Archive | SEP.2: Developing Models

# Day 63: Momentum Bar Graphs vs. Tables

College-Prep Physics: Yesterday we did the Inventing Momentum progression and developed momentum bar graphs. But today I had to arrive at school late because my own kids’ school had a weather delay (freezing rain). So I needed something meaningful for students to do with the sub. I found and modified 2 activities from The Physics Classroom and added a third.

However, the activities used momentum tables rather than momentum bar graphs. Since the kids would be with the sub, I figured a little extra hand-holding from the activity would be OK. It actually worked out well, in my opinion. Now my thinking is that bar graphs are great visual tool to introduce and develop the concept of momentum (as in the progression linked above), but for standard problem solving, momentum tables are a cleaner way to organize all the information involved. I also liked how the table also asks for momentum changes and total change. It was something I stressed during this year’s Inventing Momentum progression that I hadn’t in previous years.

Here’s my version of the activity. (I edited out bits that mentioned impulse, since we haven’t done that yet. I added the section on Explosions.) — Momentum Activity 2015

The Physics Classroom simulation and the original activities are here: Collision Carts

What are your thoughts on graphs vs. tables?

(PS: Yes, I’m back to doing momentum before energy. Why? Because despite the fact that momentum is a vector quantity, there is only ONE kind of momentum. I think kids are more easily trickiness of positive/negative momentum than they are in identifying all the types of energy present in a system at any given time.)

NGSS Science and Engineering Practices:
#2. Developing and using models

# Day 62: False-Color Images

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Astronomy: Today we did a false-color image activity.

Devise a color palette for the picture:

• You may use only 4 different colors.
• Assign each color a brightness key.

Look at each classmate’s picture and answer:

1. What is different and what is the same as you look at everyone’s picture?
2. Compare the pictures in terms of the pros and cons of using different color palettes.
3. When you choose a different color palette, do the data change, or do we just see the data differently? Explain.

Here’s a copy of the student sheet: ASTRO Color Coding Activity
(Adapted from a Hands-On Universe activity.)

NGSS Science and Engineering Practices:
#2. Developing and using models
#4. Analyzing and interpreting data

# Day 61: A Last Minute Change of Plans

College-Prep Physics: After Wednesday’s lab to introduce acceleration, I was ready to launch into the unit on constant acceleration. But then I read this modeling listserv email this morning before school:

Teach momentum early.  It allows you to leverage students’ naive conception of “impetus” – the notion that an object carries a force with it as it moves.  In many cases, they have conflated the concepts of force and momentum.

In our progression, we attempt to spiral key concepts in repeatedly.  We begin with constant velocity motion. In addition to the typical tumble buggy, there’s the motion of a hover puck and a glider on an air track to model.  It’s fairly natural to then look for the conditions when we find constant velocity – balanced forces.

In the free particle / balanced force unit, we look at forces as balanced /not balanced, and motion as CV / not CV.  We introduce system schemas, which depict the two-way nature of interactions, and introduce our students to the process of defining a system.  Hover puck and glider come out again as systems for analysis.

Next, we collide gliders on the air track to push the story line forward. We guide their focus to the change in velocity of each glider, and develop the model looking at the pattern of velocity changes observed in different collisions. Following momentum, it’s CA, and then unbalanced forces (CF) to develop N2 and get beyond “CV / not CV”.  Next quarter, we’ll look at forces in collisions, and develop N3 and the impulsive force model.

I like this approach not only because it leverages the student’s naive conceptions, but also because it spirals through core content repeatedly, pulling all of our mechanics work together in the end.

I tried teaching “momentum first” once before, but it was right after constant velocity, not after balanced forces like in the email above. That limited the amount of situations we could analyze, and there was some hand-waving about forces. But right now we are wrapping up balanced forces, so I think moving into momentum now will be more effective than it was previously. So my intended sequence for this year (slightly different than the email above) will now be:

1. Constant Velocity
2. Empirical Force Laws and Balanced Forces
3. Momentum Transfer (Conservation and Impulse)
4. Constant Acceleration
5. Unbalanced Forces
6. Energy Transfer (Conservation and Work)

So after we discussed the results from Wednesday’s speeding up lab, we looked at the forces during collisions. We used Plickers and a modified voting sequence from Preconceptions in Mechanics. Here are my slides:

Then we ended with the colliding carts demo to verify our predictions and models:

NGSS Science and Engineering Practices:
2. Developing and using models
7. Engaging in argument from evidence

# Day 57: Balanced Force Lab Practical

College-Prep Physics: Today we did a balanced force lab practical to tie together all our work on forces. It’s similar to the ones I’ve written about in past years. However, this year we used the green buggies and whiteboards instead of wood blocks and carpet/rubber. (This is because this year, in previous labs, some groups already worked with wood blocks and carpet/rubber.)

Given only a green buggy, a whiteboard, a spring, a 200-gram mass, and a ruler:

1. Determine the spring constant of your spring.
2. Determine the weight of your green buggy.
3. Determine the force of kinetic friction between your buggy’s rubber tires and your whiteboard.
4. Determine the coefficient of kinetic friction between your buggy’s rubber tires and your whiteboard.

UPDATE 2014 DEC 3: We found that when the 500 gram mass is added to buggy, the buggy rolls (rather than slide) when pulled. A binder clip on a rear wheel works great to lock the wheels so the buggy slides.

NGSS Science and Engineering Practices:
#2. Developing and using models
#3. Planning and carrying out investigations
#4. Analyzing and interpreting data
#5. Using mathematics and computational thinking

##BFPM

# Day 55: Falling Rolls Class of 2015

AP Physics C: Falling Rolls Day! Here’s video from 2 of this year’s groups. We had some synchronization issues for their first drops, but they each nailed it on their second attempt:

Click for other years and link to the activity.

NGSS Science and Engineering Practices:
#2. Developing and using models
#5. Using mathematics and computational thinking

UPDATE: 2014 DEC 4

Physics teacher Dan Hosey shared his class’s results today. I like how they use rods to drop both rolls simultaneously!

# Day 54: Kinetic Friction

College-Prep Physics: Last Thursday, students investigated the factors that might affect kinetic friction and how kinetic friction compares to static friction.

Today, students looked at the relationship between normal force and kinetic friction. Is the relationship proportional, like our previous experiments with static friction? If so, how do the slopes for kinetic friction compare to that from our static friction experiment?

NGSS Science and Engineering Practices:
#2. Developing and using models
#3. Planning and carrying out investigations
#4. Analyzing and interpreting data
#5. Using mathematics and computational thinking

# Day 52: Balanced Forces in Motion

College-Prep Physics: We’ve been looking solely at forces in static cases. Now it’s time to look at moving cases. So I asked students to complete the following chart on a whiteboard:

What’s true about the forces in the constant speed cases? In the speeding up and slowing down cases?

Then I tried to address the misconception/difficulties kids have with constant speed = balanced forces (wouldn’t it just not move?) by demo’ing dueling fan carts. First with fan carts off, then with both on — what happens after I push?

NGSS Science and Engineering Practices:
#2. Developing and using models