Physics: The Start of Momentum

physics class momentum transfer model image
In physics class, we have just come off the egg drop and are now transitioning to momentum.  We started with a day of introduction focusing on:
  1. what is momentum?
  2. the fact that we have been "secretly" using it all along as it lives in Newton's Second Law!
  3. what is a system?  why does your system matter?
  4. how do you solve problems using momentum?
We answered these questions together (the students documented the information in the space provided in the front of their freshly printed momentum packets), working through two foundational examples with the solving steps numbered and outlined.

This unit is intimidating.  There's a lot of vocabulary to understand (momentum  with a weird symbol, p  impulse, conservation, change in momentum, transfer).  Further, this is the first time since introducing free body diagrams that our entire approach to a problem has shifted from the foundation of the velocity vs time graph.  Of course, this graph is still there, but it isn't at the forefront as we are trying to practice new skills!

After the first intro day, the students arrived at our next session KNOWING that whiteboarding was coming.  In fact, they walked into the room asking, "We're whiteboarding today, right?" before dropping their bags or putting their phones on the counter.  I love how much they love whiteboarding.  On the rare occasion that whiteboards are not part of our plan for the day, their faces fall quite dramatically.  However, this whiteboarding was a little bit different.  I knew they weren't ready to solve problems yet.  While they absolutely paid attention to the introduction information, they needed time to process it.  I know this because, last year, I dove right into the problems and wow did they struggle.  I felt like I had to drag them through each scenario, repeatedly defining terms and reminding them of next steps.  This year, I really didn't want to rush them and make the solving process painful for everyone.

Following the notes section in the packet, I included questions solely about the vocabulary and concepts from the introduction day.  In groups, on their whiteboards, they used their notes and each other to answer the conceptual questions below.

sample of momentum packet questions physics
Once completed on the whiteboards and documented in their packets (see post on the whiteboarding process), the groups then moved on to solve one quantitative problem before the end of the period.

How did this go?  In some groups, the students used each other and their notes amazingly well, beginning to see the connections between the terms and asking thoughtful questions.  Other groups needed constant reminding to use their guided notes as a resource.  Some students realized they had decided not to write down chunks of information from the first day, thinking they would remember it because it made sense at the time or thinking it wasn't important.  This led to some good discussion about the importance of the notes (as we only do that type of class about once every three weeks or so).  Further, I understand more clearly who to send my notes to as an extra resource and whose packet to inspect more closely for accuracy on a regular basis.

Across the board (pun!), the students were much better prepared to tackle the quantitative problem that followed and much more comfortable using momentum terms in their discussions.  I was happy they got to work through many of their questions so early in the unit, and I feel I have a better sense of their understanding.

We still have a ways to go before they truly see momentum as a useful solving tool.  In fact, one of the problems early in the packet asks the students to solve for the net force on a ball during impact using momentum and then again using second law and a velocity vs time graph (the old way).  One of the students actually sighed happily at his velocity graph, wistfully saying, "I missed this way!"  I silently rejoiced at his words.  I honestly think I did a happy dance in my head.  I also hope he will feel the same way about momentum after a few weeks of solving!

These first few days of momentum already have me thinking about what I want to change for next year.  I'm planning on modifying the first few quantitative problems and not only have them solve for an unknown velocity, but also have them solve for the change in momentum of and the impulse experienced by each object in their system.  I want them to have to do this for each problem until they are almost sick of it and can't help but see that the change in momentum of an object IS the impulse experienced by it.  While this always seems tremendously clear to me (as well as a good chunk of the students), it isn't clear to everyone.  And me saying it doesn't help.  They have to experience it and realize it themselves!

This challenge of figuring out the ideal combination of environment and problems needed to bring the students to these skill-based conclusions has got to be one of the most fun aspects of teaching.  It keeps me truly excited for the next year!


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