# Day 119: Conceptualizing and Quantifying Work(ing)

We started the mod by taking another COE formative assessment.  Here is it: Unfortunately, most the students did not correctly solve it.  There were two issues: (1) Silly errors (like canceling the 1/2’s) and using 20m as the final height, and (2) More f a conceptual issue, not including any initial kinetic energy, just starting with gravitational.  This surprised me a bit.  Some also went two distinct steps (very top to very bottom, the very bottom to the final height).

After the assessment, we WB’ed a sheet I put together a few years ago to graphically develop the models for work(ing) and power.  To conceptualize ‘when’ working is being done a system, we used the chairs the students sit in. Hold the chair at waist level, is there working being done ON the chair? (No, there is no displacement). Raise the chair up above  your head, now is working being done ON the chair? (Yes, the force is parallel to the displacement). You get the idea. We also walked with the char at waist level, and pulled the chair across the floor. We used a WDSS as a force sensor to lift a 1.0kg mass above a motion detector (at constant speed) to build graphs of Force vs. displacement so the area trapped is the work(ing).  Then work(ing) as a function of time to build the concept of power (as the slope of the work vs time graph).

General Physics:

Today was strictly a work day. They finished analyzing the data gathered while sledding.  They also worked on creating their presentations that will be uploaded to Schoology in the form of a shared Discussion.

SIDEBAR: Interesting to note that last Friday was the last ‘sleddable’ day… it was nearly 50F here today.

# Day 97: The Radial Acceleration Experiment and The Real-Time Force Diagram for a block on a Ramp

As mentioned last week, today was a data gathering day for the radial acceleration experiment. The purpose was to determine the relationship between radial acceleration and tangential velocity.  They used a single photo gate (One Gate Timing) to measure the tangential velocity and a Vernier WDSS to measure the radial acceleration.  They set it up to measure only the x-axis acceleration.  The radius was held constant, but the experiment was completed at a second radius.  Here are a few clips of the data being gathered.

Tomorrow will be all WB’ing.

General Physics:

Today we reviewed a bit about drawing force diagrams with components.  So far, we had only worked on tension forces that had components.  Today we extended this to when we have an object on an incline.  Years ago at a Phox Share meeting, Dale Basler shared a demo using the Vernier WDSS to show the force diagram for an object on an incline.  The file is set-up with animated vectors to show the gravitational force, the normal force and a tension force, but they are really the accelerations in those directions.  It is really slick and does an excellent job of showing the kids how the forces (size and magnitude) change (or not in the case of the gravitational force) as the incline is increased.  Here is a clip of it in action.

# Day 95: Circular Motion Force Diagram Assessment and Quantifying Radial Acceleration

Today the students took an assessment on drawing circular motion force diagrams.  Here is what I gave them: You’ll see why tomorrow.

We also had a pre-lab discussion to develop and experiment that will allow us to quantify radial acceleration.  Through discussion, we arrived at looking at the relationship between radial acceleration and tangential velocity for a given radius.  Most kids decided that the radial acceleration would be affected by the angular velocity, the radius and the tangential velocity.  I asked if we could do an experiment where we varied the radius.  A few realized that it was not possible because varying the radius will also change the tangential velocity *if the angular velocity is not changed)… so simply moving out on the circular to change the radius also changes the tangential velocity.

Here is the experimental set-up being used: That’s a Vernier WDSS zip-tied to a meter stick on our record players.  The tangential velocity will be measured using a Vernier Photo gate and One-Gate timing.  The students will do the experiment twice… a small radius and a larger one.  We did not get finished, so more data collection later.  Here is a screen shot on my own data: # Day 94: A few more circular motion Force Diagram Demo’s

As I mentioned yesterday, I had a few more ways to recreate the circular motion force diagrams the students were drawing.  But first, here is the clip of the race track showing a banked corner vs. an unbanked corner.

Here is another of the situations the students drew a force diagram for:  (according my students, the drawing is a bit hard to interpret) I know that it might be a bit of a stretch to have a vertical circle at constant velocity with a rope, but it simplified the FD for the students. Here we are re-creating it:

Here is the last one. The discussion involved us getting to the point of recognizing that the tension force is not constant.  As part of the discussion, I asked the kids to sketch the tension force as a function of time graph. To check that, as well as the force diagram, we swung a Vernier WDSS in a vertical circle while capturing the video.  Really nice once the two are synched… you can see that when the sensor is at the top of the circle, the tension force is the smallest.  Unfortunately, I did not save the ones from class, but here is my practice:

General Physics:

Yep, I’m still teaching this course too.  There is not much exciting to post because much of what we are doing now, I have already written about with the Advanced class.

# Day 64: My Mess from Yesterday Successfully clean up

Yesterday I explained how I totally screwed up my first mod class in a failed attempt to develop N1L.  As I mentioned last time, I opened the class by stating that we needed to start over because I had made a rather huge mistake. At that point most of the mod oners had their interest piqued.  I explained that I thought I could develop this new idea better, but because I had not used the equipment to work it through, but had only worked ii through in my mind I made a glaring omission/mistake.  I apologized and we moved on.

Here was the set-up I used. Imagine the ‘cart’ was really two dynamics cart attached with the velcro.  Mounted on each was a WDSS set up to measure the forces.  (Sorry, forgot to take picture). The task from the handout  was to use both tension forces to get the cart to move to the right with a constant velocity.  Yes, most said to try it by making the right side force bigger.  I gathered the data and it showed constant acceleration (I should have snapped a screen shot).  The next step was to try even sized tensions.  Here is the data: Pretty nice data showing that balanced forces create constant velocity.

The next task was to get the cart to move to the right slowing down, stop for an instant then get faster to the left.  We talked about I would not be able to change tensions mid-way (which helped some kids) so they tried the left side force bigger.  Here is the data: Again, nice data showing that unbalanced forces cause a change in velocity (or acceleration).  Put the two ideas together and we can say the only way to change the velocity on an object (or to make it accelerate) is with unbalanced forces… balanced forces will not change the velocity so if it started at a constant velocity of 0 m/s it will stay ….. you know the rest… Boom N1L.

In hindsight, I should have added an acceleration graph to show that the acceleration is in the same direction as the bigger force.

General Physics:

After a bit more stack-o-graph practice, we started in on the Phan Cart predictions.  Just like we did with the Advanced class, we will use the actual graphs to practice solving constant acceleration problems using v-t graphs.

# Day 62: ‘sigh’ Action-Reaction makes me a bit sad.

We had a brief discussion about forces that included a definition.  We (ok, me) defined a force as an interaction between two objects.  We talked about the symbol (Fsubscript (on, by), the units and how we measure.  We then moved into an ILD looking more closely at comparing the size of force Object A exerts on Object B.  I’m sure most of probably do this in some way, shape or form.  The version I use had ten situations, here is a sample: The students choose A (object A exerts the greater force), or B (object B exerts the greater force), or C (they exert the same size force.  I also ask them to provide a reason.  I use the clickers in anonymous mode so we can get a feel for where the class is.

To check the situations, I use two Vernier WDSS as the force sensor.  They work much better than the dual range force sensors I used to use.  Here is a screen shot of on of the tests (and how I phrased N3L): A fair number of my students, almost on cue, say “Oh yeah, for every action, there is an equal and opposite reaction”, which makes me sad.  I just feel this statement of N3L leads to a bunch of misconceptions.  Sure they can recite the 3rd law, but have no concept of it.  For example, I walk over to a student, a give him a back hand on the shoulder (no worries, it’s a soft back hand), then the student gives me a back hand on my shoulder right back.  I ask the class, “What was the action?”, Again on cue, a fair number say ‘you hitting… and the reaction was … hitting you back’. WRONG< WRONG< WRONG.

That is one of my issues with the Action-Reaction statement… it implies that time delay… first this, then that, when in reality the two forces are exerted at the exact same instant as shown over and over again by the ILD.  I also don’t think that action and reaction makes one think of forces, but it’s all about forces.

I could go on with this rant, but I won’t, just know that ‘action-reaction’ makes me a bit sad.

General Physics:

Today we WB’ed the ten ‘stack-o-graphs’ the class had as practice over the weekend.  As an added treat, after each group was finished presenting, I had one member of the group walk it or give me a real-life -in -a-car situation that would produce the graphs.  We sent them home with a one more to practice with… a ramp set-up from the Roll-Ball App.  This takes it one step farther… decide what the motion is, then create the stack-o-graphs.