# Day 111: Energy Blocks revisited, Ediss and the 2nd Law

Today we used the graph of elastic energy as a function of change in length to develop the equation for elastic energy.  To help drive home the shape of the graph and the square relationship, I used the energy block again.  Here is a picture of  what we did:

We also developed the relationship we use for dissipated energy (Ediss =Ff*d(diss)) using this set-up:

The students did not complete this experiment, I walked them through it.  We just made use of COE (elastic energy stored when it is compressed is equal to the energy dissipated).  We determine the elastic energy using the spring constant and how mush the spring is compressed as it is pushed against the ring stand.  The energy was plotted as a function of the distance over which the energy is dissipated.  The graph is linear and the slope is approximately the frictional force.  Tomorrow the students will start in on kinetic or gravitational.

General Physics:

Today we had the post lab discussion from the Modified Atwood machine experiment that develops Newton’s Second Law.  To provide the kinesthetic experience (and so the students can internalize N2L), I used a ‘Giant Lab Cart of Physics’ (aka a delivery cart). We kept a constant force (one student pushing) but increased the mass of the system (more students riding) and also increased the net force (more students pushing).   Tomorrow we will use the giant hover craft of physics to connect all three of Newton’s Laws.

# Day 110: Hooke’s Law Research and Developing Elastic Energy

Over the last 3.5 days of class (yes, another 1/2 in there), the students completed and independent research experiment related to Hooke’s Law.  Essentially they came up with one change they wanted to make to a spring/spring system, made a hypothesis about how the change would affect the spring constant, then gathered and analyzed the data to verify the hypothesis.  They also posted their results on a shared Schoology discussion and made at least three comments regarding other experiments.  Now, truth be told, some of the experiments reached conclusions that may not be accurate, but I’m OK with this because the focus was on the freedom to just explore and be creative, not necessarily scientifically accurate.  In short, I think is was awesome. I had 2 groups compare the ‘k’ value in air and in water… yep, into our pool to gather data.  Groups look at temperature… room temperature vs. -17F (~-30F with the windchill).  I need to allow more of this.. just need to find the time.

Today we connected Hooke’s law back to energy.  We discussed how when we exert a force to cause a change in length of a spring, we are also storing elastic energy in the spring. We plotted force as a function of change in length (the traditional Hooke’s Law graph), but did not  have any energy into the graph yet.  We discussed where the energy might be hiding.  After looking at the energy bars for several changes in length and the Hooke’s Law graph, we defined the area trapped as the (working) elastic energy.

The next step was to plot a graph of elastic energy as a function of change in length.  To determine the energy values, we used the integral feature of Logger Pro to find the energy at 8 changes in length. We did not use our original Hooke’s Law graph, we used a new one plotted from the Hooke’s Law EOL and generated change in length values.  We did this to ‘smooth’ the data and to provide more to work with.  Tomorrow is the post ‘lab’ discussion on this.

General Physics:

Tomorrow we will have the post lab discussion for the Modified Atwood machine experiment that develops Newton’s 2nd Law.It is essentially the same experiment the advanced students completed, but the general students only complete one part, either acceleration as a function of mass (with a constant net force) or acceleration as a function of net force (with a constant mass system).

# Days 101-109: Playing the Catch-up Game yet Again

WOW, falling a bit behind here, I hate that! Almost two weeks since my last post, that’s bad… well enough ‘poor me’ BS.  Rather than give a day-by-day post, I’ll give the highlights…. well, highlights as I see them.

When I last posted I had used a Schoology Poll to see what problems from our Radial force and acceleration practice sheet they wanted to see.  It worked only so-so because no all the students voted. I do think I will try it agin though.

The UCM summative test followed the next day.

Into the next Unit– energy.  I used this problem to introduce it:

I used it to generate a need for our energy concepts — this is a really long ass problem using kinematics and dynamics, but pretty short with energy concepts.  When we discuss the ‘problem’, the need/desire for a shorter way comes out loud and clear.  This is one of the motivations for energy… I treat it as yet another problem solving approach that the students can use. Much like solving kinematic problems with a velocity-time graph.

We used the wealth analogy to build the basic way we think of energy.. a conserved quantity capable of producing a change in a system… that we can picture with energy pies and energy bars.  I use a snippet of Feynman’s lecture on energy conservation and the ‘blocks’ as an introduction to drawing energy bars.  We even have REAL energy blocks:

The original idea for the energy blocks was shared at one of our Phox Valley Share meetings many years ago.  I have multiple sets so when we are learning the different energy assets, we use them and I also use them with the relationships developed in some energy experiments (for a later post).

We practiced with energy bars and pies, took an assessment, then did an experiment looking at the relationship between the force exerted on a spring and the change in length of the spring.  Yep, a Hooke’s law experiment, although that name is not given until we have WB’ed the results. I use the Pasco colored springs sets.. known ‘k’ values to compare our slope to once we have defined it.

This led us into the activity we have been working on for the last few days.  It is an independent research experiment related to Hooke’s law.  Essentially, the students must come up with some change to make and use gathered data to determine how the change effected the spring constant.  For example, maybe two springs in parallel or two in series, how the length of the spring affects the ‘k’ value.  ( I made sets of springs containing 6 springs by cutting up and old snakey spring).

This is what the students are working on right now.  As part of the assignment, they will be creating a presentation to post on our Schoology course page.  They will be required to comment on at least three other experiments.

General Physics:

We finished the balanced forces unit today– summative test.  Along the way, we completed a static equilibrium with tension forces activity.  Using this set up:

the students use the force given by the force sensor (a WDSS) and the angles (measured with a protractor form a picture they snapped) to predict the mass of the candy bar.

We also completed the same frictional force experiment I posted about with the Advanced classes.

Tomorrow, we start the unbalanced forces unit.  We will be doing the Modified Atwood experiment to develop N2L.

OK, so caught up.. not pretty but caught up… at least for now.

# Day 100: Radial Acceleration/Force Problems and Some Tension in the room

We briefly discussed the answers to the Ranking Task I handed out yesterday.  Here it is:Unit 6 Ranking Task-The Merry Go Round –All Questions

Most of the students did very well on it.  In regards to the last question, I asked a series of questions aimed at gauging if the students understood a key concept about uniform circular motion and radial force.  Here it is:

Me: What is acting as the radial force in this example.

Ss:  The static frictional force.

Me: Good, which rider, D or F has a greater static fictional force?

Ss: They have the same static frictional force because they have the same mass and the we an assume he same coefficient of friction.

Me: Good. I see you have Rider D with a greater radial force than Rider C. Why is that?

SS: Ummm…. well….. let’s see…..

OK, it was not really like that for the last response, but it did take a bit of time for the classes recall that there is a difference between a required radial force and whether or not the force supplying it is larger enough.   I used an analogy. Let’s say I go to Starbucks and I want a coffee that costs \$3.  I have some dollars in my pocket, but just because I have some dollars that does not necessarily mean that I have enough dollars.  I have to actually check to see if I have enough.  If I have \$4, I’m good to go, but if I have only \$2, I don’t have enough and I get no coffee.

They students worked on 7 problems in class in small groups.  No group finished all 7, so (by design) they will have some to work independently on.  I suggested which ones were more challenging that they should work with their partners on.  This type of approach (in class time to work) is a bit new to me, I usually do not allow extended periods of time for this.  I don’t think it is a bad thing, I just want my students to realize they will have to do some independent work too.

To see which problems they want to see presented tomorrow, I set up a Schooloogy poll.  The one(s) with the most votes are the ones we’ll go through.  Here is a screen shot of it.

General Physics:

Today was about throwing numbers into the net fore equations we have been writing as we draw force diagrams.  I gave them a series of situations, they solved for the tension force, then we checked it:

Sample Tension Force Problems #2

Even though it is hard to read, the last problem set the students up to do a static equilibrium challenge tomorrow.  Here is a better picture of the last problem:

They were given one of the tension forces (measured by the WDSS) and an angle.  Tomorrows problem is very similar, but with both strings angled.

# Day 99: Checking the Toy Airplane Problem and The Mistake Game

I started the hour by sharing the results of the student survey my students completed yesterday.  I think it is important for the students to be able to see the results and especially the comments.  If provides a chance for even more dialogue.

As I  explained yesterday, we started a radial acceleration and force sample problem with the toy airplane. Radial Acceleration Sample 2 is our work from one of the sections: (You’ll notice I worked through it using the ‘Work Backwards’ approach.  This is a hugely powerful problem solving approach for students.)

Here is how I checked the answer using a Vernier laser gate and the Strobe file in Logger Pro.

Here is a screen shot of the file:

So about 4.6% difference.. I’ll take it!

General Physics:

After looking through some of the formative assessment ‘colors’ and comments, I decided to add a day of practice.  To help review and extend the idea of component of a vector I used the pHet simulation called Vectors, here it is. I like this because you can display the components as projections on the x and y axis.  After that there was an additional practice sheet.  The fourth mod class simply worked through it in small groups while I circulated and provided some small group(s) instruction.  With the 7th mod class I played the Mistake game for the first time… I made a few mistakes with it.  I had two groups do the same problems.. a bad idea because as the rest of the class tries to determine the mistake, invariably the correct answer gets discussed.  This is a good thing except when there is another group that has tried to include a subtle mistake in the force diagram yet to be presented.  Another mistake was to not limit the guesses that could be made. I had attempted to make the game into a chance to earn some candy (Smarties) if no one determined the mistake, or if it was determined.  I also handed out Dum-Dums if there was a mistake made that was not intentional.. really interesting that these showed up in the discussion and the presenting group claiming “no that is not a mistake”.

# Day 98: Survey Says…..

As part of the Educator Effectiveness project, we are required to survey the same set of students, twice during the course in which we instruct them.  So, today before WB’ing the results of the radial acceleration experiment, I ask the Advanced students to complete the second survey.  It has to be the same set of questions we gave before.  There are 10 or 12 questions we must ask and can not edit, and we are allowed to give more.  I used a google form again because I really like how the results can be displayed.  There are always some interesting stats that pop out at me. For example:  2 students (of 66) disagree or strongly disagree that I am knowledgeable about the subject I teach, a total of 6 students disagree or strongly disagree or don’t know if the lessons I plan are engaging.  I did learn that I have something to really work on… allowing students to demonstrate knowledge in a variety of ways (21% disagree, strongly disagree or don’t know).  This will be something I will try to work on throughout the rest of this year and into next year.  Any ideas on alternative assessments would be appreciated.

We WB’ed the radial acceleration experiment.  Next year I will alter how I do this… probably have the students pick a radius and monitor the radial acceleration as the angular velocity is varied.  I have not done that in the past because I have always felt that if students look up the equation for radial (or centripetal acceleration) they will see ar=vt^2/r.

We also started a sample problem.  Here is a clip of it.

The students will have the mass, the angle with the horizontal, and the length of the string.  We are going to solve for the time it takes to complete 10 revolutions, then check it of course.

General Physics:

More force diagram whiteboarding.  At the close of the hour I asked the student’s to provide me with their color. Most students in my 4th mod class were yellow (needing more practice) and in 7th most were green.  So tomorrow, with Mod 4, we will do some small group work (groups based on color) and in7th, we might play the mistake game as described by Kelly O’Shea here.

# 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.