Day 92: Circular Force Diagrams and Motion Pillars take 2

Advanced Physics:

Today we discussed the rotational kinematics assessment the kids took on Friday.  It continues to amaze me that even after a full semester with me, most of my students still reach for the equations….Here is  problem I gave them on the assessment

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It is REALLY easy to solve with an angular velocity-time graph, but tougher and longer with the equations… sigh.

We also looked at sample tangential velocity problem I asked the students work on over the weekend.

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That’s a picture of some wind turbines all my students have seen many, many times.   I asked them to determine the speed in miles/hour for the tip of the blade.  I provided them with the pdf of the design specs. The blade length is 134 ft and the operational rate is 14.4 rev/min….. the answer surprises most  138mi/hr.


After that we developed the concept of radial (or centripetal) acceleration.  I prefer radial because they are less likely to confuse it with centrifugal AND we hit a vertical circle we can discuss a tangential acceleration more easily.


General Physics:

We WB’ed the same set of motion pillars we used with the Advanced class.  I had 3 groups CYOP (create your own problem). One I used as an exit formative assessment, the other twoI will use as additional formative assessments.  We also just briefly started discussing how to draw force diagrams.  More on this tomorrow.

Days 87-91: Still Trying to Catch up

These days cover the first week of our second semester.  I’m fortunate because I really do not have any new students, because I had them during the first semester.  So here was the week in review:

Advanced Physics:

Monday — I allowed the students to see the problem set from the final exam.  Yep, that’s right, they saw the final exam after it was graded.  I feel assessments should be a learning opportunity and the final should be no different, the students SHOULD get to see the actual exam rather than a number on Infinite Campus.  We also had a brief introduction to our next unit, uniform circular motion.  We discussed the difference between rotating  and revolving.

Tuesday — We continued the discussion from the day before and built the concepts of  ‘angular motion key players’.  When we first started linear motion we talked about the key players; position, time interval, displacement, velocity and acceleration.  We developed all aspects (the conceptual, the graphical and the equations) essentially at the same time.

SIDEBAR: To review the concept of a radian I used a sweet little animated gif I found (possibly on Wikipedia?), here it is.

To accomplish this, I used a ‘mini merry-go-round of physics’ with a dry erase marker on it.  I also used a Vernier Photo gate and the Strobe setting.  I added a few calculated columns so I could display the angular position, and velocity graphs. Here is the set up:

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Next time I use this, I am going to use a thin circular piece of wood to make a bigger platform and attach a series of evenly spaced (thin) metal strips to block the gate.  The values displayed will not be the actual angular velocity, but the graphs will be much cleaner.

The angular motion (kinematic) equations were built by the students using the displayed angular motion graphs and the linear motion equations.

We also had the pre-lab discussion for the first experiment, described next.

Wednesday and Thursday: The Tangential Velocity Experiment (shared a bunch of years ago by a former member of the Phox Share Group)

I replaced the marker (that represented a single point on the rotating platform) with my Einstein action figure.  The pre-lab discussion included Albert’s angular velocity and his instantaneous velocity— a linear quantity that is always tangent to the circular path.  We defined this as tangential velocity.  We talked about what might affect the tangential velocity.  Two factors surfaced: angular velocity and the radius of the circular path. So, two parts to the experiment:  Tangential Velocity as a function of Angular Velocity (with a constant radius) and Tangential Velocity as a function of radius (with a constant angular velocity).  Now, you and I know that we would not need to do both parts of the experiment, but the students do not.

Here is a picture of the experimental set-up:

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We use two different photo gate files: Strobe (for angular velocities) and One Gate Timing (for tangential velocity). It’s hard to see, but there is a thin metal flag attached to the meter stick with a binder clip.  This is used to trip the gate.

I only have six  really well-functioning ‘mmgrop’ (mini merry go round of physics), so we broke the class in half.  On Wednesday, half gather data for the first part of the experiment while the other half did small group work on some rotational kinematic problems (solving both graphically and with the equations).  Thursday, we simply flip-flopped the groups.  The second data gathering group gathered data on the second part of the experiment.

Friday:  We started the period with a short one problem assessment on solving rotational kinematic problems, then WB’ed the results of the experiment.  I think most of you see that the slope of the linear tangential velocity as a function of the angular velocity graph has units that simplify to meters and represents the constant radius, while the linear tangential velocity as a function of the radius graph slope units simplify to the angular velocity.


General Physics:

With the start of the second semester, we begin dynamics with the general students.  We follow the same basic flow as we did with the Advanced classes.

Monday — The students also were allowed to look through their problem set from the final exam. After this, we gave them the a ‘Forces Diagnostic’ pre-test.  In both of my classes, the average score was about 10 (out of 30).

Tuesday — We discussed forces in a broad sense and then completed the Forces ILD to develop the concept of N3L.

Wednesday — We finished the N3L discussion and practiced identifying N3L pairs with balloon-o-copters.  Note to self– do not buy the cheap version, they work like ….

Thursday — We developed the concept of N1L using the Motion of a Cart with Two Tension forces.  As I wrote about with the Advanced class, this does an excellent job of clearly developing the differences in resulting motion when forces are balanced and when the forces are un-balanced.

Friday — We formalized N1L with several hands-on demos the students completed.


Whew.. all caught up again. Now if I can just stay caught up…..