# Day 68: More Force Diagrams and Graphically solving ca problems

As I mentioned yesterday, we finished the force diagram practice sheet.  Here is a picture of the set-up I used to check how the tension forces change withe length of the string and the angle they are at.

It is a bit hard to see, but the tension for on the right is larger than the one on the left because of the angle each makes, even though the string is shorter… this was a bit o a disconnect for some students.  I started this sequence with the same two strings at the same angle, so the tension forces were the same.

Here is another type of force diagram they did:

I like this one because it is open to some interpretation (cv or ca) and it requires the students pay attention to third law pairs.

General Physics:

We finished WB’ing the graphical approaches to the constant acceleration problems.  The last one we worked on in small groups was a goal-less problem about a cheetah running at cv, the ca, then more cv.  The problem they solved in small groups once the had created the v-t graph was to find an instantaneous velocity during a period of acceleration.  It could easily be solved by just looking at the graph, although some students found the acceleration, then used it as the slope to determine the velocity.  One young lady asked what the area trapped by an acceleration graph represented!!  AWESOME.  We worked through what it represented as a class and then we decided that we could also solve the instantaneous velocity problem using this approach because the initial velocity was known.  Did I mention AWESOME.

# Day 67: Force Diagrams and Solving ca Problems Graphically

Today (and probably tomorrow)was spent WB’ing a set of force diagrams.  We only made it through 3 or 4 of the eight I assigned.  It took long for a few reasons. First of all, not all the students completed the  practice (which really does not make me happy… I have not found a way to convince some of them that the MUST do the practice when they do not understand the topic) so there were many questions as they prepped the WB’s.  Second of all, it takes them a bit to explain the diagram and answer the audience questions.  Finally, I always ask a few extra questions that require some extra explanation.  For example, on this one:

I asked how the tension forces would change if everything was the same (so still the same angle) EXCEPT the right side string was attached to a ceiling that was higher so the string was longer.  Or if the angle was different.  We then use equipment to set up the new scenario and record the forces.

Here is another example:

I ask what would happen to each of the three forces (Fg, Fn, and Ft) if the ramp were made steeper.  To demonstrate this I use a set-up shared a number of years back by Dale Basler at a Phox Share group meeting.  It’s a bit hard to describe, but it uses the WDSS as the box and the 3-axis accelerometer.  Each one is displayed using an animated vector representing  one of the forces.  Fg is the y-axis that is calibrated to -9.8m/s/s; the Ft is the  x-axis and the Fn is the x-axis. It’s all wrapped up in a pretty like Logger Pro file, but here is a picture of the set-up:

So as the right side is elevated, the animated vectors show that Fg does not change, but the Ft and the Fn do.

General Physics:

We discussed the kinematic graph assessment they took on Friday and then started WB’ing the graphical solutions to some constant acceleration problems.  Tomorrow there will be more of the same.

# Day 66: Motion Pillars, Force Diagrams and Kinematic Stack of Graph Assessment

Today we finished WB’ing the practice on motion pillars and began to practice drawing force diagrams.  I asked the students to watch a screen capture  (here it is incase you’re curious, note: it is about 15 minutes long: )

of how we draw force diagrams as homework so we could get right to answer questions about the process and working in groups to draw some.

Unfortunately, not enough of the students took the time to watch the movie.  The ones that did , knew exactly what questions they wanted answered, the others struggled and did not get much completed.  Should be interesting to WB the practice sheet on Monday.

General Physics:

We took a kinematic graph assessment today, then worked through a sample problem involving constant acceleration that we solved using a v-t graph.  The problem involved a clip from the original Back to The Future… where the DeLorean gets up to 88miles per hour for the first time.

# Day 65: N1L including the Giant Hovercraft of Physics

In the first part of class we looked at some examples of Newton’s 1st Law.  I did a few classic N1L (inertia) demo’s.  I’m sure most of you have seen this one:

I also had the students get involved.  I made  I ‘heart’ Physics book markers the students could earn by getting it out from underneath the dry erase marker without touching or disturbing the marker.  Sort of a mini version of the table cloth demo.

After that, they rode our giant hovercraft of physics (sorry, no photos).

We also did a bit of WB’ing on a practice sheet that made use of  motion pillars.  Here is an example:

We did not quite finish, so that is where we will pick up tomorrow.

General Physics:

We finished the phan cart predictions and continued to practice solving constant acceleration problems with a v-t graph.  As of yet, the generals have no kinematic equations for constant acceleration.

# 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 63: A failed attempt to develop balanced vs. unbalanced forces (and N1L)

My mod one class was really awful today, or rather, I was really awful in mod 1.  To begin the class we played with balloon-o-copters to identify third law pairs.  That went well enough except for one little item…last year we purchased  about 300 of them for a very cheap price, thinking we’ll give one to each student, they can play with it and use it to teach others about N3L.  The only problem is that they really did not work so well, or at all.  But that’s not the really awful part.  My usual sequence is to develop N3L and the paired interaction, then explain that to explain the motion of one object, we need to look at the forces acting ON that object, not the forces exerted by that object, so one side of the force pair.

Over the course of my career, I have tried a wide variety of activities to develop the concept: guided discovery with large chunks of dry ice, guided discovery with bowling balls and pvc mallets or rubber mallets, hover pucks, the bowling ball grand prix.  I was never really satisfied with how the activities went and my post lab discussion to bring it all together.  For the last several years, I have used this activity from Kris Troha on Twitter a year or two back.  Essentially it uses the set-up shown to connect balanced forces to constant velocity and unbalanced to constant acceleration.  I like it because it is easy to set up and does exactly what I want it to.  One draw back is that it is not a kinesthetic experience for the students, and it does not really develop the concept of inertia as the bowling ball (and replacing it with a basketball) does.

Well, on Sunday, I read a pretty good article in the most recent issue of The Physics Teacher by Joshua Gates, called Experimentally Building a Qualitative Understanding of Newton’s Second Law, you should read it.  It describes an experiment about using a WDSS and a 2 pulley Atwood Machine.  I liked it and thought I could use it to develop the balanced vs. unbalanced forces, add a motion detector and connect it to motion.  I thought through it very carefully last night and typed some things out (no handout though) and went for it.  My HUGE mistake that I figured  out before I made a really HUGE mistake was adding a second WDSS as the other mass.  I showed the mass (one wdss) moving vertically and measured both forces… yep  reading the same value so balanced…. motion detector showed constant velocity … done.

UGHHHHHHH I hate it when I screw up. I knew it right away as I was writing it on the front whiteboard.  In my mind, all I really showed was that the tension is the same in the string…. I never defined my system as one of the WDSS’s and looked at the forces exerted on it (as our system) as I had talked about merely 10 minutes before this.  Constant velocity because the forces acting ON the system are balanced… namely Fg(WDSS, Earth) and Ft(WDSS, String). NOT the two blasted Ft values!!!!

Luckily the bell rang so I could re-group. I talked to one of the students in my class (OK, my daughter) and she did not even know it was a screw up… that does not make it ok, but she did say it was confusing.

So, what to do tomorrow… well besides owning my screw up, which I will do publicly in class, I’m going back to the cart with two tension forces, but adding two WDSS’s and a motion detector to provide proof.

SIDEBAR:  I can not find the link to the original twitter post, but if you want me to share the handout based on Kris’ idea, let me know.

General Physics:

A bit more practice on the Stack-o-Graph’s.  We tried to use the Roll Ball applet hosted on the Labout Loud site, but because it is an old version, the Java Security prevented us from having our students use it on laptops. Instead I used it on my laptop ( I fixed the java security so I could use it).  We took an assessment that I used the results from to put the students into groups for tomorrow —>Phan Cart activity.

# 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.