Day 26 – Wow, forces are difficult!

Today we tried a little something different….  the past two years the kids have REALLY struggled drawing force diagrams.  There are so many problems…  So this year we decided to break it down a bit.

We started with only one object, I drew an example last class of a book on a table, we did the force diagram of the book.  Today we did three more scenarios – mass hanging from spring scale, and then same thing, but pulling down or pushing up a little on the mass.  I am also trying to call it “cylinder” since calling it a “mass” always seems to cause issues.

I drew the interaction diagram and force diagram with them for the hanging cylinder.  I told them when they read the spring scale, it is telling them how hard the spring scale is pulling (Ft).  Then I let them go.  One class did great, one ok, and one (as expected) struggled a lot.

They first had to figure out that the spring scale reading equals the Fg since the cylinder is not moving, and the forces must be balanced.  Then, for the next two, most classes at least drew three forces, though the had lots of ideas about how to draw them… this was new, I usually just show this, but I let them struggle with it for awhile.  Also they didn’t have much idea what to do with the numbers, so there was all kinds of variety there…  I tool many pictures of their (incorrect) boards.  In the struggling class, many groups didn’t even draw three forces, often left off the Fg…  we have a lot of work to do.

We circled up for a board meeting, and I walked around and pointed things out, starting with the Ft=Fg for the first one.  Then we talked about the Fg interaction between the cylinder and earth…  and decided nothing about THAT interaction changed (the fact that my hand was now there on the other two doesn’t involve the cylinder-earth interaction) – so the Fg should stay the same across all pictures.  Then they started to figure out how to balance the forces.  We also had a discussion about whether to put arrows side by side, or to stack them, and decided stacking them made it easier to see if they were balanced or unbalanced.

After this, we went back to the book-table diagram from yesterday and now looked from the table’s perspective.  We drew the force diagram for the table, and then identified the  N3L pair…  must be the same force type, same feeler-dealer,but swapped positions.

Lastly, we did an example (originally taken from @fnoschese I think)…  and made it quantitative.  Wow this is hard.  Really really hard.  A couple of kids came after school and I remembered all of the problems…  they don’t always mark the weight as Fg, they mess up the force types…. the have trouble identifying what is happening.  Well, it is the hardest unit of the year, I have been warning them.  We will press on and hopefully the way we are breaking it into smaller pieces will help them.

Overall, I really liked the new sort-of-exploration way to move forward with force diagrams!  We will see if it pays off…  time will tell!



Day 25 – Learning Diagrams

Today we continued on a little with our gravity discussion, using the idea of Atlas dropping earth and moon “balls” onto the actual Earth.  Another good time to discuss the newton’s 3rd Law pair, and the equal forces on both sides (despite a different effect!)

Next we learned to draw interaction diagrams.  I still can’t believe it took me so long to switch over to teaching forces this way!  Now I can’t see that there is any other way to teach forces without completely focusing on Newton’s 3rd Law and the force interaction pairs!

We practiced drawing interaction diagrams together through a series of examples I set up, and then they did a few on whiteboards.  We had a circle board meeting all together and looked at them and discussed any issues.  For the most part they looked really good.  We had one picture of a train car (4 actually) and it was cool because the kids said, well it depends where the engine is, whether it would be normal or tension.  Well done!  They will do these again for homework – I decided to give each group just one to whiteboard before they did the worksheet, and for this, I think it worked out pretty well.

Next we learned how to draw force diagrams, with “feeler-dealer” labeling.  Again, something I used to think was a pain in the butt, and now I can’t imagine it any other way.

The last two years the kids have struggled quite a bit on the first few days of drawing force diagrams.  So this year we are adding another activity (next class) to just draw vertical ones, before drawing any N3L pairs with two objects.  Though it was super interesting because in 2 of 3 classes, when we started drawing the book-table force (book was the feeler) – they were like – why?  Why is the book the feeler? Couldn’t it be the other way around?  What about the table???  Brilliant.  I was like, hang on, it’s coming!  But GREAT insight!!!  🙂

We have a few other changes coming up to hopefully smooth this early unit out a bit, stay tuned! And man, I am really falling down on the picture taking… had to pull one old one…  I need to get better about this next year!!


Day 24 – Types of Forces Discussion

Today we went through the Types of Forces discussion…  loosely based on the Preconceptions in Mechanics book.  I like these discussions, as it sets the stage for the whole Force  Interaction – stressing Newton’s 3rd law above all others!  It was a pretty long day – in previous years this has been broken up into two days since the timing and activities before were a little different… but not to be this year.  It was long, the last 15 min were especially long…but overall it went well!

We showed a book on a  meterstick, and then increased the number of metersticks until it more resembled a table.  We pulled each other with spring scales, and then pulled toothbrushes (these were thanks @fnoschese).  We developed concepts of normal, tension and friction – also referring back to the interaction stations of last class.  I must have asked 1000 times, which pulls harder?  And CONSTANTLY stressed the forces being equal, taking TWO guys to make it happen – even though the effect of those forces may look different.

We also brainstormed what they thought they knew about gravity, and watched a few video clips – one @fnoschese’s Cavendish experiment, and also one about Einstein’s gravity.  Mind blown there!

I am excited to start drawing some diagrams next class!

** note:  We presented a lot of this at NTSA last year – here is a link to those slides!

Forgot pics… I seem to always forget on this day!


Day 23 -Interaction Stations

Today we took the acceleration quiz – I have them about 45 minutes, seemed to be enough time!

Next we really started into the Forces unit using Kelly O’Shea’s Interaction Stations…  original post I saw here, though I think there may be more current ones:

I had them go around and look at these, the first one they struggled a bit and then they got the hang of it.  I walked around and peppered questions like – What two objects did you pick? (rubber band and fishing weight)   Are they compressing, stretching or shearing?  (Stretching).  How do you know?  (I see the rubber band stretching.)  Is the fishing weight stretching?  (No.)  Hmmmmm…. and walk away.

I kept going around asking that – basically if the less-obvious object was compressing/stretching too….  (Yes the foam is compressed, I can see it indent.) Is the cylinder compressing too?  (no…  well…  maybe…. no….  no, right?)…..  this frustrated the heck out of them, which of course, I loved.  They will have to stew in that until next class!

Day 22 – Bowling Balls and Review

Today we spent half the period doing the intro bowling ball activity for the Forces unit, and half the class reviewing for the acceleration test.

Bowling balls went well this year!  We had about 35-40 minutes and got it done!  They used the Motion Shot app again, and drew how they had to hit the ball to cause certain motions.  We used the last few minutes for me to demo all of it again, especially the constant velocity = NO HITTING!  When we go back to class we reviewed …  hitting = CHANGE  in motion… no hitting = NO CHANGE in motion.

I think they are in pretty good shape for the acceleration test!  We shall see!


Day 21 – Putting it all together!

Today was our last real day on acceleration  We started by whiteboarding a few from Practice 7 to make sure we were in good shape…

Next we did the BIG template where they have to put all of their skills together.  All of these were speeding up and starting at a position of zero…  next year maybe we should add some slowing down and non-zero starting positions and velocities!  Overall, they seem to be doing well for the most part!


Day 20 – Learning to use a velocity graph

Today we finally moved on from the lab!  We started doing a little bit of whiteboarding, comparing the motion of different position and time graphs.  This went really well, and was very necessary, as there were still lots of misconceptions around!

Next we learned how to use the area under a velocity graph to find the displacement, and then use this to construct a position graph.  A few years ago we tried to find the position every second, but for most kids that proved too mathematically taxing.  So we now find the total displacement and then just connect the initial and final points with the right shape.  Some kids still choose to do the intermediate points, which is great if they can!