# Day 139: Impulse-Momentum Problems and A pretty good sample COE problem.

It’s the day before a summative test… so we finished Wb’ing some conceptual questions about impulse and momentum and then WB’ed some impulse-momentum problems.  The practice sheets did not ask for the IFF charts, but I asked each group to add them.  For the most part they all struggled with them.  I’m wondering if this is because of how we developed or I presented them?  The middle ‘F’ graph was the easiest to get the students to see how to add to the IF charts.  Part of the issue was that they really did not pay attention to the sign o the impulse or the direction (sign) on the force exerting the impulse.  For next year, I will have to think of a way to present this in a better way.

SIDEBAR:  I will be out of my building tomorrow… planning a test for the Advanced kids…  hope this works out OK????

General Physics:

We WB’ed the gravitational energy experiment and again, lousy data.  I’m just not sure if it is a faulty experiment or my students a just plain rotten in lab?  From there we completed and sample COE problem.  Predict the velocity of a pendulum at the bottom of the swing, given the change in height.  We then used a laser photo-gate from vernier to check it.  It is the same set up I wrote about here.

# Day 138: Conceptualizing Impulse and Gravitatonal Energy

Yesterday I gave the students a set of conceptual questions about impulse and momentum. I don’t know about you, but I find many of my students can easily hide behind the quantitative problems and think they really ‘get’ the topic.  In other words, they are really petty good at putting the numbers into  the correct equation.  Granted, sometimes a sign messes them up, but for the most part they are pretty solid at that.  They still struggle at times to use the models/equations (sans numbers) to explain things.  I will not bore you with all the questions on the assignment, but we did add a few demo’s that I hope helped solidify the concepts.

One question dealt with a clay ball hitting a wall and then a same mass super ball hitting the wall. Which had the greater change in momentum. Not too bad on this one, but then asked which imparts a greater impulse on the wall… that made a few think.  Here is the demo I did to help:(will add later, sorry)

Another question was the traditional question about why airbags work.  I did the classic egg thrown in to the bed sheet, but then also showed them the cup and ball toy. We talked about how it is easier to catch the ball, if you draw your hand down and away as you catch it.  We then made our own version:

We also had a series of questions about comparing aspects of a collision between a Hummer and a VW bug.  Compare the forces, the impulse, the change in momentum, and finally the acceleration.  I tried for about 45 minutes to figure out how demonstrate this with two dynamics carts and WDSS force sensors.  Each time the forces were about 2-3 newtons off.  It was just not good enough for me… especially with the maximum force only being about 20N.  Tomorrow we will WB some quantitative problems.

General Physics:

All data gathering today!  The purpose was to develop the model for gravitational energy.  To get at it, we simply allowed a dynamics cart to roll down a shallow incline from various starting heights and measured the velocity with a photogate near the bottom of the ramp. We calculated the (kinetic) energy from the velocity  and plotted this as a function of the change in height.  Tomorrow we will discuss it… here’s hoping the slope turns out to be the weight of the dynamics cart!

# Day 137: WTH, this impulse data is UGLY.

The plan for this class period was to WB the results from the impulse vs. change in momentum experiment we conducted on Friday.  The results were overwhelmingly UGLY.  I had one or two groups (of 7) in each group that ended up with a slope of approximately 1.0 as was expected.  I re-ran the experiment during my prep hour. Here is a screen shot of that the data gathering looks like:

and now a shot of my impulse vs. change in momentum graph:

My slope is 0.984…. so close enough to one for me.  So why did most of my kids get ugly data?  I think I know what happened.  First of all, they did not did not use an instantaneous velocity at the time the collision started, that’s what I did.  But a bigger issue was how we measured force.  I screwed up and let them put the hook or bumper in to the WDSS.  The issue was some of them screwed the extender or the hook/bumper in way too tightly.  Some of the force sensors were reading 15-20 N before they even zeroed it. Now my concern is that they might have damaged the internal sensor 😦

General Physics:

Today was practice with the kinetic energy model.  We WB’ed a practice sheet, worked in small groups on the first energy conservation problem, and did a pre-lab discussion for the gravitational energy experiment we will do the next class meeting. Here is a screen shot of the COE problem:

# Day 136: Feeling a bit ‘impulsive’!

As I explained in the last post, today was all about gathering data for the impulse-momentum experiment.  Here are two short video clips that show how we set-up the equipment:

and

The initial momentum was varied by having different initial velocities and also by adding mass. I like having two slightly different experiments.  The impulse – change in momentum graphs obtained all have a slope that should be approximately 1.0. The part with just the cart alone has a negative impulse and negative change in momentum, but the one with the rubber band has a positive impulse and positive change in momentum.  Makes for an interesting discussion in the post-lab discussion.

General Physics:

The bulk of today was spent having the post-lab discussion for the kinetic energy experiment.  The slopes were not quite 1/2 he mass for all the groups, but thankfully, all the groups did have the square relationships.  We used the energy blocks again to reinforce the concept of a square relationship:

We started with a velocity of zero, so no kinetic energy.  Then I said I gave the cart a velocity , v  and that gave us 2 ‘blocks’ of energy.  Form there, we went to a velocity of 2v, then 3v,  and so on.  I really like how the non-linear nature shows up using these blocks.

# Day 135: I’m tired… why? .. labs all day… hell yeah!

Yep, I’m tired, but it is a good tired.  Labs in all 5 classes. That means the kids are doing and thinking. So why am I tired?  Well, I like most of you, I constantly circulate around to answer any questions and give a few helpful hints.  The tiring part is sometimes resisting the urge to fix mistakes outright rather than use a series of questions. Plus, we only have 53 minutes classes WHICH IS NOT NEARLY ENOUGH TIME FOR ANY EXPERIMENT.

We discussed the COM assessment from yesterday, then finished the pre-lab discussion for and impulse-change in momentum experiment.  I’ll shoot some video tomorrow as they will have to finish gathering the rest of the data.  Essentially, the students are either running a dynamics cart with plunger button taped down into a WDSS acting as a force sensor, or pushing a cart that has some elastic material attached to it away from a WDSS that has the other end of the elastic material attached.

General Physics:

Data was all about gathering the data for the kinetic energy experiment. As described yesterday, we are using the Pasco Spring Launchers.  It is a bit of a complex experiment for this group of kids.  Record the compression of the spring, (to be used to calculate the elastic energy that is equal to the kinetic energy) and record the max velocity at the point where the spring is no longer compressed.  Plot kinetic energy as a function of velocity. From some of the data I saw today, the square relationship comes through.  I’m just hoping the slope of the linearized graph is close to 1/2 the system mass!

# Day 134: Final Thoughts on the Physics Face-off

The student groups were given 15 minutes to present their solutions tot he Physics Face-off to each other.  After words, I asked for some feedback: (1) I liked the Physics-Face off because…  (2) I did not like the Physics Face Off because…. (3) If the Physics Face off is done again, I would change…, because…

Nearly everyone one of my students liked it.  Most said because of the creative nature, or the competition.  The main dislikes were because of group dynamics (partner problems, or the other group did not correctly solve its own problem…) As for changes, one that caught my eye was to give the problem to a group that wrote a problem for the other video I provided, that way they did not already ‘know everything’ about the video/possible problems.

The activity is definitely a keeper!!

The students also took a formative assessment on COM.  I wrote the assessment based on the pHet Collisions simulation.  (I actually got this idea when looking for COM problem in the on-line test I’m using, OpenStax College Physics.  Here is it:

Tomorrow when we discuss it, I will simply set it up in the simulation and allow them to see the collision and the answer.

The few minutes we had remaining was spent just starting a pre-lab discussion for an impulse-momentum experiment.

General Physics:

Today the bulk of the time was spent having a pre-lab discussion for our kinetic energy experiment.  We will again use the Pasco Spring Launchers:

The main difference with the General students is that we will have one station manned by me to determine the spring constants.

# Day 133: You NEED to do this with your classes!!

Today was essentially a work day on the ‘Physics Face-Off’, that I wrote about in the last post. It is an awesome activity.  Once again, here is the link to the activity developed by Peter Bohacek that uses the AWESOME Direct Measurement Videos.  Today the students worked to create their challenging yet solvable problem for another group and then worked together to solve the problem given to them.

The level of discourse today was absolutely incredible.  I should have recorded some audio!    Two suggestions I have to improve it (at least for my kids) that I will make sure to do next year are: (1) Have my students start writing problems earlier in the year. As you know, writing a good logical problem is as easy as it sounds.  I have had my students do some of this, but not enough. (2) Make Use of Direct Measurement Videos sooner/more often.  They are truly awesome.

Tomorrow we will wrap up the activity and I will get some student feedback.  For the bits I over heard, they really liked it.

General Physics:

The students worked in small groups on a set of Hooke’s Law and Elastic energy problems.  The assignment had 6 problems. We told the students one of the 6 will be scored as an assessment… seemed to motivate them a bit more because they could work together.

# Day 132: COM and Developing Eelas

NOTE: This post is being written one day late… I watched the NCAA tourney game last night, so this post was preempted.

We started the class by discussing any questions the students had about some 1D conservation of momentum problems.  There were a few, but not many…. meaning the problems were too easy or they did not do them.  As it turns out, it was half and half; those that did the problems found them pretty easy, those that had not bothered really did not know where to start.  As part of the solution each problem required and IF Chart.  If (not pun intended) you do not know what IF charts are, take a look at this post from Kelly O’Shea.  The charts re pretty awesome.  My experience has been that most students do not have any trouble solving 1D COM problems… especially since the equation is always the same.  TO me, the real power if the IF chart is in answering conceptual questions… like after two objects collide inelastically, which way and how fast…  It was here that there were more questions.

Anyway, after a few minutes of questions I put the students in to groups of three work on a single 2D collision and an inelastic collision that combines with energy concepts to answer a question.  I let them work for about 30 minutes. With about 10 or 15 minutes left, I introduced the activity that we would work on tomorrow, but that they needed to get ready for.  It is the ‘Physics Face-Off’, developed by Peter Bohacek.  Here is a link to it.  In short , it uses Direct Measurement Video’s to have student groups create a challenging problem for each other.  More in the next post.

General Physics:

Today we graphically developed the model for elastic energy from the area under a Hooke’s Law graph (force applied vs. change in length).  We had already established that the area trapped was the elastic energy, today the students plotted the elastic energy as a function of change in length graph.  To make it go a bit faster and to conceptualize it for the students, we had the start with Logger Pro made Hooke’s Law graph.  From here, they choose a change in length off the graph,  used the examine feature to find the corresponding force, the calculate the area of the triangle (elastic energy).  Al this was explained in the Logger Pro file.  We simply added a text box with the basic instructions:

Following the steps creates the non-linear elastic energy vs. change in length graph:

Linearizing it:

Notice how the slope of this linear graph has the same units as the spring constant, but the value is 1/2k… and there is it is…. in linear form:    Eelas =1/2kΔx^2

Some may ask, why not just give the equation to the students or derive it for them??  Then it is me doing the work and thinking, not my students.  Nuff said!

# Day 131: Yet Another Shortened Day….

That’s right folks…. only 30 minute classes and an early release for some professional development; the day before Good Friday (so no school for us). Now normally I tend to be critical of our ‘professional development’ on these days. BUT, I was looking forward to this one because we were offered a choice of 8 different breakout sessions put on by colleagues in my building!  Here were the choices:

Meaningful implementation of the College and Career Readiness Report Card – Room 314
In this session, we will discuss the ways that the College and Career Readiness Report Card has been used and how we might make the use of this tool more effective in assessing our students appropriately.
Developing a standards-based scope and sequence – Room 316 (Department Chairs only)
In this session, the English Department chairpersons, will provide a tool that they have developed to assure appropriate implementation of course standards and how the department has engaged in the discussion to refine standards that will be accessed from a departmental perspective
Our process of implementing department-wide re-takes – Room 330
In this session, the Math Department will discuss how they have effectively implemented re-takes to assure students meet the standards before moving on.  Facilitators will discuss struggles on the way to implementing this process department-wide.
Developing benchmarks that are tied to assessments – Room 332
If you missed this session last year, you will have a second chance for the Math Department to discuss the way that they have tied benchmarks to assessment and the growth they have seen in our students.
Grade Less, Accomplish More: Changing the way you (and your students) think about homework – Room 326

In this session, one of our educators will share how replacing “homework” with “practice work” has not only made grading easier, but students more accountable and receptive to completing assignments on time.
Re-taking performance-based summatives – Room 328
In this session, we will provide examples of how you might provide students the opportunities for re-takes on non-traditional, performance-based summative assessments.  If you use labs or projects to assess your students, you may want to attend this session to discuss the process of re-takes using these tools.
Alternatives to traditional assessment – Room 336
In this session, our problem based learning (PBL) team will discuss the tools they use to assess students as alternatives to traditional assessments.
Want to make exemplary rubrics in a short amount of time? -Room 324

Transform your classroom assessments and come learn how to access, edit/revise, and create your own rubrics.  These strategies and online tools will be ideal as we continue to develop our common assessments.
I chose the two options in blue.  Why show all the options? Well besides thinking you might be curious, I am proud that my building provided a chance to get a glimpse into our colleagues rooms… GOOD Professional Development in my mind.  Sign me up for more!
With the 30 minutes available, we worked through a sample conservation of momentum problem as a class.  We used the Pasco Mini-Launcher and Projectile Cather Assembly that attaches onto a cart:
A very typical problem, predict the final velocity (verified with the motion detector shown). The predicted value was 0.190m/s, as shown in the work here (COM Sample Problem (14-15)).  Here is a screen shot of the verification:
Awesome right!
SIDEBAR: Because there was a number of students absent (remember, this was a 3.5 day week and an early release on a Thursday), I made a screen cast of the class solving the problem.  I used QuickTime to do a screen capture.  It occurs to me that I should really be doing this each time we do one of these sample problems regardless if there a a bunch of missing students.  It is easy to do (wish there was a way to show the actual demonstration..) and might be helpful as my students review for finals.  I know I could also use student created screencasts as an alternative method of assessment…. something else to work toward!  Anyway, here is the movie in case you are curious.  Feedback welcomed.
General Physics:
We used the 30 minutes to discuss the four short quantitative problems they were assigned as practice.  After that, we developed (via discussion) that the area trapped by the Hooke’s Law graph (linear Force applied vs. Change in length graph) represents the working done on the spring by the person pulling AND thus the elastic energy.  They are poised to graphically develop mathematical model for elastic energy by plotting the elastic energy as a function of Change in length.

# Day 130: Finalizing the Collision Experiment; Samples of COM; Variations on Spring Constants

We used the first ~15 minutes of class to discuss and decide (based on the student calculations) if kinetic energy is conserved in each type of interaction.  We easily saw that it WAS NOT conserved in explosions. In one class that had poor data to start with, it was difficult to see if the kinetic energy was conserved in the elastic collisions and NOT conserved in the INelastic ones.  In the other two classes, this was much easier to see from the data.

After this discussion we looked at a classic example of conservation of momentum… Newton’s Cradle. I really like using this for a few reasons. First, it helps the student conceptualize the difference between inelastic and elastic collisions. It forces them to explain why only one ball swings away when one ball is started, rather than one ball swinging twice as fast… this does not conserve the kinetic energy.  Finally, it sets the stage for this:

Our ‘Giant Newton’s Cradle of Physics’.  We also looked (qualitatively) at Conservation of Angular Momentum with another classic demonstration:

General Physics:

We tried something different this year to help the students conceptualize spring constants.  We created a short practice sheet that involved a few questions and ranking tasks related to spring constants and systems of springs. We used a “Think, Pair, Share” approach.  Here is the initial information:

1. If the same mass is hung on each one, rank the springs in terms of the change in length.

2.  If 200g are hung on Spring C, how much mass needs to be hung on the others to create the same change in length?

3.  If two identical Spring A’s are connected end-to-end (in series) how does the overall change in length for this system compare to when the same mass is added to one Spring A?  Does the spring constant for the system change? If so, how?

4.  If two identical Spring A’s are hung side by side(in parallel) and connected with a rod on the free end  how does the overall change in length for this system compare to when the same mass is added to one Spring A?  Does the spring constant for the system change? If so, how?

We used the colors spring sets from Pasco to verify all four answers.  The remaining questions on the practice sheet were ones that required the student to use Hooke’s Law to solve a few simple problems.