Conservation of Momentum

I’m not sure what other teachers have noted, but I’ve been surprised at the difficulty that students have when learning about momentum. I think that for a year and half I sort of assumed that kids found momentum an easy topic. You know, they take some mass and multiply it by velocity and bam! Momentum. $$\vec{p} = m\vec{v}$$

Except I noted some problems.

Now, this is where I’m supposed to drop some pearls of wisdom and reflection on you. Unfortunately I can’t quite remember the things I noticed last year that led me to believe the kids had a harder time with momentum than I anticipated. I think it boiled down to them not having much concrete hands-on experience with it. Yes, the formula is simple. But using a formula and linking it to a deeper understanding are two different things. Because of this, I wanted to implement a lab where the students had a chance to develop their own idea on what momentum and the conservation of momentum are.

A word of warning: if your schedule is such that the idea of conservation of momentum is covered in 30 minutes through direct instruction, this is not the place for you.

Because my school last year lacked enough motion sensors for 10 groups (each group needing 2 sensors), we couldn’t do a hands-on inquiry. Oh, and we didn’t have computers for hooking up the sensors anyways so there was that. Instead, I opted to implement a video analysis of collisions. I managed to record some OK-ish videos of different collisions and tested them out in Video Tracker. A proper analysis of them should allow the students to record decent data.

Warning #2: if your school has 6 year old computers that sucked even 6 years ago, you may want to re-think this. The first time I ran this activity in a computer lab, video tracker would run out of memory. In fact, the computers we were using weren’t even able to properly playback the HD videos of the collisions.

Below is the lab outline we used. It’s a bit cookbook-ish for me, but for many kids this analysis is not easy. I would allow two days for this, some students may finish in a day. The meat and potatoes of this lab is the CER section. I used the Claim/Evidence/Reasoning paradigm a few times in class prior to this lab, so my students were aware of what was being asked.

CartCollide.mov
Cart1Mass.mov
Cart2Mass.mov Explode.mov
ExplodeAddedMass.mov
HeadOn.mov

If you’ve tried something like this with momentum, I’d love to hear about.  I’d also like to hear back and find out how things went if you use these resources.