Day 32: Interaction Diagrams and Force Diagrams

Day 32: Interaction Diagrams and Force Diagrams

College-Prep Physics: Now that we have gravitational forces, spring forces, and normal forces under our belts, we can analyse more complex situations. Today was a direct instruction lesson* on drawing interaction diagrams and forces diagrams. You might notice some changes from how I drew them from last year.

I’m using agent-object notation on the force diagrams, rather than last year’s force type + agent, in order to combat the misconception that the force diagram represents what the object is doing, rather than what is being done to the object. This also helps with getting the students to focus on the objects that are exerting the forces, because “every force has a source.” To make the force diagrams easier to read and label, we’re not including the force types on the force diagram vectors. Force types are labeled on the interaction diagram only, to help reinforce that a force is a single interaction between objects.

I’m also starting with complex scenarios early, and also asking students to draw more than one force diagram for a given situation. Last year, some students had the misconception that there must always be one force up, down, left, and right. The didn’t realize you could have 2 forces in one direction or no forces at all.

Drawing multiple force diagrams also allows for identifying 3rd Law pairs (the two vectors with circles in #4 above, though we haven’t formally called them 3rd Law pairs).

We also started with numerical values early. Although the scenarios don’t ask a specific question, we determined the values for as many forces as we could based on what was given.

In hopes of avoiding another common misconception, you’ll see that in both scenarios the normal forces aren’t equal to the weights of the objects.

We are only looking at static cases right now. Up next is tension, then friction. After friction, we’ll consider the dynamic cases.

The two scenarios pictured are taken from Preconception in Mechanics, though PiM doesn’t have the students draw interaction diagrams or force diagrams — a fault I found out too late last year. You can get the entire handout here: ForcesSchemaFBDDevelopmentStatic2015

PS: I haven’t been using the HW sheets from PiM at all. Rather, I’ve been using the occasional PiM HW problem as a bell ringer/do now/warm up.

*If you have a more engaging way of introducing interaction diagrams and force diagrams, please share!

##BFPM

NGSS Science and Engineering Practice #2: Developing and Using Models

Day 32: Interaction Diagrams and Force Diagrams

Day 41: Electron Theory Review

Today AP Physics 2 started the Electrostatics unit. The first topic was electron theory, which is review of last year’s class for all students (regardless of which class they took last year). Rather than a review lecture which would have little engagement, I finally had the idea to leverage peer instruction in this context. I presented a few notes on electron theory, emphasized the theme of explaining macroscopic phenomena from an atomic perspective, and then we spent the rest of class discussing conceptual questions via peer instruction. Students were much more engaged and I had much greater confidence in what they did and didn’t retain from last year.

Day 41: Electron Theory Review

  ##electrostatics ##peerinstruction  

Day 31: Equality of Normal Forces

College-Prep Physics: On Friday, we established that the table must be pushing up on the book. Today, we explored a different scenario to determine if normal forces between objects we equal in size. (Based on a similar sequence in Preconception in Mechanics.)

VOTE #1: Compare the forces between the wood stick and the car. (target)

Day 31: Equality of Normal Forces

I set up a slow buggy driving into a wood dowel that is hanging down from a ringstand clamp. If you remove the tire treads, the buggy wheels will continue to spin, showing that the buggy is continuously pushing against the dowel.

Day 31: Equality of Normal Forces

Some students says the forces are equal, some say the buggy is pushing harder because it’s trying to roll into the stick, and some way the stick is pushing harder to keep the buggy in place.

I don’t give the answer, but give them the next scenario instead.

VOTE #2: Compare the forces between the hand and the spring. (anchor)

Most kids say they are the same. It helps to think of a small, motionless board in place between the hand and the spring. Since the board is at rest, the hand and the spring must be pushing equally on the board. Now gently slide the board out from between the hand and the spring. Have any of the forces changed? So how do the forces compare? If I push harder on the spring, what happens? Are the forces the same now? How does the spring know how hard to push? (A lot of kids talk about the spring adjusting or compensating until the forces are equal. Some even refer to the spring lab we did previously. While the forces are ALWAYS equal, even while the spring is moving, I let that detail slide because we’ll return to the dynamic case in another lesson.)

VOTE #3: Compare the forces between the stiff and loose rubber band. (bridge)

Day 31: Equality of Normal Forces

Again, most kids got that the rubber bands pull equally because the ring is at rest. How is this possible when one rubber band is stretched more than the other? What happens when you try to make one of the rubber bands pull harder? What happens if the ring is removed and the rubber bands are tied together? Are the forces still equal?

VOTE #4: Compare the forces between the rubber hose and the car. (bridge)

Day 31: Equality of Normal Forces

Now I have the slow buggy drive into a piece of flexible rubber hose. The slow buggy works well because the hose will visibly flex and while keeping the buggy in place.

Day 31: Equality of Normal Forces

Again, students say the forces are the same. How does the hose “know” how hard to push? What would happen if we replaced the slow buggy with the fast buggy?

VOTE #5: Compare the forces between the wood stick and the car. (target)

We return to the first scenario and re-vote. Students make the connection that the wooden stick still bends and the force between the car and the stick must be equal. Then I quick run through the book scenarios from the previous lesson and ask them to compare the forces (the same, the same, the same, …)

##BFPM

NGSS Science and Engineering Practice #2: Developing Models
NGSS Science and Engineering Practice #6: Constructing Explanations

Day 31: Equality of Normal Forces

Day 40: Thermodynamics Exam

Today AP Physics 2 concluded the thermodynamics unit with an exam. I didn’t bother to take a photo. Instead, I’ll share a photo from my 8th hour AP Computer Science class. They were having trouble focusing minutes before the bell and also discovered there were buttons on the monitors with which they could play.

Day 40: Thermodynamics Exam

Nice rainbow!

  ##thermo  

Recap of Gravity, start Fn

So today I decided at the spur of the moment to have some more discussion on gravity…  I had them all get mini whiteboards and asked a few questions – let’s see if I can remember them…  1.  I have a book and crumpled paper, which one has a bigger force of gravity?  (they mostly said same)  Even though they also pretty much all agreed that the book was heavier.  2.  Which one would hit the ground first – and why?  (they mostly said at same time, but they also said it was because they had the same force of gravity… despite discussing it again in #1.)  3.  Looking at just the book and the Earth, which force is bigger, earth on book, or book on earth.  They mostly picked the book has more force acting on it… although some remembered the prior discussion and said same,  4.  On the moon which has a bigger Fg, on the book or on the paper?  Now they were getting better, book has more Fg because it has more mass (but less Fg when compared with Earth since the moon is less massive).  5.  On the moon, which has more mass?  (still book, same stuff in it).  6.  Then I flicked the paper across the table, and asked if i did that on the moon would it go less far, same, or further?  Answers were all over the place…  but then we discussed same because the mass didn;t change..  we will revisit this later!  After this discussion we had to discuss Normal Force, which I did with a book, table, metersticks spanning 2 tables, and the Pasco matter model.  This seemed to go ok – we did interaction diagrams, and free body diagrams, and I set them off on the bathroom scale activity…  We kind of needed some more time to discuss, but honestly it was the longest I have ever stood in front of the class and talked, and they were losing interest and my voice was hurting, so we just went for it!  We will finish that activity and discuss next class!

Oh I forgot, I also showed these two videos after our discussion….

Recap of Gravity, start Fn

##bfpm

Recap of Gravity, start Fn

Day 30: Does the Table Push Up on the Book?

College-Prep Physics: Today we did another round of voting (a la Preconceptions in Mechanics) to answer the question “Does the table push up on the book?”

One snafu that happened this year that didn’t happen last year: Because we studied gravitational forces first, kids were confused by the question and thought about the gravitational attraction between the book and the table. This was something I did not anticipate. So I had to clarify the scenario (explaining that table’s gravitational force on the book pulls the book down rather than push the book up as per the question).

Last year, that confusion wasn’t an issue because we did normal forces first, which is the suggested sequence in preconceptions in mechanics. But I was dissatisfied with that sequence because there were questions about normal forces between individual objects that are stacked on top of each other. We were talking about the object at the bottom of the stack having to support the weight of the objects on top. Those complex scenarios are easily analyzed using system schema and free-body diagrams, but we hadn’t talked about gravitational forces yet.

So, despite the confusion this year, I still think gravity should be done before normal force. So for next year, I’m revising the questions. I’m going to start with the hand on the spring question, since the answer is obvious and we just wrapped up the spring lab. Hoping that question puts kids in the proper mindset, then I’ll move to the table on the book question. And instead of the foam question, I’ll replace the foam with springs. (My foam never really deformed much anyway.) Day 30: Does the Table Push Up on the Book?

Here’s the revised slides I’ll try next year:

##BFPM

NGSS Science and Engineering Practice #2: Developing Models
NGSS Science and Engineering Practice #6: Constructing Explanations

Day 30: Does the Table Push Up on the Book?

Gravity Lab

So we started into our Balanced Forces unit with the gravity discussion and here is the packet ( BFPM Packet ).  After the kids got their data and made their Force vs. Mass graphs we discussed them.  First we just looked to be sure all the regular stuff was written correctly, which they seemed to do a good job with this time.  Then we looked at the trends between the boards.  Everyone agreed that the graphs were all the same – so we looked at the slopes.  In one class, they were close to 9.8, but also had 0.9, .009 etc….  a factor of not converting grams to kg correctly.  We then had a discussion about mass vs. weight, and how more massive things are pulled harder by the earth.  I then dropped some objects, and of course many of them thought the heavy book would fall faster than the ball.  And then I did the paper, and they all said the paper was falling more slowly because of “air resistance”, but then when I put the paper on top of the book and dropped them together, many of them were shocked to see the paper fall right with the book.  One student was so confused and said “What is even going ON here??” which I thought was funny!  One student suggested crumpling the paper, so I did, and then they fell together which surprised many of them.  We then determined the book had more Fg due to more mass, and the paper less, so how can the effect be the same?  (we have not yet discussed acceleration, which made this a bit trickier!).  I’m trying to get them to see that ratio of Fg/m is the same for both…  I flicked the crumpled paper across the table with my finger, but when I do that to the book, nothing really happens – it is harder to more more massive things.

Gravity Lab Gravity Lab Gravity Lab

##bfpm ##paradigmlab

Gravity Lab