Face to Face + Progress on Momentum

Thomas and Robin
Thomas and Robin

I had found an excellent purveyor of T-shirts and as a joke got a couple made up with “I’m at the bottom of the exponential” for Robin and I to wear. This necessitated a visit to his house so we decided to have a face to face chat (so far we have done them virtually using zencastr). Moving a laptop to his living room, plugging in another microphone and getting it all to work proved something of a challenge, even for two physics teachers. Once we got it working we realised that my voice came out pretty poorly at times. Hopefully it is not too bad in the final podcast.

Recording face to face was another challenge. Usually we have an idea of what we are going to say, but this was pretty freeform, and there was the tension of having a chat with a good friend with the need to get some content you would be prepared to share.  Our numbers keep growing so we must be doing something right.  (Geeky aside: I worked out the URL to get regular stats from the excellent BluBrry podcasting service/WordPress plugin and have made a beautiful graph for Robin and me of “listens” that gets updated every 6 hours. It tries to fit a linear and exponential to the line and currently is a close fit (listens vs days) with exponential.)


I went for a long bike ride on Sunday and this gives you a long time to think. I mulled over a way of teaching momentum quantitatively using pea-shooters. I thought about a gas rocket I built once and how it could be converted in to a large pea shooter. Maybe if I made very simple rockets out of one sheet of A4 by rolling it up and stapling the top shut I could explore impulse by shooting them vertically? I then remembered that the department actually has its own pressure gun for exactly this that was used with Year 7 a few years ago for a rocket challenge. Happy days.

So, if I assume the force is constant then the impulse given to the rocket is Ft = Δmv. F and m are constant so this shows v ∝ t (just as v = u + at does of course, a is constant too from Newton’s 2nd Law, F=ma). You can use suvat  to show the length of the rocket, L ∝ t2 ∝ v2 and then in exactly the same wayto show that the height the rocket flies, h ∝ v2 which means that h ∝ L !  At least, that is the theory. I am very aware of Ben Rogers’ ideas about cognitive science and not overloading the students, so this will take a little more thought before Wednesday.