Modeling Unit 8

Unit VIII: Central Force Particle Model

Jon started today by giving a brief demo.  He had a rubber stopper tied with a string attached to a hanging mass.  In between the two was a plastic tube (think very sturdy straw), which he held in his hand.  He asked us where he would need to release the ball in order to hit a certain object.  He then asked where he would need to release it to hit a different object, in a different part of the room.  He then socratically questioned us to say that the speed of the rotating stopper was constant, but the velocity was continuously changing.
He then asked us, what causes a change in velocity? (answer: unbalanced force)
What direction must the force be? (answer: towards the center*)
What direction is the acceleration? (answer: towards the center)
*Chris showed us a demo we can do if the class doesn’t agree that the force/acceleration is towards the center.  He grabbed the bowling ball (yes, the bowling ball, again) and a broom, and asked one of the students to make the ball move in a circle.  She first started out inside the circle of the ball and was constantly pulling the ball towards herself.  Chris then had her stand outside the circle and put a cup as a reference point for the center of the circle.  She again had to constantly push the ball towards the cup.
From there, Jon led us to derive an equation for the average speed of an object in circular motion:
\large \overline {v} = \frac {\Delta x}{\Delta t} = \frac {2 \pi r}{\Delta t}
From there, we walked through the tradition questions for paradigm labs:
What do you notice?
What can you measure?
What can you manipulate?
Then Jon helped us to create the purpose:
To determine the graphical and mathematical relationships that exist between the speed of the stopper and the amount of mass hanging on the string.
{We did not study the mass rotating, however you could have part of the class investigating this, and the radius if you want to “kick it up a notch” – Bam!}
We found that this lab was very tricky and had lots of error.  A couple points to minimize the error:
  • Have the lab members keep one job: timer, recorder, twirler
  • Make marks on the string to help see where it needs to be to keep a constant radius
    • This is huge!
  • Possibly use a force sensor held against the table.
  • Possibly use video analysis to determine the actual radius
    • as the ball drops, the length of the string is no longer the true radius
    • cut a slit in a tennis ball and squeeze over stopper to make a more visible point.
We also discussed what to do if a group has “bad” results.  We agreed that early in the year, make sure you are doing a thorough job of checking the groups while they are experimenting to avoid this.  However, as the groups get comfortable with the whiteboarding process, letting mistakes slide into the meeting can make it more interesting.  Think through when you want to call on those groups.  We also agreed that we need to remind students that the data measured isn’t wrong, the procedure to keep multiple variables may have been insufficient, but the data is the data.  Encourage the students to discuss the subtleties of their procedures to determine where groups differed.  If the class is getting bogged down, don’t be afraid to say, “Let’s come back to this after all the groups have presented.”
If the students didn’t already, have them create graphs of F_{hanger} vs $v^2$ instead of m_{hanger} vs v^2.  When they do so, ask what the slope represents.  If they aren’t sure, ask what the units of the slope are (kg/m).  Since the slope is constant, what mass and distance are staying constant?  To which, they should reply the mass of the stopper and the radius of the circle.  From there you should be able to derive the centripetal force equation:
\large F_c = \frac {m v^2}{r}
When we came back from lunch, Jon again attempted to shoot his ping pong launcher.  See the results in this blog post.
After that, we began work on Unit VIII worksheet 1 & worksheet 2
A couple great ideas from one of our cohort to help students “see” circular motion:
  • Cut a wedge out of a disposable pie pan, then roll the ball roulette style
    • ball will come out in a straight line
  • Have student’s run down multiple flights of stairs as fast as they can
    • may need to make this a “mental” experiment not an actual one.
    • ask students what they must do to turn from one flight to the next while at a landing
Before the workshop started today, I saw a very cool video on youtube that I’ll show, just because I thought it needs to be seen:

We started to day whiteboarding our summaries of Arons’ Chapter 5.  For those that haven’t read it, it’s a fantastic book with sharp insight into the shortcomings of teaching physics.  It’s written at a very high level, but once you get used to it, it has a lot to tell you about how you should be teaching physics.

From there we finished up Unit VIII
What worked?

  • We liked the demo with making the bowling ball move in a circle
    • Especially the person outside the circle
  • Getting insight into what to do (and what not to do) during a lab
    • once members determine the job they can do, stick with it
    • POGIL
  • Student discussions help them get understanding as to what lab was showing
  • The idea that data isn’t wrong, the method of isolating variables may not be sufficient
  • The fact that we (the students) are always finding the graphical and mathematical relationships
    • once you get the hang of it, you know what to do when the models get more difficult
    • new lab, same analysis

What didn’t work?

  • Teacher notes require editing/more detail on graphs
    • Centripetal force lab


  • Even though we knew what the outcome should be, struggling through labs is very helpful
  • For labs that fail (class completely lost), come back as a teach demo and explain how you are doing the experiment differently
    • demo vs lab less time if you don’t have it (due to lost period of failed lab)
  • If you have problem students or limited supplies, split the class and have half do the lab and the other half work on problems & switch part way through.
  • Use record player and put a thin piece of wood (less than 1×4) across the deck, have students measure coefficient of sliding friction \mu_{k}, and predict what is the greatest radius to place the penny such that it won’t slip. (Find \mu_{k} from maximum angle with no slip).
    • vernier has a lab for accelerometer and turntable
      • difficult to due with calculators, not too bad with computers
  • Would be nice to see a paradigm lab for universal gravity
  • One member mentioned that this graphical analysis is very important as the next generation standards will implement a lot more graphical analysis.

I would love to hear your thoughts, please leave a reply

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