I was wrong . . .

One of the fun concepts I have been teaching for years has to do with relative velocity. It is easy when one object is stationary and the other is moving. The relative velocity of one to the other is simple the speed of the moving object and its direction. However, it gets more interesting when both objects are moving. If they are both moving in the same direction, you simply subtract the velocity of the faster one from the slower one.

Then here comes my angst. For years I have done a demo where two students walk toward each other and clap hands in the middle as they meet. I then ask what is the relative velocity of the one student moving at 5 m/s to the other moving at 5 m/s. Most everyone answers 0. And then there is that special student who gets it and answers 10 m/s. They get their name on the superstar board for a few days.

And then I say if two cars have a head on collision with the same speed, it is as if one hits a brick wall at the sum of their speeds. Then this year at my new school the student with the top score on the last physics test raises her hand. "Mr. Banks, I remember a mythbusters episode where they did that with two cars and it was not the same as hitting a brick wall at the sum of their speeds." Ouch. Send it to me and I will look at it. Before I got home good she had already emailed it to me.

Here is the link if you would like to see it.

I called in two of my former students (one with a PhD and one almost) for help.

Here are their responses.

First Dr. Foster
(I need to give a brief introduction. He is my first former physics student to get a college major in physics. He invited me to the Air Force Academy for his graduation and I surprised him and went. He surprised me several years later. The doorbell rang one day and there was Michael's mother with a copy of his dissertation. On the fly leaf he thanked me for getting him started in physics and said it was still fun. One of the proudest moments of my teaching career.)

Ok. I've watched the video. Fun stuff.

If I were king for a day, I'd run an additional crash test: 2 moving objects -- 1 car traveling at 50 mph (like before), and one moving wall (50mph also) with extremely high shear strength (titanium?), thickness/volume selected to match the mass of the car while maintaining impact point commonality with prior tests.

What I'd like to test is their premise that "the energy of the crash is transferred to twice the mass, halving it, resulting in a crash that looks like just 1 car, into a wall at 50 mph." By minimizing the kinetic energy transfered to the moving wall (i.e. high shear strength), the impact of doubled KE would be seen in the car (pun intended). My only point is the subtleties of inelastic energy transfer are lost in the quote above.

I agree that the crash test results are initially counterintuitive; but at the end of the day, I think the tenets of relativity are still in tact.

And now for soon to be Dr. Sadjadi.
(In the eleventh grade he was bored with school. He came to my class and never looked back. As a senior, after doing a junior project on a cloud chamber, he told me he wanted to do it on the string theory. At the time I had no idea what he was talking about. I tried to talk him out of a theoretical paper since no one had won with that kind of project. He insisted and I wisely let him go. He won and his class stood and cheered like he had won the super bowl - later I got to hear him present his paper at the AAAS meeting in Boston. Another very proud moment.)

The summary is this: summing the velocities works perfectly. The collision experiment is confusing because both of the cars are destroyed. If you look at the system in terms of energy, there are two ways to frame the experiment: you can be standing on the ground, watching the cars race towards each other, or you can be in one of the cars.

The ground observer sees two objects of mass m moving with velocity v. The total kinetic energy is 2E, where E is the energy of a single car traveling at velocity v. When the cars hit, all of that energy is lost in the heat/sound/damage of the collision. Since the two cars are of similar construction, each car takes half the energy. So the damage is identical to a single car hitting a wall (energy dissipated is E for each car).

From the perspective of one of the drivers, the second car is moving at 2v, while he himself is at rest. So the total energy in this moving reference frame is 4E (energy goes as velocity squared). After the cars hit, the two vehicles come to rest in the ground frame, which is equivalent to the two cars moving backwards in the car's original moving reference frame. So the energy post-collision is 2E (twice the mass, moving at velocity v). This leaves another 2E to be dissipated in the collision itself. This agrees with the ground observer.

I hope that helps.

Thank you for great students past and present and the opportunity to be wrong so I can learn even more.

Are we going to blow something up?

Volunteers to clean up (not after an explosion)


That is the question every chemistry students wants to know.

We have not blown anything up yet but I did do a cool demo found on stumbleupon.com

The challenge is to pick up a nickel (without touching the matches) resting on a match box with a match's base leaning on it and the business end of the match resting on the business end of a match that is place vertically on top of the match box. If you do it right it looks like this:



If you want to just see it here it is:



here are a few shots along the way with our density column lab:







And our physical and chemical changes lab.








Separation of sulphur and iron filings by magneticism


Separation of sand and salt by filtration

New Action Hero Coming to a School Near You

It started with one of my students drew an emblem of a B within a shield for Super Mr. B. This student and I got off to a difficult start but now we are making progress. This Super B kind of reminds me of that transition and other transitions we make as educators in the lives of others.

Anyway at MC, Pep rallies are on steroids. Almost everyone dresses out in some form of orange and blue craziness.

The attire above is just a beginning. Eventually I will have an royal blue cape with an orange super b logo designed by my "star" students. And eventually a super hat, boots, leggings, etc.

One has to get up very early in the morning to outjag me.

Caring in the Classroom

I have to not include names to protect privacy but I had something happen yesterday in class that gives me a warm fuzzy feeling (none of the students described are pictured in this post). Trust me, that is a rare thing in teaching. My students in Biology were given a project that is new for me (I borrowed from a workshop I went to a couple of years ago at NABT in Denver). Each student is given a cutting from an African violet. They dip it in root hormone and then plant it in the pot they brought from home.

How its done

They created a bar graph on notebook paper. They are to record the growth every two weeks and to take a picture with ruler behind the leaf at the same interval.

If they keep these records along with some journal notes about its location amount of watering, conditions, etc and if there plant is still alive at the end of the nine weeks, they get a 100 test grade. Points are deducted at the rate of 10 per week if the records are not kept. If the plant is still living at the end of the second nine weeks and all records are kept, the student receives two 100s.

One of my students who always struggles a bit brought his pot happily (more or less) to class. As we were doing the ready,set,go (my version of do now or bellringer), he inadvertently knocked his clay pot on the floor, and it shattered into about eight pieces. Another student spontaneously got up gathered up all the pieces, took it to his table and asked if I had any glue so he could glue it back together. As we continued class and prepared for and planted the cuttings, he carefully glued the pot back together so it could receive a new plant. The fact that the students were of different social, academic and racial groups did not matter. What mattered was a pot was broken, and it needed repair. One student cared and it made all the difference.

First Annual Frog Races or the Jumping Frogs of Madison County

First, let me say, nothing is done small or halfway at the flagship school in the state.

My first annual frog race in my two physics classes was no exception. Students were to fold a card out of an index card with the handout provided (borrowed from gpb.org/chemistry-physics). Then each team was to measure the race track (I had put a piece of masking tape with s for start and another with f for finish) on each of seven lab tables. One class had 27 students! I published the actual measurements with significant figures on the web site (they used these numbers in their lab report to calculate per cent error). They were to measure and record the length of the track, the times for two time trials of each of the group member's frogs, and then to calculate the speed using the measured distance divided by the average of the two times.

I will let the videos speak for themselves. First, two that show the students working out their calculations.





First and Second Place winners in Third Block

First Place


Second Place

First, Second and Third Place Winners in Fourth Block


First


Second



Third

First Day of School Finally Arrived

Tickled Pink to be at MC

This shirt and tie were purchased four years ago (at a Magnet School Conference in Chattanooga at a very swanky Men's Shop) in anticipation of wearing on my first day of being a principal. I did not even take them out of the box until I finished my degree and passed my licensure exam. I never found the right calling to be a principal.

When I thought I had a physics job at Clinton, I decided to wear them to the interview. When I did not get that job, I thought I would never wear them again. I decided to break them in right at the Jungle. As is the tie, I am tickled pink to be here. Instead of a principal, I am a principle teacher maintaining the standard of excellence at The Madison Central.



This is my famous lab coat and my introduction. There is a longer version later.

There were two affirmations that I am in the right place at the right time. One came from one of my best ever students at Velma - when I told her the news about teaching at MC, she very perceptively said I am so glad you are in a place where you will be happy.
The other came in those anxious few moments before the students actually came to class for the first time. I was walking down the hall yesterday and saw a former student from Velma walking down the hall. This guy struggled in one of my classes my first year at Velma and he and his mother and I had many conferences. Eventually he hit his stride and passed the class with flying colors. He left Velma for better opportunities at MC (kind of like his teacher). When I asked how he was doing he said he was in one of my classes. How cool is that?

Finally I was able to get fresh cut roses from my garden to the two front office angels, the attendance gems, the tough love lieutenant general of the school and to her excellency (the paragon of principalship - I should know I have worked for eleven).

Stay tuned for great things to come, I am in my element.

Let me introduce myself