Archive | September 24, 2014

Day 16: Relative Motion


College-Prep Physics: This year I decided to bring relative motion into my curriculum. It’s a unit in Preconceptions in Mechanics, a book I used a lot last year for introducing different types of forces. My hope is that vector addition of velocities (which can be easily demonstrated, see below) will help some kids understand that vector addition of forces act the same way.

I modified the lesson cycle from the Preconceptions in Mechanics, Unit 2 Day 1 Lesson.

I started off the lesson showing the first 15 seconds of of this Japanese video in which a baseball is shot at 100 km/hr out of the back of a truck moving in the opposite direction at 100 km/hr (you could even do the first 3 minutes if you’re evil):

They’re hooked. “What happens?”

Next, I handed out the voting sheets. Here are the slides with my questions for each stage of the voting:

For the first vote, students write down their vote, an explanation, and a “makes sense” score. Then we share out responses. I don’t tell them the right answer, but just move on the the next voting question.

“WAIT! What happens? Why are you moving on?” they ask.

“Don’t worry, we’ll come back to that question later. But first I want you to consider these situations.” I say.

So we go through votes #2-#4 on the slides. After writing their vote, explanation, and makes sense score on the sheet,  we share out responses, and try to come to a consensus. After consensus is reached, I demo the scenario using buggies and a short Pasco dynamics track (pictured above). The track is clamped to 2 flat-top constant velocity cars from The Science Source, which have the same motors and wheels as the typical red and blue buggies, meaning they go the same speed. For questions 2 and 3, I use a slow blue buggy (1 battery) to represent Adam running east and west and fast flat-top cars (2 batteries each) to represent the faster train moving east.

The best is when we get to vote #4, in which Adam is running at the same speed as the train. So we use a fast red buggy (2 batteries) to represent Adam. The results were perfect:

Then vote #5 returns to original question: What’s the velocity of the ball as it leaves the truck? We share out, come to a consensus, and then watch the rest of the video from Japan.

I also follow-up with a short MythBusters clip in which they replicate the same experiment, but use a soccer ball instead of a baseball. Great results:

As a check for understanding, we did the HW sheet for Day 2 (not Day 1) in class. They knocked it out of the park, so I don’t think doing the Day 2 or Day 3 lessons from Preconceptions in Mechanics would be good use of time.

We didn’t do any of the voting questions about non-parallel velocities, and I don’t plan to with my college prep kids. If I did, I’d make that an entire lesson with its own set of voting questions, rather than stick it at the end of Lesson 1 like PiM did.


NGSS Science and Engineering Practice 6: Constructing Explanations and Designing Solutions