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28 February 2014

Free College Physics Textbook at OpenStax -- Try It and Let Me Know

For decades, it's been fashionable for college students to whine about the price of their physics textbook.  Really?  $250 for this thousand-page tome that I'm only going to use for the problem my professor assigns?  Does it come with a diamond ring?

High School students are largely (seemingly) insulated from that cost, because most schools buy and distribute the books.  Don't think your district is getting much of a discount.  Read Surely You're Joking, Mr. Feynman for just one behind-the-scenes anecdote about state-level textbook adoption committees.  All that money comes out of your tax dollars, and (if you teach in the public school) money that could otherwise be part of your salary.

Of course, one could make similar complaints about highway construction costs.  $10 million per lane per mile?  That makes a physics textbook seem like a drop of fish poop in the Gulf of Mexico.  The question "is that too much?" makes no sense.  Rather, one relevant inquiry to start with is, "By building an extra lane on this freeway, do the taxpayers get a benefit worth $10 million?"

The more important question, though, is one of opportunity cost: "Would it be possible to get a comparable benefit to a freeway lane for less than $10 million?"  Could we, for example, build a high speed rail line for that kind of money that better serves the people driving that route?  Or, would less than $10 million worth of improvements to the air transport system put fewer people on the road, and thus accomplish a similar goal?  I don't know the answer to this question, because I'm not a transportation expert.

In physics, though, I know straight-up:  The commercial physics textbook is not worth your money.  Take a look at the OpenStax College Physics book.  Leaf through it a bit.  The problems are similar in number, style, and difficulty to those in Serway, Giancoli, or Cutnell and Johnson.  The explanations are also on-par with the leading textbooks: not great, not the way I would have written them, but perfectly acceptable.  The student and teacher ancillary materials are also comparable -- OpenStax provides students the complete solutions to every third problem, and they offer teachers who register a full solutions manual.

So why, oh why, is anyone paying for a classic text?  I'm more than willing to hear and acknowledge that your favorite text is superior to the OpenStax version.  Is it $250 per student superior?  Is it ∞ times better?  If it were your personal money, would you be enthusiastic about buying the quarter-of-a-thousand-dollar text while a free equivalent or near-equivalent was available?  I doubt it.

I'd love to hear comments about this or other free online texts.  


26 February 2014

Useful App: Coach's Eye

Erik, my physics teaching colleague who also coaches track, showed me a fantastic iOS app.  He takes high-speed video of his runners using his iPhone 5, then uses the app "Coach's Eye" to analyze what he sees.  The big advantage of Coach's Eye over all other video software that I've seen is the stopwatch overlay.  He touched the screen, and a stopwatch accurate to 0.01 s appeared.  Then he scrolled to advance the video frame by frame, and the stopwatch synched to the video.  The applications for track are obvious; but we both saw how this application could revolutionize motion measurements in the introductory physics lab.

Now, my ipads aren't the latest; they take video at only 30 frames per second, as opposed to the 120 fps that Erik has.  So what -- I can still get 0.03 s accuracy, which is plenty for a projectile problem.  

THE QUESTION posed as homework to my freshmen:  A ball is shot horizontally from a cannon from a cliff above level ground.  When the ball's initial speed is doubled, what happens to the time of the ball's flight?

While many students use appropriate facts to find that the time of flight doesn't change, many others do not -- their instinct is that the ball that goes farther and faster must also spend more time in the air.  Even those who get the answer right don't really believe.

So I re-pose the question in lab, using a PASCO projectile launcher.  I handed out eight ipads, asking the students to open the Coach's Eye app and prepare to take video.*  Everyone recorded video of the ball's path.

Don't worry -- while it took me 20 minutes and several questions to Erik before I was comfortable using the app, my 14 year olds -- all of them, not just the top students -- were ready to shoot video in less than 30 seconds.  

Then we clicked "analyze" and placed the stopwatch on the screen, synched to the initial launch.  The screenshot above shows the ball in flight, 0.26 s after release -- you can see the ball as a yellow streak in the middle of the frame.  (With a 120 fps camera the streak would resolve into a dot.)  Everyone advanced the frames until the ball hit the ground, and wrote the stopwatch reading on the board.  The average time of flight measured by the class was 0.45 s.  

On the board, I carried out the kinematics calculation to predict the height of the launcher -- I got 99 cm.  I handed a meterstick to a student, who measured... 98 cm from the floor to the mouth of the launcher.  Physics works.

But there's more!  I had everyone get in recording position again, and shot the ball with a much faster speed.  We again analyzed the video to find the time of flight, writing the results on the board... and the average time was once again 0.45 s.  Amazing.

Tomorrow, I'm going to follow up with one more problem, as suggested by my colleague Alex.  I'll ask on a quiz, if the height of the launcher were doubled, would the time of flight double, less than double, or more than double?  Of course they'll all say the time should double to 0.90 s.  But then I'll pile two desks on top of one another, fire the projectile while everyone records video, analyze the video... and we'll see what physics says.

Notes about the app:  It costs $4.99 for the app, plus another $4.99 in-app purchase to get the stopwatch tool.  Worth every penny, at least for a single class ipad.  And the Coach's Eye developers, whoever they are, haven't given me a dime for this review.  If they'd like to change that sorry state of affairs, they can contact me through Woodberry Forest School.  :-)


19 February 2014

Why do both blocks experience the same normal force? Don't allow misconceptions to slip in.

This table [not pictured] lists the coefficients of friction for four materials sliding over steel.  A 10 kg block of each of the materials in the table is pulled horizontally across a steel floor at constant speed.  Which block would require the smallest applied force to keep it moving at constant speed?

Virtually all of my students will recognize to use the equation Ff = μFn.  They've also been well trained in using such an equation to first identify and justify which variable is the same for each of the blocks. 

Most students recognize that the normal force Fn is the unchanged variable.  The question is, how should they justify that the normal force is the same for each block?  Consider some incomplete answers:

1. The normal force is the same for each because they both weigh 100 N.  No, sorry... Consider one 10 kg block hanging from a string in midair, and another on the floor.  These blocks experience different normal forces, because the hanging block experiences no normal force whatsoever.

We have to get to the idea that the up forces equal the down forces, so the normal force equals the block's weight.  Why is that?

2. The normal force is the same for each because neither block moves vertically.  Still not quite there -- it's not motion that relates to forces, it's acceleration.  Try again.

3. The normal force is the same for each because neither block is changing speed vertically, so both experience zero vertical acceleration.  This makes the vertical net force zero for each, meaning up forces equal down forces.  Both blocks thus experience a normal force equal to their weight; since they weigh the same, they experience the same normal force.  Now we're cooking with ethanol.*

*True story: during my senior summer research, my nerd friends and I tried to grill, but we were out of lighter fluid.  No problem, 'cause the chemists had a ready supply of ethanol from the lab.  Two hours and zero flames later, we ordered pizza.

Point is, even though I've moved on from explicitly discussing the difference between movement and acceleration, I've gotta be vigilant that we never slip into the "acceleration = motion" habit.  And, even on a problem so straightforward, it's important for students to write out the reasoning to justify determining the normal force.  Otherwise, they fall into the habit of "normal force ALWAYS equals weight", which we know to be bogus.

12 February 2014

Open Lab 2014: July 27-29 at Woodberry Forest

I spend much of my summer running official College Board AP Summer Institutes.  I encourage you to join me for one of these... the dates and locations for 2014 are posted in the sidebar.

While I love AP Physics, and I love the Summer Institute format, I also recognize that there's more to physics teaching than can be discussed in a week devoted specifically to the College Board's courses.  What about conceptual physics?  General physics?  Research?  And how about college-level physics that doesn't correspond to the new algebra-based AP exams?  These topics deserve some attention in serious professional development workshops.

On July 27-29, 2014, I invite you to Woodberry Forest for a Summer Institute that is emphatically NOT devoted to AP physics.  I will share my own materials related to non-AP courses; we'll talk about and actually do some activities and laboratory work focused at all levels of physics, from conceptual to research and everywhere in between.

Also joining me will be Staci Babykin Murray, of McIntosh High School in Peachtree City, Georgia.  Her particular expertise is in using (in my words) "new-ish" technology in the classroom: iPads, Learning Management Systems, flipped classes, etc.

Between Staci, me, and the other folks who show up for the open lab, you should be able to have a good discussion about any physics teaching related questions you might have.  Hopefully we'll all leave on Tuesday the 29th with a bunch of new ideas to try out.

I'll post more logistical information shortly.  For now, know that there is no charge for the open lab, but there's no grant money, either.  You'd need to pay for food and lodging.  Arrive on Sunday midday; we'd have a late afternoon formal* session followed by dinner together and an (informal) evening "session" at my house.  We'd work all day on Monday, and until mid-afternoon on Tuesday.  You'd want to stay Sunday and Monday nights at the Holiday Inn Express in Orange, VA -- that's a five-minute drive** from campus.  We'll eat together in Orange for meals.  As those of you who have been to my summer institutes know, just being around other physics teachers is professional development, whether we're in the lab, walking around campus, at dinner, in the pub, etc.

* (or as formal as anything I do ever gets)
**(Or a 1.5 hour walk, or a 50 minute jog... I've done all of these.)

There is no "registration," -- just tell me you're coming and make a hotel reservation.  Spread the word.  And if anyone can navigate the bureaucracy of CEUs or whatever, please let me know.  I don't intend to provide any more than a certificate that you were here, but if someone who has the knowing can do the background work to get CEUs for all attendees, that would be awesome.

10 February 2014

I pull a block across a surface at constant speed, then double the speed. Does the pulling force also double?

Problem set question:  When you apply a force of 2 N to the right, you cause a wooden block to slide across a rough surface at a constant speed of 3 m/s. Now you want to push the block so it moves at a constant speed of 6 m/s.  How much force should you apply?

(A)          6 N
(B)          4 N
(C)          2 N
(D)          1 N

The most common answer by far, even from a pretty darned skilled class, is (B).  The usual reasoning is tautological:  "If 2 N gives 3 m/s of speed, then I must double the force to double the speed."  Boux.

Of course, I explain in class that Newton never said "F = mv."  He said "F = ma."  Since the acceleration is zero in either case, the pulling force will still equal the friction force.  And since the force of friction doesn't depend on speed, the pulling force will be unchanged.  

Few students believe me.  Only an experiment will convince the class that I'm not full of crap.

So I had Brendan use the experimental setup shown at the top of the post to verify this answer.  A force probe on a PASCO variable speed cart is attached to a string pulling a block along a surface.  First, Brendan set cart to a slow speed, and recorded the force probe reading.  Then, he set the cart to a very fast speed, and recorded the force probe reading.  Results:

This cart was moving slow.

At a slow speed, the force probe reads 0.69 N plus or minus about 0.05 N.

This cart was moving fast.
Same pulling force, though...

And at a fast speed, the force probe reads 0.70 N plus or minus about 0.05 N.

Conclusion:  Doubling speed does NOT double force.  I was right: physics works.  Imagine that...

05 February 2014

Paper Football and a force exercise

Ever play Paper Football?  Only one student out of my 45 freshmen had not.  As today was our first day
back to school after the Superb Owl, my colleague Alex Tisch created a force-and-motion exercise based on the game of Paper Football.

How do you play:  You get four tries to tab the triangular "football" (pictured) across the table such that it comes to rest with part hanging off the edge of the table.  After scoring, a team may attempt an "extra point" by flicking the ball such that it becomes a projectile that soars between the "goalposts" created by the opponent's fingers.

What does this have to do with physics?  Consider the football in four situations:

(1) While your finger is in contact with the football during the tap
(2) After your finger loses contact, but while the football still slides forward
(3) When the football is in position for a score, at rest with part off the table's edge
(4) While the football is in the air during an extra point attempt

In each situation, students are asked:

(a) Draw a free body diagram of the football.
(b) What is the direction of the net force on the football?
(c) What is the direction of the football's acceleration?

This produces some fascinating responses.  This is a good exercise for continuing to bust silly misconceptions such as "the acceleration is to the right, because the football moves to the right."  But the four situations provide ample opportunity to work on advanced problem solving skills, too... 

In situation (1), the acceleration direction must be determined through the motion (speeding up, so acceleration is forward).  Only then can the direction of net force be justified as the same as the acceleration.  Many students started with the net force being forward because "the force of the finger is greater than the force of friction."  But how do we know?  The problem never gives values, and there's no way to know a priori which horizontal force is bigger.  Only facts about motion and acceleration lead to the net force direction.

But in situation (2), only one horizontal force -- friction -- acts on the football.  So it's entirely legitimate to start with the direction of the net force straight from the free body diagram.  Then, the acceleration must be in the same direction as the net force.

These two similar situations must be approached with opposite-ordered logic.  Cool... 

When a student finishes answering all four situations correctly, I hand him a paper football and assign him an opponent.  They play a game in the remaining few minutes of class.  Winners get candy...  Thanks to Alex for creating this activity.

03 February 2014

USIYPT 2014 results

This weekend, nine teams from all over the world converged on San Jose, California for the 7th US
Invitational Young Physicists Tournament.  The Harker School, the tournament hosts, won the championship. Participating schools in order of finish in the preliminary round include:

Shenzhen Middle School, China
Woodberry Forest School, Virginia
The Harker School, California
Rye Country Day School, New York
Nanjing Foreign Language School, China
Pioneer School of Ariana, Tunisia
Guilderland High School, New York
Georgian English-Spanish School
Pioneer School of Manzeh 8

The Clifford Swartz Trophy for the best poster was awarded to Guilderland High School.

In the semifinals, Shenzhen defeated Rye, and Harker defeated Woodberry.  The final physics fight saw Harker narrowly edge out Shenzhen.  Harker is the first two-time champion, having also won in 2011.

The 2015 US Invitational Young Physicists Tournament will be held on Jan. 30-31 at Woodberry Forest School in Virginia.  If your school would like an invitation to participate, please contact Greg Jacobs, president, USAYPT, via the Woodberry Forest School website.

USIYPT 2015 Problems:

#1 --  Avogadro’s Constant: Measure Avogadro’s Constant in as many different ways and methods as you can (at least three, more preferably; the Board knows of approximately twenty methods). Assess the uncertainties of each method and compare your results.

#2 -- Gauss Rifle: Build and investigate the so-called “Gauss Rifle” that consists of a sequence  of magnets and steel balls placed on a rail system.  Shortly after one ball is rolled forward at one end of the rail the ball at the other end shoots off with a large speed.  Optimize your design to produce the greatest speed of the final ball for a rifle length up to 1 meter.

#3 --  Parametric Resonance:  Below are diagrams of two compound oscillating objects that  exhibit parametric resonance.  Build one of these devices, and observe and explain the subsequent motion.  Explain the relationship, if any, with children pumping their legs on a playground swing.

#4 --  Teapot Effect, Windowsills, and Drip Kerfs: Recent research results claim to have settled the physics of the “teapot effect” where a fluid flows around an edge and then flows along the subsequent surface. Carpenters have defeated this effect for windowsills by cutting a groove, known as a “kerf”, underneath the edge of windowsills to prevent rain water from flowing back onto the building under the sill, and instead drop onto the ground.  Evaluate the recent research, experimentally apply the results to windowsill drip kerfs, and tell the carpenters where the kerfs should be created, and to what depth and width to protect buildings from the teapot effect.