The RaspberryPi is one of those things that captures the imagination of some people. While many people are using it for useful purposes, I have decided to use mine for nefarious ones.

I have set up a Pi on our school's network, but in the office of the faculty member who has the responsibility for training the rest of us in technology use. I will sneak into his office early Friday morning and attach some speakers to the Pi.

Since I can access this computer from my laptop, I will start to play some songs from the Bob Rivers site at what I hope will be a semi-inappropriate time.

The Roller Coaster and the "Natural Path"

A classic example for HS physics teachers when discussing circular motion is the roller coaster loop. And the classic difficulty is getting students to understand the role the track plays in exerting force on the roller coaster car. To help with that, I have developed the idea of the "natural path".

After the basic discussions of circular motion and the emphasis that the force needed for circular motion  is not a real force, but the result of other real forces (I seldom call it centripetal force), we go to the roller coaster. However, I start with a typical loop cut in half vertically. We discuss what path a car would take when it came to the top of the loop and flew out into space. We get the typical projectile path. We draw one for a car moving at high speed and then another for one moving at low speed. I call those the "natural paths" that the car wants to follow.

I then draw in the rest of the coaster loop. We look at the high speed case and note that the natural path lies outside the loop. What prevents the car from following its natural path? The track. It must exert a force downward on the car to make it move in a circle.

We then look at the low speed case and see that the track must exert an upward force to keep it moving in a circle. We discuss what a rider would feel for each of these cases, with my in-class examples all starting with a roller coaster engineer specifying the force she wants a typical 75 kg rider to feel and then calculating how fast the car has to be moving to satisfy that requirement.

So, what will you feel?

Now comes the task of getting students to understand what the roller coaster passengers will feel. It starts with what they are feeling now. We talk about the forces they feel as they are sitting on the classroom stools. We decide that "normal" is when they feel the seat pushing up against them with a force equal to their weight. It usually takes some more discussion to get them to realize that they would feel "normal" when something is pushing against their butts, whether is it up or down. So, when the track (and thus the seat) is pushing with a force against your butt, you feel attached to that seat; you don't feel like you are falling. If that seat no longer pushes against your butt or is even pulling your butt down, you no longer feel attached to that seat; you feel like you are falling.

For advanced students

If you have students with a good calculus background, you can have them compare the curvature of the natural path to the curvature of the loop. And they can then calculate the force exerted by the track from the differential equation F=dp/dt.

 

A Use for those Old Whiteboard Marker Caps

Shoot for your grade

Many physics teachers have a variation on the "Shoot for your grade" lab. It is the lab where students predict where a marble rolling off a table will land on the floor. My variation is where the students get the speed from using photogates, essentially the Vernier-developed version, but with their target an egg. As expected, my AP guys perform better than my regular classes. However, I hope one year to have a splattered egg for each team, both regular and AP.

This year, when lab time came around for my AP class, we were in the middle of the collision section and had just derived the equations for an elastic collision with one stationary object. I came up with the idea of using one marble being hit by another as the projectile. Obviously, the case of a marble being hit by one of the same mass is trivial. Fortunately, thanks to my late father, I have a few ball bearings that are about 5 times the mass of a typical glass marble. So I have the students roll the steel marble down the ramp which will then hit the glass marble. Students first measure the speed of the steel ball at the bottom ramp using photogates and then calculate the speed of the glass marble using the analysis of an elastic collision. Those speeds are then used to determine the projectile distances of both the glass marble and the steel ball. The problem is that the balls are of slightly different radii. For a good collision, you want them to collide center to center (more precisely, you want them to collide so that the radial planes at the point of contact are co-linear). Here is where the caps come in. 

If you look carefully, you see the glass marble on the marker cap with the steel ball on the ramp. It is not a perfect fit, so the students have to shim the ramp to make the contact spot perfect.

Does it work?

As an experienced physics teacher, I know that a lab that looks great on paper can fail spectacularly in the hands of typical students.  I can report that with my AP class, all got landing spots within the max-min predicted spots with one team getting very close to the spots predicted on the average speeds.

      

A replacement for the Lance question

What is Your Power?

In a previous post, I had lamented that I would be unable to use a standard question I like on a test. I have now found a suitable replacement.

At this time of the year, I have my students get a measure of their personal power output by running up some stairs.  While everyone gets a different result, most of my guys get around 0.5 - 1 horsepower for the 3-5 seconds of effort. On the next test, I would ask students to calculate the power output of Lance Armstrong based upon his time cycling up Alp d'Huez, one of the most famous in the Tour de France. We would then compare his 0.5 hp to the student's 1 hp, noting that his effort was over 38 minutes, whereas the student's was over a few seconds. However, with recent revelations, I cannot with good conscience ask that question again.

In my search for a replacement, I had thought about changing to a question involving a cyclist in the Mount Washington Auto Road Bicycle Hill Climb, but since many cyclists have been tainted by the doping scandal, I thought better. I then remembered the Empire State Building Run-Up. With a few minutes on a search engine, I was able to get some good data. The last winner, the 71 kg German Thomas Dold, ran the 1050 feet of stairs in 10 minutes and 28 seconds (I was unable to find weight data for the 9 minute and 33 second recold holder, Paul Crake). This will now be the basis for my new question.

Why I still use the Imperial System

American Students Don't Think Metric

As physics teachers, we know that measuring and calculating with kilograms, meters, and seconds, is better than doing so with pounds, feet, and minutes. However, our country is still an Imperial one. Students have an intuitive idea that 60 mph is fast, but have no idea that 30 m/s is a little faster. For this reason, I still use Imperial units along with metric ones in my class.

There are some complications. Some students mix the systems together; others want to always convert from one system to the other. These are hassles in any class, but part of the learning process.

How much horsepower?

Over the Thanksgiving break, I have my students do the classic problem of calculating the average horsepower generated by a car's engine from the 0 to 60 mph time and the curb weight. We have covered work, power, and energy in class, and sometimes have discussed a homework problem where the power output (in watts) of a Porsche is calculated. Since the published data for most cars sold in the US are already in Imperial units, I have my students keep those measurements in feet, pounds, and seconds. 

The day before the break, I remind the guys that, while I have not given them a one-line formula to do this calculation, they do have the necessary tools to do a successful analysis. But, the first time or two they try, they will make mistakes. This is normal, part of the learning process. If they get an unrealistic result, they have made a mistake. Set things aside for a couple of hours or overnight, but keep thinking about it in the back of their minds. Come back to the question later and try again. I want them to make mistakes (the typical ones are using 60 mph not 88 ft/s and using weight in place of mass). About this time of year, most of my students start to understand the place of making mistakes in scientific analysis.

Letting Students Discover Torque

Letting Students Discover Torque

After a couple of labs where the data was messy and students were unsure of how good their experimental technique was, it was time for a lab where things are more straight-forward and students could regain some confidence. I scheduled my "beam equilibrium lab". The set-up is simple. Take two force meters and lay a meter stick on top. A mass is then placed on the meter stick at various places and the forces are recorded. 

I usually do this lab after having introduced the concept of torque and that a static situation means that the sum of the forces is zero and the sum of the torques is zero. Students make calculations based on the static conditions and confirm them with this experiment. This year, I changed things. Torque was not mentioned beforehand.

 This year, I started by having my guys place the meter stick on the force meters at 20 and 80 cm, and then place a mass at 30, 40, 50, 60, and 70 cm. We then look for any patterns. Most see right away that the 30-70 readings and the 40-60 readings are reversed. Some will see that the close the mass is to a meter, the higher the recorded force, so it looks like how far the mass is from the support point is important. We then set the meters at 10 cm and 90 cm and place the mass at various places in between. Again I ask if they see any patterns. Some teams do, but I give the hint of pairing the force and distance measurements. After a few minutes, I can see "the light-bulb moment" for most. They are then to make predictions about what they would see when the meters are placed at 25 and 75 cm. Success! The class wraps up with a discussion of the concept of torque and that they discovered that for a system to be in static equilibrium, the sum of the torques has to be zero as well as the sum of the forces. 

Sometimes, letting your students find things out themselves is easy to do.

A Wow feature from Vernier

Sharing Made Easy

Sometimes you have a demo that is tricky to set up or that requires equipment such that you can only make one setup. In the past, you would have a couple of students run it in front of the class and then share the data with the others. If you are fortunate, you could show the collected data on a big screen in front of the class where they could copy it as it comes in. But, if you want your students to do data analysis in a spreadsheet application, they would have to manually enter such. Vernier has a solution.

DataShare

The newest upgrade for the Vernier LabPro software has added a new feature called DataShare (it is also on their newest data acquisition system LabQuest2). DataShare allows any device running a web browser connected to the same network to view a live graph of the data and to download the data.

Once you have downloaded the upgrade and installed the Bonjour software that comes with (the Bonjour software interacts with your computer network, so you will want to speak to your school's network administrator before you activate the DataShare feature), you need to start DataShare.

The software searches your network and assigns an appropriate IP address that you will share with your students. Naturally, you will need to enable Data Sharing, and I suggest you disable the "remote control of data collection" unless you want your class wise-ass "helping" you.

 Once you have things running, a student just has to type your IP address into any web browser to see this.

 They can also change how they view the data.

On a computer, students can download the data for more advanced analysis.


Students can also view things with Vernier's iPad app, Graphical Analysis ($3).

 One advantage that this app has over web browsers on the iPad is that students can download the data for later analysis. I have been unable to download the data with web browser on the iPad (no problem with a real computer), a bug that I hope will be fixed in the future. (Update: Vernier is aware of the problem and is working on a fix.)

(Update 21 Sept; it appears that with Apple's update of their operating system iOS7, the bug has been squashed. I am now able to download data using the Safari browser on my iPad and then open that data in the spreadsheet app I have installed. I can now have my guys do some more advanced data analysis that they are unable to do on the LQ2 or on the Graphical Analysis app).

I plan on using this new feature many times in my class room in the future. Thank you Vernier.

You can do the same with the LabQuest2

You can also do the same with the more portable Vernier LabQuest2 DAQ, but it will cost you $350. However, if you are equipping a new lab, I would suggest getting the LabQuest2 instead of the LoggerPro system. Since my students will all have iPads this year, I will be starting to use these.  This year I will use my AP Reading stipend to purchase a handful of them (unfortunately not enough for a full classroom set).

But my school's network is unavailable

Some schools have a very restrictive computer policy. I know of one school system where the computer hard drives are wiped clean every night and the standard applications are re-installed for the next day. Asking for anything special requires a long paper trail and many days. If this sounds anything like your school, create your own wireless network. All you need is a cheap wireless router. Then connect your LB2's and your other devices to that network.

Since your wireless access point doesn't have to be powerful to get to the other side of your classroom and you don't really need encryption, you can use an old or cheap router. Maybe someone at your school who has upgraded has one you can use. Or you can find on sale for $10-$20. It is not that hard to set up your own private wireless network. Just give it a good name. Mine is Charon (one student each year gets the reference).