Why don't you have a New York Public Library card?

Why don't you have a New York Public Library card?

 Ebooks are becoming a feature in the classroom. And libraries are a good source. While my local public library system has made it easy to get ebooks (and to search for dead-tree books and to have them sent to my nearby branch), it is somewhat limited. At a recent meeting of the Western New York Physics Teachers Alliance, I learned that I have another resource.

Do you live in New York State?

If you live in NYS, you are eligible to get a New York Public Library card. While you may not get to visit an actual branch, you can download from their large ebook collection. The shot below is the beginning from a search for "quantum". Many books are on the graduate student level, so the selection is deep.

The actual process of getting the card will take a few weeks.  You first request one on their website and wait a couple of weeks until it arrives in the mail. You then need to send in a picture that proves you reside in NYS (unless you want to drive to Gotham and visit one of their branches). Wait a few days and when you get an email confirming your acceptance, you are set to go. It is free, so why are you waiting?

If you are an iPad user, the Overdrive Media app allows you to access the NYPL and read books on that device.

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.