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13 February 2016

Block sliding down a frictionless incline -- two different articulations of energy conversion.

A block of mass m is released from rest from the top of a frictionless incline a vertical distance h from the table underneath.  The incline is fixed to the tabletop.  Obviously the block speeds up as it moves to the bottom of the incline.  

In AP Physics 1, students will be asked carefully about energy conversion.  Depending on the system we consider, what are the "external" and "internal" forces?  How should we articulate the energy conversion that allows the block to speed up?

1. On one hand, consider the block alone.  It has zero kinetic energy to start, and some KE to end.   Thus, some force must have done work on the block.  (Since the we are considering the block alone, all forces acting on the block must be "external.")

The incline can't do work on the block -- the force of the incline on the block is perpendicular to the block's motion, so does no work.  The only work done on the block is from the force of the earth on the block -- gravity.  The amount of that work is the block's weight mg times the displacement parallel to the force's direction, i.e. the vertical displacement of the block h.  That work mgh is converted to the block's kinetic energy.  

2. Consider the block-earth system.  Now the earth cannot do work on the block-earth system because the earth is part of the system.  But the block-earth interaction produces a potential energy mgh at the beginning.  No work is done by external forces -- the only force external to the system is the force of the incline on the block, and that's perpendicular to the block's motion, and so can do no work.  The system loses mgh of potential energy, which is converted to kinetic energy of the block. 

09 February 2016

Rubric for a conceptual physics question for grading by peers

The table 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, if any, would require the smallest applied force to keep it moving at constant speed?

I asked this of my freshmen last week, as we come to a close in our Newton's second law unit.  We've done a large number of this type of problem, explaining the multi-step logic involved in justifying the answer.  This is not easy for a physics student of any level -- rather than simply applying a single fact or equation, this seemingly simple question requires students to make a number of connections.

An answer I'd expect:

Moving at constant speed means zero acceleration and net force, so the applied force is equal to the friction force.  The friction force is Ff = μFn.  The normal force on each block is the same, because with no vertical speed change Fn = weight, and all weigh 100 N.  So by the equation the smallest Ff takes the smallest μ -- that's copper.

I had students grade each others' work on this question to a five point rubric:

5 points:
¨  1 point for use of the equation Ff = mFn
¨  1 point for recognizing that the normal force is the same for each
¨  1 point for explaining why the normal force is the same for each (i.e. vertical net force or acceleration is zero)
¨  1 point for correct arrows OR several false calculations showing that we want the smallest m

¨  1 point for the correct answer

(That fourth point is phrased in language my freshmen understand and have seen before... for other physics teachers I'd say "1 point for showing how the equation supports finding the smallest μ.")

Now, you might come up with a bobzillion other valid rubrics.  I didn't give an explicit point for pointing out that the applied force must equal the friction force, for example; on another day I might have.  Five points for a single question on a problem set is a lot.  My students are expected to do four to seven problems of this type each night; I don't expect a thesis, just a few sentences using facts and equations which hit the important logical progression.  By the time I or classmates have looked at every problem most nights, plus all the similar in-class work, each student gets the feedback he needs to improve his understanding.

That is, each student gets that feedback if he looks at his graded work and gives a crap why it's right or wrong.  That's the holy grail of high school teaching, of course... leading our horses to water AND making them drink.  We need to deal positively and appropriately with the significant but frustrating population who keep making the same mistake over and over again, never listen to or care about feedback, but tell the universe with hangdog eyes how impossible physics is.

So that's why I have students grade each others' work.  Not only do they see the statements I'm looking for to award credit, but they have to read carefully for themselves to see whether someone else's paper meets the appropriate criteria.

I didn't "go over" this problem set -- because I would have helped just the one of the fifteen students in the class who was paying attention while I talked.

I didn't just post the solution -- because I would have helped the 0.3 of the fifteen students in the class who would take the time to look at the posted solution.  (And even then, I would have helped only five percent of the students who looked at the solution, because only that five percent looked more carefully than at the answer.)

I made the students grade each others' work, and I made them responsible for grading correctly.  The questions I answered in class were about how to apply the rubric.  Occasionally I would get a content question; my answer in those cases was carefully followed.  When I required corrections to the problem set, the students did excellent work, much better than if I had graded the set and passed it back.

Making students grade is one of the best ways to foster collaboration and a team atmosphere.  They realize quickly that correct answers aren't a matter of my opinion, that points aren't awarded randomly or arbitrarily.  The teacher is not the opponent -- rather,the class is working together to understand this mysterious natural world.  And I'll use every trick in my repertoire to foster such a positive and useful attitude toward physics class.

01 February 2016

US Invitational Young Physicists Tournament -- results from 2016

Shenzhen Middle School, USIYPT Champions 2016
The eighth annual US Invitational Young Physicists Tournament was held last Friday and Saturday, January 29-30, at Randolph College in Lynchburg, VA.  Eleven teams from around the world competed; results are listed below.

Phoenixville Area High School of Pennsylvania was atop the standings after the six preliminary rounds.  In the final rounds, Shenzhen Middle School, of Shenzhen, China, earned the championship trophy.

Champions: Shenzhen Middle School (pictured)
Second Place: Rye Country Day School, New York
Finalists: The Harker School, California; Phoenixville Area High School

Clifford Swartz Poster Session Champions: Nanjing Foreign Language School, Nanjing, China

Poster Session: 
Pioneer School of Ariana, Tunisia
Woodberry Forest School, Virginia
Phillips Exeter Academy, New Hampshire
High School Affiliated to Renmin University, China
Princeton International School of Mathematics and Science, New Jersey
Nanjing Foreign Language School, Nanjing, China
Cary Academy, North Carolina

I know that I and the USAYPT board thanks Randolph College, and physics department chair Peter Sheldon, for their extraordinary efforts in hosting and supporting the tournament.  

For 2017, the USIYPT will be held January 28-29 at the University of the Sciences in Philadelphia, hosted jointly by the university and by Phoenixville Area High School.  The problems for 2017 are listed below.

If you'd like information about participating -- either as a juror, or as the leader of a team -- please contact me.  A "Young Physicists Tournament" involves not just the presentation of research, but extensive discussion and conversation between teams about that research.  Teams are judged not only on the quality of their own research, but on their ability to communicate their understanding through asking and answering questions.  You will love it.

USIYPT Problems 2017

Granular materials
Build an apparatus that performs the following procedure: a rectangular container is placed on a vibrating base. The container is split into two equal parts by a vertical wall that is shorter than the outer walls of the container. An equal number of beads are placed in both containers at a level slightly less than half the height of the middle wall. As the base oscillates up and down at a constant frequency the beads jump above the middle wall from side to side; eventually they will all be in one side of the container. Explain this phenomenon and estimate how long the process takes for your apparatus.

Investigate the motion of a projectile inside a blowpipe.  Determine the conditions for maximum exit velocity when blown by the mouth.

Support a long, vertical tube containing water.  Heat the tube directly from the bottom and you will observe that the water erupts.  Arrange for the water to drain back into the tube to allow repeated eruptions.  Investigate the parameters that determine the eruption frequency.

Planck’s constant
Use LEDs to measure Planck’s constant, and explain the theoretical basis for your experiment. Measure the wavelength of the LED light directly, without relying on the manufacturer's data.  Describe the precision of your experiment and discuss if your margin of error covers the currently accepted value of the constant. You must build the experiment yourself from standard electronic parts, without relying on a commercially available Planck's constant apparatus.

23 January 2016

What does a 5 on an AP Physics 1 exam mean? It still means an A, but read on...

Back in July, I posted about the results of the first ever AP Physics 1 exam.  Executive summary: The percentage of students earning each grade dropped.  But since the pool of students taking the exam nearly doubled, more students than ever "passed" the algebra-based AP physics exam.

The comment section of that post has been active and interesting.  One comment in particular deserves an extended response.  Aaron Shoolroy points out a seeming disconnect in the College Board's statements.

On one hand, Aaron saw that university physics professors -- the ones who are ultimately responsible for awarding credit for AP exams -- were "pleased at the depth of knowledge that the test assessed."  That's correct.  The College Board has done an excellent job communicating with physics departments, explaining why this exam is harder, showing them that students who do well on AP Physics 1 really, really know their stuff.

On the other hand, though, Aaron points out that the College Board has marketed the AP program as a college equivalent.  That is, they claim that a student who earns a 5 on the AP exam would have earned an A in the college course.  "Not a chance," to paraphrase Aaron, and others who have made similar comments.  "No professors are only giving 4% of their students As.  I know my students will be doing much better in college physics than they are doing on this exam.  What gives?"

What gives is a subtle shift in philosophy from the College Board.

For decades, the College Board has aimed their AP exams at the typical introductory university course.  They have done detailed statistical analysis to demonstrate that AP exams match their college equivalent in content, in skill evaluation, and in student performance.  That analysis included -- as a matter of College Board policy -- a vast cross-section of institutions of higher education, all the way from community college to state school to ivy.  All introductory college physics courses are different; previously, the AP exam was aimed dead-center.

That philosophy has changed with the redesigned science courses.  Now the College Board designs AP courses to be equivalent to the "best" college courses.  The increased emphasis on skill development over content means that AP exams evaluate skills taught with the "best practices" of science teaching.

When the cutoff scores for a 5, 4, 3, 2, 1 were set last year, the College Board sought input from high school teachers and college professors about what standard they would expect their own students to attain on each of the exam problems to earn each grade.  They used statistical information from pilot exams in some of these "best practices" courses to correlate scores to grades.  They did NOT start with "what percentage of students should earn 5s or As?"  

So Aaron, you're right -- most professors are not giving only 4% of their students As; and many of our students who earned 3s and 4s on AP Physics 1 will go on to earn As in college physics.  But neither of those facts are relevant to the score setting process.  The relevant question is, given this exam, what level of performance would earn a student an A in the best college courses in the country?  

I take no position as to whether the College Board's new philosophy is right, or good, or good for the general state of physics education, or good for your or my students.  I have some opinions, both positive and negative, that don't belong in this post.  Today I'm simply explaining the philosophy behind the AP score setting.

What I can say is that the College Board's new philosophy is internally consistent, and that the score cutoffs were derived fairly and openly based on the criteria developed by the redesign committees and the College Board's executives.

14 January 2016

Demo: How can something move down if I'm pulling it up?


The picture above is one frame of a video from class yesterday.  Hanging from the string is a PASCO cart with the "visual accelerometer" attachment.  Before this demonstration, I spent some time convincing the class that the lights on the cart correspond to the direction of acceleration.  Thus, everyone is well convinced in this screenshot that the cart has an upward acceleration.

We've used Vernier force probes before, too.  I showed the class that the cart weighs 7 N.   They can immediately see in the screenshot that the force probe reading, which is also the force of the string on the cart, is more like 11 N.  They're well convinced now, experimentally, that the upward force of the string on the cart is greater than the downward force of the earth on the cart.  They know that the net force must be 4 N, upward.  

The class is, unfortunately, also well convinced that the cart is moving upward.  Oops.

"BOUX," I've said for years.  The net force is not in the direction of motion -- the net force is in the direction of acceleration.  And we just spent a whole unit discussing how acceleration is in the opposite direction of motion when something slows down.  This cart could be moving down but slowing down.

For the first time, though, I can give immediate, undeniable experimental evidence.  I can just run the video forward for a few frames -- the class sees that the cart was, undeniably, moving downward.  Even though the acceleration and the net force was upward.  Cool.

Hints:  This was one of my early attempts at obtaining this kind of video.  I discovered I needed to use the "slo-mo" function on my iphone 6.  Try to lower (or raise) the cart such that the string never goes slack -- I didn't do that well in this trial.  

There is, in fact, a couple frames of lag between the appearance of the green dots and the force graph displaying greater than 7 N.  That's the computer processing and displaying the output from the probe, which is not instantaneous.  You can even see screen flicker in the video, since I'm recording at ~100 fps while the screen refreshes at 60 fps.  But the video is still convincing.

And finally, you can certainly get the students themselves to make this video.  Tell 'em to pull out their phones and record.  Chances are that at least one student gets a good recording.  Then have that student show a screenshot like the one above to classmates, explaining carefully the relevance of the green dots, the relevance of the force probe reading, and how the video shows the cart moving down, not up.

31 December 2015

Does AP Physics 2 include thermal expansion?

Joanne, a veteran of two of my summer institutes, writes in with this question.  Before responding, I did a search for "thermal" and "expansion" in the curriculum guide.

Nothing shows up for "expansion."   Lots of hits for "thermal".  

My understanding is that that everything about "heat" in physics 2 is about energy transfer via heating -- transfer by conduction, convection, radiation (looks like nothing but qualitative for the latter two, maybe some semi-quantitative for the first).  And then a bunch about energy transfer in gasses, with kinetic theory, the microscopic source of the ideal gas law, PV diagrams, etc.  

Be very, very sure that students can describe and explain what it means, at a microscopic level, for energy to be transferred via heating.  They've gotta be able to say more than a vague "the molecules move around more."  Manipulation of equations, and rote problem solving techniques with PV diagrams, will not be of any particular use on the AP Physics 2 exam.  The understanding has to be deep and thorough.

But no thermal expansion, as far as I can tell.


29 December 2015

Notes from observing an English class

As part of my school's faculty development program, we're asked to observe a teacher of our choice outside our department.  I asked to watch John Amos's English class.  I chose him because I knew him to be an outstanding, experienced, creative, intelligent teacher.  More to the point, he teaches 9th grade like I do, but his style and personality are very, very different from mine.  I thought it would be useful to see how another craftsman uses a different skill set to achieve the same general goals.

So many "Physics Educators," and "educators" in general, have the Soviet attitude that if only everyone did things my way, students would learn better.  I disagree.

I've always been open at my workshops and on this blog: my ideas, philosophies, and suggestions are mine alone, developed in the context of my personal strengths and weaknesses, and shaped by the ecosystems of the three schools at which I've taught.  What I do cannot work for everyone.  Yet, it's still worth sharing my thoughts, techniques, and ideas.  Not in the sense of "do these things and you will become a great physics teacher;" but rather, "here are a few ideas you may not have considered; try them, and then either throw them out or adjust them to make them your own."

So here's my extensive reaction to John's class.  I will not be adopting wholesale any of his particular techniques; but I appreciate the exposure to some different ways of approaching my craft.  Many of John's ideas are in the back of my brain now, ready to manifest -- consciously or subconsciously -- in my own classes.  In other words, I bought myself a few new tools.  Whether and how I use them is discourse for a future time.

What happened in the class?

John included three or four segments in a 45 minute class:

1. Discussion of vocabulary words
2. Discussion of previous night's chapter in Bradbury's The Martian Chronicles
3. Instruction about responding to passage identification questions
4. Practice responding to passage identification questions

This was a 9th grade general English class that included many of the same students I've worked with this year.

My reaction:
I always thought I hated kibbe.* I dreaded when my mom tried to make it. Around age 30, I realized that I liked kibbe just fine -- what I hated was my mom's cooking of the kibbe.**

*Kibbe is a Lebanese dish with bulgur wheat, ground meat, onions, and spices.

**Mom's laham mishwe (little bites of spiced lamb and onions) is wonderful. Don't tell her I said anything about the kibbe.

Similarly, my personal experience with the classroom study of writing and literature has been universally negative. I've always been aware that it's been the poor teaching and poor classroom atmosphere that turned me off to English, not the discipline of English itself. But John's class brought the source of my negative reaction to the forefront.  Last week, I watched a master at work.

I've advocated to physics teachers that we give a short quiz at the beginning of every class. The purpose is as much to settle the class down as to use the quiz as a device for review. John likewise recognizes the necessity for a start-of-class routine, but does it differently. He throws vocabulary words, the ones that will be on the upcoming test, up on the screen. As soon as the first boy arrives, he begins a relaxed, informal discussion about the words.  Thus, even the boy who is second to arrive feels he's joining class in media res, and so gets his materials settled and ready for business right away. John's vocabulary discussion -- which is interesting and captivating anyway --  serves the same teaching purpose as my quiz, but is better suited to John's personality than mine.

It almost goes without saying that we moved through four activities before any of the four had a chance to get old or stale. This class could easily have gone on for 90 minutes. Like the best entertainers, John left the class wanting more -- better 5 minutes too short than 1 minute too long.

Now, part of what made the class great was the enthusiastic and substantive participation of the students. Most, including some I know not to be A+ students, jumped in with excellent and interesting things to say, listening to each other and advancing the conversation. It helped that we weren't reading Jane Eyre, we were reading about Martians.  Most of the class was invested in the book, and in the class discussion; those few who weren't sat quietly, listening, without causing distraction. Those who participated did so authentically, never playing a game of one-upsmanship, never ignoring a classmate's comment.

I'm well aware that John's done considerable behind-the-scenes work over the first half of the year to set up the class I saw. At some point he's had to assert himself as alpha dog. For example:

One student -- he's from Vietnam, and in my AP physics class -- put forth some ideas which were initially confusing to the class, and even to me and John. The confusion came from many sources -- his language barrier and accent made it tough to follow him. This student's general intellectual level is well beyond that of most of the class, so his thoughts were more complex than we had yet considered. The class discussion was about linguistic metaphors for time, which necessitates some common cultural and idiomatic ground which isn't necessarily shared between Dixie and Southeast Asia.

I couldn't be more impressed with the class's reaction. I learned from experience in my own English classes: if I have an interesting but different take on a subject, keep my dang mouth shut, because if I don't explain it perfectly clearly right away such that everyone agrees with me, I'll have to deal with withering scorn from my classmates. Only occasionally was said scorn verbalized ("Oh, my gawd, the book's not that deep. Okay, I get it, you're smarter than we are.") Usually the negative response was manifested in body language, subtle dismissive gestures that ostracized me. My teachers either didn't notice, didn't care, or cared but didn't know how to take action.

In John's class, though, this student's classmates tried valiantly to get his point. No one made any rude snorts or eye rolls. Even those who were generally disengaged simply remained disengaged; they did not take the opportunity to get a nonverbal jab in at the smart nerdy kid.

How did John do it? How did he establish and maintain this atmosphere of genuine intellectual curiosity among 9th grade boys?

I mean, I do it... it takes every trick and tool I've ever learned, but I do it. I pounce on any student who makes a dismissive gesture, hollering loud enough for the sewage plant down the hill to hear me. I give out candy to the first student to give me a confident yet wrong response. I set up collaborative situations in which students must work with randomized class members. I have students grade each others' work so that right and wrong answers are transparent -- it's hard to make fun of someone when you know your own wrong answers will be out there for someone else to see.

But my strengths in setting tone -- my loudness, my subject's black-and-white nature -- are not in John's toolbox. He's softspoken, teaching a subject in which shades of gray are mandatory. So how does he do it?!?

(John did share one thing had done -- a different student, John says, had a difficult attitude for a while. John realized that this other student needs to be front and center, always with something to do or say; then he can be a very positive contributor. So, when John had the class read a passage out loud, he carefully appointed this student to read a major part. That kept him involved and invested, and less likely to turn to the Dark Side.)

I know there's more to say here... I wish I could have come to class the next day, when he was planning to give specific feedback to students' writing in response to reading passage identification questions.  But this should give you an idea of what I saw, what I thought about, as I observed this class to which I wish I could transport my 14 year old self.  No, I wouldn't have become an English major, but that's not the point.  :-)

23 December 2015

Momentum bar charts: worked-out examples

A large circular disk is initially stationary on a horizontal icy surface.  A person stands on the edge of the disk.  Without slipping on the disk, the person throws a large stone horizontally at initial speed vo relative to the ground from a height h above the ice in a radial direction, as shown in the figures above.  Consider the x-direction to be horizontal, and the y-direction to be vertical.  Consider the system consisting of the person, ball, and disk.  “Initial” refers to before the ball is thrown; “final” refers to the instant before the ball hits the ground.

The picture and some of the description is from an old AP Physics C exam question, which asked for detailed calculations of various quantities in terms of given variables and fundamental constants.  In AP Physics 1, there's no need to do the calculations;however, it's critical that we teach how to set up those calculations, or at least how to explain what is conserved and why.

I pose this and eleven other interesting situations in my energy and momentum bar chart exercises.  These are comprehensive, end-of-course activities that will challenge most physics teachers.  There's no algebra, none at all; just the requirement for careful understanding of the meaning of a "system", and then of an external force acting on that system.    

Can you make a qualitative impulse-momentum bar chart for the x-direction?

Of course you can.

Initially, nothing moves; so nothing has momentum.  No impulse acts on the person-ball-disk system.

What about the impulse due to the force of the person on the ball?

Since both the person and the ball are part of the system, the force of the person on the ball (and its Newton's 3rd law companion) are internal to the system.  The impulse column requires impulse applied by the net force external to the system -- only the net external force can change the momentum of a system.  In this case, there are no external forces in the horizontal direction.

The point of the bar chart is that it shows by inspection how the total system momentum is distributed: the bars on the left side plus the bars in the middle equal the bars on the left.  In this case, there must be zero total momentum after the throw.  How is that accomplished?  The ball moves right, so has what I'm calling positive momentum.  To maintain zero total momentum, the person and disk move left, giving them negative momentum.  The person and disk move together, giving them the same speed -- not the same momentum.  Since the disk is more massive than the person, the disk has a larger share of the system's negative momentum.  Note that the bars representing the person and disk add to about the same size as the bar representing the ball, showing that the total momentum remains zero.

How about the y-direction?

Sure, though it's a bit trickier.

Again, initially no movement or momentum.  Think for a moment: what causes the impulse in this case?

It's NOT the person pushing the ball.  That's a force (and thus an impulse) in the horizontal direction only.  And, that's internal to the system, anyway.

In the vertical direction, two external forces act on the system: the normal force of the ground on the system, and the force of the earth on the system.  The force of the earth is equal to the weight of the entire system; the normal force here is equal to just the weight of the person-disk part of the system (because the normal force is a contact force, and the ground is only in contact with the person-disk part of the system; the ball is in free fall).  So the net external force is equal to the weight of the ball.  That causes a downward impulse, represented by the bar in the chart above under the J.

Now inspect the bar chart: zero bars initially plus the impulse bar must equal the bars of total momentum when the ball is about to hit the ground.  The person and disk still don't move vertically, so they have zero momentum.  The ball must have a momentum equal to the impulse provided by the earth on it; that's represented in the chart by the ball's bar having the same size as the impulse bar.

That's enough for today.  But you can answer many, many more questions involving this situation.

What about an energy bar chart?  (Energy is a scalar, so you don't have x- and y- direction charts for energy.)  What if the earth is part of the system?  What if the system is JUST the person and disk?

See, the situation is rich, rich, rich with subtle questions.  Have fun with these.  Post thoughts in the comments.  Assign them to your students, and post the common misconceptions.  Go nuts...


14 December 2015

Cart on an incline: what qualifies as an "external force?"

When teaching about energy for AP Physics 1, one of the trickiest bits is defining an appropriate system, and then applying the work-energy theorem correctly to that system.  The question:

Hi Greg. From my understanding, an external force for a cart going down a [frictionless] incline would be the normal force acting on the cart. 

The weight is an internal or conservative force, so none of the external forces on a frictionless incline do work? I still consider Fg parallel to be an internal force for the system. Is this a correct assumption?

Not sure... gotta define your system first.

If your system is just the cart, then two external forces act: the weight (i.e. force of the earth on the cart), and the normal force.  Both are "external" forces because the forces are applied by objects that are not part of the defined system.  The normal force is perpendicular to displacement, so does no work.  The weight does work, because mg is parallel to the vertical component of displacement. This work is mgh, where h is the vertical component of displacement.  The cart acquires kinetic energy by the work-energy theorem -- the work done by the earth is equal to the cart's change in kinetic energy.

However -- if your system is the earth and cart together, then the only external force is the normal force, which does no work because it's perpendicular to displacement.  The work done by the earth on the cart is internal to the system, and conservative; so the system potential energy (equal to mgh) changes.  The system acquires kinetic energy by reducing potential energy, without any work done by external forces to change the total mechanical energy.

11 December 2015

Starting a Physics Lab From Scratch -- What Equipment Do You Buy?

In their December 2015 issue, the journal The Physics Teacher attempted to answer an important question that new teachers -- and teachers new to a school -- regularly ask:  "I don't have any equipment at all.  What do I need to order?"

Problem is, TPT asked the question of a university lab manager, who had ideas as far removed from a typical high school teaching situation as the troposphere is from the mantle.  No, sorry, you should NOT order $1400 AC power supplies, infrared cameras, or cloud chambers as your first purchases, unless, say, your top priority in setting up a banking office from scratch would be purchasing lie-flat seats for the executive jet.

No, folks, you want fundamental equipment to start your high school lab, equipment that is simple to use, durable, and (where possible) multi-use.  You want equipment that allows you to do demonstrations and laboratory activities in line with the first-year physics curriculum you cover.  

Here's my rough list of equipment, with caveats below.  You may think of other things; great.  Post a comment.  But be aware of my goal, here -- I'm not trying to be truly comprehensive in this list, and I'm not listing equipment for everyone's pet experiment.  

Rather, I'm answering the question: What would I buy for a high school's introductory physics program, given a one-time, not that big, start-up budget?

Enough for multiple lab groups:
PASCO carts and tracks with pulleys
PASCO hanging mass sets
Vernier Labquests, with motion sensors
Ohaus spring scales (just the 2.5 N and 5 N sizes)
Cheap breadboards, digital multimeters, resistors, and connecting wires
Lenses / curved mirrors
Batteries, miniature light bulbs with holders

Demonstration equipment*
Variable DC power supply, up to 20 V*
"Decade box" variable resistor
PASCO fan cart
Happy/sad balls
Vernier force probe*
Vernier force plate
Vernier light sensor*
Laser/fish tank
PASCO projectile launcher
PASCO string wave generator

*Where marked with an asterisk, it's worth getting enough for multiple lab groups if you have the money; otherwise, just get one unit for use in demonstrations.

Things not to get
Stopwatches (phones and watches will perform this function)
air tracks (PASCO tracks work better for 1/4 the price and 1/1000 the noise)

I'm assuming basics like metersticks, rulers, protractors, ringstands, string, computer printer with projector, copy machine, white or chalk board, desks, etc.  

I'm also not including things that can be found around the school, or jury-rigged for cheap: like using PVC pipe for waves or rotation demonstrations/labs; clear rectangular plastic containers filled with water instead of commercial plastic blocks for refraction labs; tennis balls and marbles; etc.

And finally, I'm assuming topic coverage approximately equal to the AP Physics 1 exam, regents exam, or my conceptual physics exam.  Obviously if you're not teaching lenses and mirrors, don't buy them; if you are teaching magnetism in your first year course, you might include other materials (like magnets, perhaps).

I'm sure I've left out some things.  Post a note in the comments.  Perhaps I'll edit based on your suggestion; regardless, readers of this post would benefit from other folks' different perspectives.