air?
c.  How high (max) will it rise above the ground?
d.  What is the other angle that would yield the same range?

4.  Review these ideas:

a.  SI standards - what they are, what they were, why was there a change?  You don't need to know specific numbers, but rather things like:  the meter is now based on the speed of light, though it was once based on the distance between north pole and equator.

b.  Unit conversions

c.  Odd numbers rule (Galileo)

d.  The weirder problems (drowsy cat, problems with quadratics)

e.  Difference between distance and displacement, speed and velocity

f.  How to use all equations of motion

g.  How and when to use horizontal and vertical components of motion

h.  The related demonstrations (ballistics cart, etc.)

HW

E and A - try this problem.  Consider a ball kicked at a 40-degree angle, with an initial speed of 38 m/s.  Find:

- initial components of velocity

- time in air

- max horizontal displacement

- max vertical displacement

- the other angle that would give the same horizontal range

HW and upcoming test

A -- practice the problems you've already seen.  Have a look at those we've already done, and review your notes.

E -- two things:

1.  Review problem.  A ball is released horizontally with an initial velocity of 15 m/s.  If it lands 22-m m (horizontally) from the launch point, from what vertical height was it launched?

2.  Think about how you would incorporate angles into a projectile problem?  In other words, if a ball was kicked with a speed of 25 m/s, but with an angle of 30-degrees (with respect to the ground), how would you deal with this?  How would you find time in air, horizontal displacement, max vertical displacement?  You probably won't be able to fully solve this yet, but that's ok -- think about the things you'd need to know, or ways to approach the problem.

BOTH CLASSES:

Expect a test the week after next.

Fun.

http://www.xamuel.com/inverse-graphing-calculator.php?phrase=LALLY

Local gravity, in gory detail.

Some thoughts on the acceleration due to gravity - technically, "local gravity". It has a symbol (g), and it is approximately equal to 9.8 m/s/s, near the surface of the Earth. At higher altitudes, it becomes lower - a related phenomenon is that the air pressure becomes less (since the air molecules are less tightly constrained), and it becomes harder to breathe at higher altitudes (unless you're used to it). Also, the boiling point of water becomes lower - if you've ever read the "high altitude" directions for cooking Mac n Cheese, you might remember that you have to cook the noodles longer (since the temperature of the boiling water is lower).

On the Moon, which is a smaller body (1/4 Earth radius, 1/81 Earth mass), the acceleration at the Moon's surface is roughly 1/6 of a g (or around 1.7 m/s/s). On Jupiter, which is substantially bigger than Earth, the acceleration due to gravity is around 2.2 times that of Earth. All of these things can be calculated without ever having to visit those bodies - isn't that neat?

Consider the meaning of g = 9.8 m/s/s. After 1 second of freefall, a ball would achieve a speed of .....

9.8 m/s

After 2 seconds....

19.6 m/s

After 3 seconds....

29.4 m/s

We can calculate the speed by rearranging the acceleration equation:

vf = vi + at

In this case, vf is the speed at some time, a is 9.8 m/s/s, and t is the time in question. Note that the initial velocity is 0 m/s.  In fact, when initial velocity is 0, the expression is really simple:

vf = g t

Got it?

The distance is a bit trickier to figure. This formula is useful - it comes from combining the definitions of average speed and acceleration.

d = vi t + 0.5 at^2

Since the initial velocity is 0, this formula becomes a bit easier:

d = 0.5 at^2

Or....

d = 0.5 gt^2

Or.....

d = 4.9 t^2

(if you're near the surface of the Earth, where g = 9.8 m/s/s)

This is close enough to 10 to approximate, so:

d = 5 t^2

So, after 1 second, a freely falling body has fallen:

d = 5 m

After 2 seconds....

d = 20 m

After 3 seconds....

d = 45 m

After 4 seconds...

d = 80 m

This relationship is worth exploring. Look at the numbers for successive seconds of freefall:

0 m

5 m

20 m

45 m

80 m

125 m

180 m

If an object is accelerating down an inclined plane, the distances will follow a similar pattern - they will still be proportional to the time squared. Galileo noticed this. Being a musician, he placed bells at specific distances on an inclined plane - a ball would hit the bells. If the bells were equally spaced, he (and you) would hear successively quickly "dings" by the bells. However, if the bells were located at distances that were progressively greater (as predicted by the above equation, wherein the distance is proportional to the time squared), one would hear equally spaced 'dings."

Check this out:

Equally spaced bells:

http://www.youtube.com/watch?v=06hdPR1lfKg&feature=related

Bells spaced according to the distance formula:

http://www.youtube.com/watch?v=totpfvtbzi0

Furthermore, look at the numbers again:

0 m

5 m

20 m

45 m

80 m

125 m

180 m

Each number is divisible by 5:

0

1

4

9

16

25

36

All perfect squares, which Galileo noticed - this holds true on an inclined plane as well, and its easier to see with the naked eye (and time with a "water clock.")

Look at the differences between successive numbers:

1

3

5

7

9

All odd numbers. Neat, eh?

FYI:

http://www.mcm.edu/academic/galileo/ars/arshtml/mathofmotion1.html

Fun problems to play with

In light of the recent tricky class problem (falling object spends 1 s to fall the seconcnd half of its motion), I submit a couple of other fun ones for you to try:

1.  A juggler can toss a ball a certain height H.  To what height must the ball be tossed to spend twice as much time in the air?

2.  If you drop a ball on the Moon (where g is 1/6 that of Earth), how does the time to fall compare to the time of fall for the same object on the Earth?  How would the answer be different on Jupiter (where g is around 2.3 times that of Earth)?

3.  Drowsy cat problem.  Imagine a drowsy cat sitting in a window (2-m tall).  The cat sees a ball fly up the window, past the top sill, and back down.  The ball is in view for a total of 0.5 seconds.  How high above the top of the window sill does the ball go?  (Answer:  2.34 m.  This is a tricky problem.)

HW

A -- Lab draft due Wednesday

E -- Lab draft due Tuesday


Problems for E and A.  Assume no air resistance.

1.  An object is dropped from rest.  How far could it fall in 3 seconds?

2.  How fast would it be moving after 3 seconds?

3.  If you drop a rock from a 30-m high bridge into the water below, how long will it take to hit water?

4.  Imagine throwing a ball straight up into the air, with an initial speed of 25 m/s.
a.  How long will it take to reach apogee?
b.  How high will it rise?
c.  How long will it take for the ball to return to your hands?

Quiz practice

For the quiz on Wednesday (E) and Thursday (A):

1.  Consider the earth revolving around the Sun.  The average distance between earth and Sun is about 150 million km.

A.  What is the Earth's average speed around the Sun?

B.  Convert this to ft/second and show the conversion process.

2.  From Park School, you travel 2 km east, 4 km south and an additional 3 km east.  Find:

A. Your displacement, including angle from start.

B.  Your average speed and velocity, if the trip takes 25 minutes to mountain bike.

Quiz upcoming

A - next Thursday (5/1)

E - next Wednesday (4/30)

Yay!

Velocity Lab

Formal Lab – The Measurement of Velocity

In this experiment, we will determine the velocity of a cart by 2 methods:

·              Photogate timer

·              Ticker tape timer

Each method can be quite accurate, though what is actually being measured by each is worth some discussion. 

Recall that velocity is calculated by knowing the displacement and the amount of time required to traverse it:

v = d / t

Strictly speaking, this is average velocity.  In theory, the average velocity is a mathematical average of all (if that were possible) instantaneous velocity points throughout the trip.

Instantaneous velocity is the type of velocity you receive from a speedometer – it is the velocity at that instant.  In the case where the object moves at a constant rate, the instantaneous velocity (at all points) is equal to the average velocity.  That should be the case (approximately) for this lab.  We will determine the extent to which this idea is true in this lab.  In this lab, you may work in cm OR m – be consistent.

Procedure

1.      Set up a path for your car to travel – 1 meter should be long enough.  Place your motorized car on it.

2.      Attach a piece of timer tape to the card and ready the cart for motion.

3.      Place a photogate timer at some point along the cart’s path.  Place a flag on the cart – it must break the light gate fully.  Ready the photogate for timing.  Measure the width of the flag for future reference.

4.      Set the tape timer and note the frequency of operation.  Turn on the tape timer.

5.      Turn on the car and allow it to run the length of the path.

6.      Remove the tape and write the time value from the photogate on the tape for future reference.

7.      Repeat for 2 different cart trials, using new tapes each time.

Analysis I – the Ticker Tape Timer

·              Examine the ticker tape.  If the car is traveling at a uniform velocity, how should the dots appear?  Verify that this does occur. 

·              Starting with the first clear dot, measure the distance that each consecutive dot is from the first dot.  Recall the frequency of the timer – this determines the time intervals.  For example, if it is set at 10 Hz, the time between each dot is 1/10 of a second.  With this in mind, write down the first 30 or so total displacements from the first point.  The corresponding times (for 10 Hz) are 1/10, 2/10, 3/10, and so on.

·              Plot total displacement versus time on a graph.  What type of relationship is it?  Does this seem correct?

·              Find the slope of the graph.  What does this represent?

·              What would a (displacement vs. time) graph of an accelerating car look like?  How about a decelerating car?  How about a car moving backwards with constant velocity?  Draw these in your lab.

Analysis II – the Photogate Timer

·              Calculate the instantaneous velocity of the car using the time and width of flag.

·              Compare, by means of % difference, the velocities from both methods.  Percent difference is found by taking one value minus the other value, divided by the average of the two values, and multiplied by 100.

In your conclusion, discuss the relative accuracy of the two methods and give methods for improving the lab.

HW for A

1.  Draw the following graphs (roughly) for a car that starts from rest and speeds up consistently for 10 seconds, then holds its speed constant for 10 seconds, then hits the brakes and comes to a halt in 5 seconds:

a.  displacement vs. time
b.  velocity vs. time

2.  Think about the lab - you'll be comparing the velocity measurement of a toy car using 2 techniques:  photogate timer and ticker-tape device (getting the slope of the graph).  Which might you expect to be more "accurate"?

3.  Find the definitions (mathematical and conceptual) of acceleration.  What are the units?

HW reminder for E

1.  Make sure that you have your graph of d vs. t for class tomorrow - they should be individual graphs, NOT group graphs.  Make sure to have the program find the slope, too.

2.  Also, be sure to find the speed using the photogate data.

Lab questions will be forthcoming after tomorrow's class.

HW for E

1.  Draw the following graphs (roughly) for a car that starts from rest and speeds up consistently for 10 seconds, then holds its speed constant for 10 seconds, then hits the brakes and comes to a halt in 5 seconds:

a.  displacement vs. time
b.  velocity vs. time

2.  Think about the lab - you'll be comparing the velocity measurement of a toy car using 2 techniques:  photogate timer and ticker-tape device (getting the slope of the graph).  Which might you expect to be more "accurate"?

3.  Find the definitions (mathematical and conceptual) of acceleration.  What are the units?

HW for A

I mentioned this in class but didn't update it on the blog.

Find out something about the SI kilogram - what is it based on now, and how might this standard be changed over the next few years.

Also - do the homework marked "HW for E and A" below.

HW for E and A

1.  Calculate the values of the average speeds in m/s.

2.  Convert to miles/hour (MPH).

3.  Create a factor for converting from m/s to furlongs per fortnight.

4.  How many seconds is a microfortnight?  Recall that micro means one millionth.  Show the work for this.

5.  How great a distance is a nano-light year?  Work it out.  Recall that nano means one billionth.

6.  If interested, create a similar problem and solve it.

SI Units info

Some comments on standards.

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

>

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

HW

Map out your trip to school, using Google maps, etc.  Print out the map, making sure that there is a legend/scale on it.  You'll need that to find the displacement.

Find your:

- travel time
- total distance
- total displacement
- average speed (distance / time)
- average velocity (displacement / time)

Bring this to class (Wednesday for E block, Thursday for A block)

A block physics - papers due Friday

You can still submit them tomorrow if you wish, but I'd like to chat about e/m induction a bit - and your devices might use this idea, so your paper may benefit.

Pick a device (paper)

Pick one of the following devices to research and write about in a 1-2 page paper.  Make sure to include a helpful diagram or two.  It is ok to talk about the history, but be sure to go into the physics - how does it work?

speaker or headphones/earbuds

telephone (regular, not cell)

guitar pickup (standard electromagnetic, not piezo)

microphone

transformer

MRI

magnetic tape recording
ruining electronics with magnets
hard drive
generator
turbine - wind, steam, water
metal detection
telegraph
phonograph (record player)
maglev trains

If you think of other ideas, let me know so I can tell you if you're in 'over your head'.

This will be due in 2 classes..

A and E hw

1.  Explain how/why the motor works.  It will be important to think about magnetic fields in coils.  In other words, why was it important to wrap the red wire into a coil?  What effect does that have on the magnetic field in the coil?  Is it similar to the electromagnet seen in class?

2.  Real motors are a bit more complicated.  Find images and/or descriptions of real motors, and comment on how they compare to yours.

E hw

Research electromagnetism and Oersted's law.

A hw - somewhat late

Find out about:

Electromagnetism

Domain theory

Sorry for the delay in posting this.

Test answers

E HW

So, obviously something is up with this North/South business.  Well, what is it?  To recap:  the needle end of the compass pointed toward the South end of the magnet.

- What does this mean?

- What ramifications does it have for the Earth's magnetic field?

Other questions for research:

Where is "magnetic north" on Earth?

Magnetic south?

Yet more things to think about:

Must there always be at least one north pole with every south pole (and vice versa)?  Can magnetic monopoles exist?  This is not something you'd necessarily know, so feel free to research.

How are magnetic fields different from electric fields?

Test practice

1.  Consider three resistors:  2, 5, and 10 ohms.  They are in series with a 30-V battery.  Find total resistance, current and individual currents and voltages.

2.  Repeat the above problem if the resistors are in parallel with the battery.

3.  Resolve the combination circuit below:

4.  Charge review.  Consider a chunk of charge:  -15 C.  

A.  What kind of particles are these?

B.  How many of these particles are there

C.  If a 10 C charge is brought nearby, 0.1 m away, what is the force between the two chunks of charge?

D.  Draw the electric field between the charges.

Test heads-up

Test on circuits (and earlier charge stuff) is next Thursday and Friday, right before spring break.  Sorry for the final push - cant be helped.

A block - review combination circuit

E block

Lab draft due next Wednesday - final lafy due on Friday.  

Attention A block

Draft of lab will be due on Tuesday.

I'd like you to have a little more time to make it great. 

Final lab due next Thursday.

That said, review series and parallel circuits.  Make sure you understand them.

Ok?  Ok!

Lab Guidelines (general) reposted

Lab Guidelines

The lab writeup should have each of the following items:

Title of Experiment - this is up to you
Your name
Lab partner(s)
Date(s) performed

Purpose - the purpose of the experiment, as it appears to you

Data - in table form, with units.  Give table a title as well.

Graph(s), where relevant - for this harmonic lab, graphs are optional.  They may make your point(s) stronger.

Answers to lab questions - see lab handout

Sources of error and ways to eliminate/reduce error

General conclusion - Talk about what you learned in the experiment.  Analyze data.  Give thoughts and reasoning, where appropriate.  Talk about applications or places where this new knowledge applies.

Make sure it is neat.

Series and Parallel - for A block

Folks in A block -

The earlier posting is about the lab stuff.  Here are a couple of review questions related to yesterday's class,

1.  You have 3 resistors:  3, 5 and 12 ohms.  They are in series with a 30-V battery.  Find all currents and voltages in the circuit.

2.  Consider the parallel problem from yesterday:  1, 2 and 3 ohms in parallel.  Now that you know the total resistance (6/11 ohms), find all voltages and currents in the circuit.

Bring circuit and/or lab questions to class Wednesday.

Yay!

lab stuff

A - Draft due Friday

E - Draft due Monday

Final lab will be due the class after the draft is due.

Lab Questions for "Ohm's Law" lab - please change the title.  If you use "shocking" as part of your title, I'll deduct a point for lazy punning.

So far, you've done 2 graphs: I vs. R, V vs. R. They will be included in the lab. Don't forget titles, units, etc.

Lab questions

1. Examine the 2 graphs. Do they make sense? Why? What's going on in them? Do they appear to obey any mathematical relationship/equation?   Is one graph stranger (or more unexpected) than the other?  Discuss.

2.  Calculate experimental resistances for each pair of V and I - use the equation R = V/I

3. You've just determined experimental resistances for each trial. Are they within 1% (the supposed tolerance) of the expected/theoretical values (the ones on the box)? Should they be? If not, why are they not so good?  Don't forget sources of error, in general.

4. What does it mean exactly if something follows Ohm's Law? Do all electrical devices follow this law? Are there substances that definitely are not "ohm-ic"?

5. Other than the batteries "dying," what might happen as the batteries are connected to the resistors? Would the V and I values change?  Discuss.

6. What is meant by "internal resistance" of the battery, and how does it affect this experiment (your results)?

7. Anything else you want to conclude or talk about.

8.  Hooray!

HW

A-finish graphs (V vs. R, and I vs. R).

Examine the data and see if things make any sense to you.  Consider how and why the V and I change (if they do) and what kind of relationships are at work.

E - investigate the concept of resistance. Prepare to run a lab comparing voltage, current and resistance (next Tuesday).

HW

E block - quiz Wednesday

A block - investigate the concept of electrical resistance

hw

For Tuesday's E block class:

Research L. Galvani's frog experiment.
Find out how this relates to the battery - it may be useful to read about A. Volta.

Quiz Thursday

For Monday's A block class:

You still have a quiz.  Sorry.  Have a look at the practice problems posted earlier.

pre-quiz practice

1.  Recall the electric field concept.  Draw the field that you would see under these circumstances.  The first two were shown in class:

a.  a positive hunk of charge by itself

b.  a negative hunk of charge by itself

c.  2 hunks of charge close to each other; both are negative

d.  2 hunks of charge close to each other; both are positive

e.  a hunk of positive charge close to a hunk of negative charge

If you want to know quickly if your answers a close to correct, do a google image search for "electric field".  The first few images that show up will have these among them.

2.  Review for quiz:

a.  Calculate the force between 2 charges (5E-6 C, -15E-6 C), when separated by a distance of 0.004 m.

b.  If the distance between these 2 charges were changed to 5 time the original amount, how would the force be different?  Do this without a formal calculation.

c.  Consider the 5E-6 coulomb charge.  How many protons is this?  Recall that 1 proton has a charge of 1.6E-19 C.

d.  What does it mean to be a fundamental particle?  

hw

For Thursday's A block class:

Research L. Galvani's frog experiment.
Find out how this relates to the battery - it may be useful to read about A. Volta.

Quiz Monday

For Wednesday's E block class:

Apparently, the Electric Field applets (nor any applets) are working on most computers (even if you update your java).  Try to find out how to draw electric fields.  If time allows, do the homework above.

Quiz next Tuesday

electric fields

Electric Fields

Play around with the applets, so you can investigate electric field lines.  Recall the sign convention:

Field lines point away from positive charges
Field lines point toward negative charges

With this in mind, draw field diagrams for some/all of the scenarios described below, as well as some of your own:

•    Single positive charge
•    Single negative charge
•    One negative and one positive charge (equal magnitude of charge)
•    Two negative charges and/or two positive charges
•    One negative and one positive charge (unequal charges) – try a couple different configurations, with at least one attempt having very different magnitude than the other
•    Multiple charges in weird configurations – go for broke here, and make several drawings (at least five)

The applets below may prove useful – or at least cool to play around with.  Also feel free to do a Google search for ‘E-field’ and ‘applet’.  This should prove fruitful.

Also, don’t confuse E-field lines with Equipotential lines (lines of constant electric potential, or voltage).  Some applets will display both if desired.


http://www.cco.caltech.edu/~phys1/java/phys1/EField/EField.html

http://falstad.com/vector2de/
http://falstad.com/vector3de/

http://www.cbu.edu/~jvarrian/applets/efield1/elefi_z.htm

http://www.colorado.edu/physics/phys1120/phys1120_sp99/Applets/Efield.html
(This is useful for seeing the field vectors at work.)

Coulomb's law hw

Now that we have reviewed Coulomb's law, play with these:

1.  What is the force between an electron and a proton in a hydrogen atom, separated by the Bohr radius of 53 pm (x 10^-12 m)?

2.  Is the force in the above problem attractive or repulsive?

3.  How many protons are required to give a charge of 5 C?

4.  Consider a sodium ion with a net +2 charge.  Keeping in mind that chemists usually drop the 'e' part of the charge, what is the total net charge on that atom?

5.  Read about the concept of electric field.  How do we draw them?  What do the field lines represent?  A good place to start is physicsclassroom.com, and look under the static electricity section.

Physics - yay!

FYI

HW

1.  Find out Coulomb's law.  Write the equation and identify the variables.

2.  Coulomb's law is an "inverse square law" - what does that mean?

3.  Investigate the "Standard Model of Particles and Interactions."  Write something about it.  Enjoy!

HW questions about charge, etc.

Questions to ponder.  Please write out your answers in your notebook.

1.  Write some type of definition of "charge".  It's ok to look something up, but try to formulate your own definition of charge first.

2.  Find the actual distance between an electron and a proton in a typical atom - it's ok to use hydrogen as your example.  Find out the average speed, too, if you can.

3.  Are protons, electrons and neutrons are "fundamental"?  That is, can any of them be broken into something smaller?  If so, talk about it.

4.  Review the rotating 2x4 demonstration from today - why does the board rotate?

5.  What is the official standard unit for charge?  How is it defined?  This may be tough to put into words.  Try your best.

6.  How do the mass of proton, neutron and electron compare?  Look up their values, if that's helpful.

*7.  (If time allows.)  Look up the Heisenberg Uncertainty Principle.  How does it relate to the measuring of particles?  If you don't follow it, keep in mind that it was new and challenging enough to win Heisenberg a Nobel Prize (so don't feel bad).

for fun.

http://sploid.gizmodo.com/the-sum-of-1-2-3-4-5-until-infinity-is-so-1503066071/@caseychan

(Thanks, Kyle!)

http://geekdad.com/2014/01/1000000000000000000000-years/

quiz practice

1.  Make sure you finish the diffraction (informal) lab for wavelength of laser light.  There will be a related question on the quiz.

2.  Speaking of which..... A 400 nm laser hits a diffraction grating (750 slits/mm).  The wall/screen is 2-m away from the grating.  Find:

a.  diffraction angle for a first order image
b.  distance between the central/primary (n = 0) image and the first order image
c.  how many images are produced

3.  Consider a 40-cm focal length mirror.  An object (5 cm in height) is 100 cm in front of it.  Find:

a.  image location
b.  magnification of image
c.  image characteristics
d.  Give the do(s) that would yield only virtual images.
e.  Give the do(s) that would yield NO images.

During the quiz day, I'll also ask you to write a short self-reflection/"what I've found interesting" paragraph.  This will be included in your semester grade report.  Feel free to start writing this now.  It needs to be emailed to me before the end of the week.  Thanks!

Ripple Tank applet FYI

http://falstad.com/ripple/

Diffraction homework

1.  Consider the following:

A 632 nm (red) laser hits a diffraction grating with 100 slits per mm.  A screen/wall is 1-m away.

a.  What is the spacing (in m) between each of the slits (or openings)?

b.  What will be the angle of diffraction for a first-order (n = 1) image?

c.  How far from the central image (n = 0) will the first-order image be from the central image?  Hint:  use trig (SOH CAH TOA).

d.  Is there a second order image (n = 2)?  If so, what is the angle of diffraction?

e.  Is there a third order image?  What is the maximum number of orders of images seen?  Hint:  consider that the maximum diffraction angle is 90 degrees.

2.  Find out something about X-ray diffraction and what it is useful in determining.

Homework

Have a look at these pages:

http://www.physicsclassroom.com/Class/light/u12l1a.cfm

http://www.physicsclassroom.com/Class/light/u12l1b.cfm

The expressions are treated in a somewhat complicated fashion (in the section mentioned below).  If you can, try to find an equation for diffraction.  Something that is easier to make sense of.

If you have time, check out Lesson 3 (at the bottom):

http://www.physicsclassroom.com/Class/light/