Day 67: Marshmallow Puff Tube

 

College-Prep Physics: We did an exploratory activity as an introduction to impulse. Another single-sentence lab:

LAB 13 — Marshmallow Puff Tube
Design several mini-experiments to determine the factors that affect the speed of the marshmallow.

Here’s the results from one group.

Sadly, no one thought to change the mass of the marshmallow:

More info about building marshmallow puff tubes:
http://www.exploratorium.edu/snacks/marshmallow_puff/

NGSS Science and Engineering Practices:
#3. Planning and carrying out investigations

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 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 131: Discovering Momentum Conservation

College-Prep Physics: This year, I taught energy before momentum for two reasons…

• In my opinion, the conservation of a scalar quantity (like energy) is simpler than conservation of a vector quantity (like momentum).
• Collisions are an easy way to “break” the energy model, i.e., it’s not useful for analyzing collisions. There are several collisions which all start with the same amount of kinetic energy, but have different outcomes.

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So yesterday we analyzed the energy transfers in various collisions:

• Red & Blue Pasco carts and tracks
• Logger Pro with Motion Detector
• Air Track Simulation from Tony Wayne

Elastic collision: 1 kg @ 4 m/s collides with 1 kg @ rest
8 J of E_k –> 8 J of E_k

Inelastic collision: 1 kg @ 4 m/s collides with 1 kg @ rest
8 J of E_k –> 4 J of E_k + 4 J of ΔE_th

Inelastic head-on collision: 1 kg @ 4 m/s collides with 1 kg @ -4 m/s
16 J of E_k –> 16 J of ΔE_th

Kinetic energy not always constant before/after. Outcomes ranged from all kinetic transferred to none transferred.

YET — there seems to be a pattern. In the inelastic collision, the speed drops by half. In the inelastic head on collision, the speeds “cancel out.”

Students naturally asked what happens if we change the mass of the carts. So we demoed a few examples in the remaining time:

• What happens in head on inelastic collision between a light cart and a heavy cart going the same speed? (Goes slower in direction of heavy cart.)
• How might we get them to stop after collision? (Make the lighter cart travel faster.)

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TODAY

Explored a careful sequence of collisions where students had to draw before/after pictures, bar charts, and look for what remained the same (based on a similar progression from Etkina).  Again, we used the Red and Blue Pasco carts and tracks to see the collision and used the Air Track Simulation to collect quantitative data. To get a feel for how the conversation flowed, it’ll be helpful if you read a post from 2 years ago called Inventing Momentum.

What’s new this year is that I’ve created a worksheet to help students keep organized notes for the discussion (snapshots below).

Here’s the progression of collisions:

A 1-kg red cart moving right at 4 m/s collides elastically with a 1-kg blue cart at rest. We draw speed bar charts and we see that speed is constant before and after.

A 1-kg red cart moving right at 4 m/s collides inelastically with a 1-kg blue cart at rest. We draw speed bar charts, but see that total speed is constant before and after. Does this new pattern still work with the previous scenario?

A 1-kg red cart moving right at 4 m/s collides inelastically with a 1-kg blue cart moving left at 4 m/s. Total speed is NOT constant, but total velocity is constant. So now our bar charts become velocity bar charts. We draw the negative velocity bars downward. Does this new pattern still work with the two previous scenarios?

A 2-kg red cart moving right at 2 m/s collides inelastically with a 1-kg blue cart moving left at 4 m/s. Total velocity is NOT constant. But the total “mass-velocity blocks” are constant before and after (see Inventing Momentum).  So now our bar charts become mass-velocity bar charts. Does this new pattern still work with the all the previous scenarios?

A 2-kg red cart moving right at 3 m/s collides inelastically with a 1-kg blue cart moving left at 3 m/s. Total “mass-velocity blocks” are constant before and after.

A 2-kg red cart moving right at 3 m/s collides elastically with a 1-kg blue cart moving left at 3 m/s. Total “mass-velocity blocks” are constant before and after.

You can download a Word version of the worksheet: MomentumFindAPattern 2014

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