Day 60: Forces Diagnostic and WB the Ramp Lab

Advanced Physics:

Today I gave a (the) 30 question Forces Diagnostic Assessment. Although I am not using the results for part of my SLO this year, I am using it to help me plan instruction. The most missed question dealt with N2L and the summation of forces.  The overall pre-test average for the all three sections this year was 14.  Seems higher than in the past.

After this was completed, we finished discussing the conceptual part of the projectile motion summative test.

General Physics:

Today we WB’ed the ramp lab results.  The instructional goals were: (1) qualitatively develop the  concept of acceleration as the slope of the v-t graph and how the velocity changes each second, and (2) qualitatively develop the concept of instantaneous velocity as slope of a line tangent to a non-linear position-time graph.  We are not going to develop any of the kinematic equations for constant acceleration until the end of the unit.

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Day 59: Summative Projectile test take 2, the ramp lab

Advanced Physics:

Today was only two problems for the problem set part of the summative test over projectiles.  One horizontal launch and one angled launch.  The horizontal launch was the popular Speed (movie) problem— assuming a real horizontal launch, how far does the bus drop?  One of the angled launch choice was not overly challenging IF the students thought carefully about the situation.  They did not need the vertical velocity at all to solve it…. how far away should the launcher be to hit the falling monkey when it is 15 cm above the table.

 

SIDEBAR:  Some students really like having the two parts of the test split… I ay have to think about this a bit more.

 

General Physics:

Today was all about gathering data for the ramp lab.  We may not have gathered the best data.  Dynamics carts a re a bitt too fast with the short ramps, and some of our toy cars are not the best.  Might be time to build some of the constant acceleration disks or invest in the Motion Encoder system from Vernier?  Tomorrow we WB the results.

Day 58: More Ring, Testing and Another Ramp

Advanced Physics:

In each section, we had a few groups that still needed to set-up and test the Ring of Fire.  After they were finished the students completed the conceptual part of the summative test.  There was 11 MC (even some angry bird questions) and one set that dealt with a projectile launched at a smaller angle (less than the 45 degree first launch) .. sketch and explain the new horizontal position and vertical velocity graph.  This was harder for my students than I though it should have been.

General Physics:

We discussed the constant velocity summative test and started the pre-lab discussion for the ‘ramp’ lab.  The version of the ramp lab we do with the general physics students is not the full-blown ramp lab we do with the Advanced students.  We do not have the use photogates or watches to gather position – time data.  We use a motion detector to go right to the non-linear position -time graph and the linear velocity-time graph.

Day 32: No, the slope is NOT the acceleration and Mirror Mirror

Advanced Physics:

Today we started the WB discussion of the ramp data.  As explained yesterday, I assign the direction and initial position because it makes the post lab discussion more interesting and allow us to address a few things.  Here is a screenshot of the two graphs we ended up with:

Screen Shot 2014-10-15 at 7.50.53 PM

So far so good for the students.  There were a few that had to think really hard about what the blue graph was telling us… the (magnitude of the) slope increases in the negative direction so the object is getting faster in the negative direction as it rolls down the ramp.

Next we went to how to make it linear… yes, I do linearization. I find it helpful to work on proportional reasoning , to work with variables only, and to identify what the slope and intercept mean.  So, here is another screenshot of the linear graph:

Screen Shot 2014-10-15 at 7.51.44 PM

Ok, we have established the square relationship between time and position.  Now to identify the meaning of the slope and intercept:

Screen Shot 2014-10-15 at 7.52.00 PM

We always focus on the units.  The slope units are m/s^2.   I explain that the meters are easy,  they represent a displacement. But the square seconds…. we have no real conceptual basis for that.  What is a square second?  We understand a square meter, it represents an area.  A ‘second’ represents a time, but what about a square second?  By now the math kids are nearly foaming at the mouth.  Finally one of the kids will say, “Yeah, but m/seconds squared is the acceleration, that’s what the slope is”. I simply reply, No, the slope is NOT ‘the acceleration’.  And now some want to argue, some have their confidence shaken and some just want to be told what the hell the slope is (hell is my word, not theirs).

I explain that we are at a dead-end because of this, and when we are in a car, and we hit a dead-end, we go back to where we know something. or us, we go back to our original non-linear position-time graph.  This is where I help the students develop the idea of instantaneous velocity.  Another screen shot:

Screen Shot 2014-10-15 at 7.52.31 PM

To help explain the idea of instantaneous velocity, I used the ‘selfie’ analogy (I added Snapchat) I read about on the modeling listserv a while back.  It worked pretty well, especially since I snapped a self in each class that I will start with tomorrow as a review.  I am careful to define instantaneous velocity as shown above and include the position piece.  This is because of where we are headed tomorrow.  Notice not a single mention of acceleration from me.  It will be officially defined tomorrow.  I LOVE THIS STUFF.

 

General Physics:

After we discussed the test they took yesterday, I shut the lights off and all of a sudden some laser dots floating around the room became visible.  I sprayed some Fog in Can around the round and the beams burst into view.  From a laser on my desk, to a plane mirror on the front board, to a plane mirror up on top of a cabinet, to our disco ball.  GLORIOUS.  So now we are on to mirrors.  Following the same flow as we did with lenses, we start with looking at how the incidence angle compares to the reflected angle.  We use our magnetic laser levels on the main whiteboard and a plane mirror to quickly establish the two angles are equal.  From there we look at how the object distance compares to the image distance and how the object height compares to the image height.  All of this in accomplished by using a CD case and a two identical legos.  I read/heard about the AWESOME idea from Frrank Noschese here.  This is the BEST way I have seen to build these concepts.  GO ahead and watch it, I’ll wait.

 

See what I mean?  It works so well.  Tomorrow we tackle drawing a ray diagram to show how that image forms.

Day 31: The Ramp Lab

Advanced Physics

As mentioned yesterday, today was all about gathering data for the ‘ramp’ lab.  This is the (usual) experiment that develops several concepts, that I’ll discuss tomorrow. I have set this up a number of different ways.  This year I gave the students much more freedom and choice that I have in the past.  They could choose from 3 different objects to send down the ramp.  Their choices were a hot wheels car on its track, a constant acceleration dune buggy (this is the pull back kind, but they free wheel if you do not wind them up), or a large steel marble on a channel made from physics (duck) taping to 10 foot pieces of conduit together.  These are the same pieces of conduit I use a guard rails for our buggy bash.  I like the conduit ramps because it really nicely extend the ramp so the students can get more data, and allows for shallow angles to make the data easier to gate.

In a sense, we repeated the position vs. time experiment we did in the cv unit.

SIDEBAR: When we introduce the lab, it is not one of the ‘does the cv model still work in this situation” intro’s.  I do not do this because I have found the majority of my students already know the cv model will not apply.

I like the similarities because we are still measuring positions and times to describe the motion.  The students were also able to choose a method of timing; stopwatches or photogates (Pulse Timer – Two Photogates).  It totally blows me away that after all we have talked about in terms of our inability to make accurate time measurements by hand, some groups still reach for the stopwatches first.  Maybe it’s because they know them or think it is easier.  The only condition I set is this:  the odds groups (I use playing cards to group my students into lab grops) will put the origin or ‘0’ at the bottom of the ramp and start at some initial positive position, while the evens will put ‘0’ at the top and roll in the positive direction.  I love this when it comes to whiteboarding tomorrow.  It allows me to see who is still stuck on distance travelled instead of position and it awesome when they try to explain the meaning of the slope of the linear graph… stay tuned….

 

General Physics:

Lens and refraction test today… nothing too exciting.  Tomorrow we start in on reflection and plane mirrors.