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Kicking a Rock on the Moon

Physics for You

In Episode 6 I mentioned a thought experiment with a concrete-filled rugby ball. I have used this model for years and knew it came from a text book long ago and I have since been searching for it. I was convinced it came from Peter Warren, the head of department where I did my teacher training and author of one of the books that was used to teach me back in the early 80s. It turns out that Pete is still teaching, but did not draw the cartoon.

It actually came from Physics for You by Keith Johnson, and it is still available, new from Amazon in the 5th addition. I actually own a Johnson (it has sat on a shelf in my office untouched for years) and did not think to look in it until I had sourced a couple of Warrens from amazon.  The character kicking the rock (in this case) is Professor Messer.  I assume Johnson drew them all himself, as did Pete in all the cartoons in his books.  I excitedly looked up @ProfessorMesser on twitter to see if he had an online presence but it turns out to be a real person.

One of the Warren books I sourced is a gold mine. It is the advanced physics laboratory book and this would be a super resource for anyone new to A level teaching. I got it for 20p just a week ago but a quick check on amazon shows it is now £28+! It contains many practicals that I consider routine but has given me some new ideas too. For example, my school does not have a position sensor/force sensor etc. so I have not been able to datalog damping. This book has a beautifully simple way of doing it: attach a magnet to a hanger boinging on a spring, bounce it up and down inside a coil and measure the current in the coil. 

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Entries! and Momentum Round-Up

We put out the competition not really knowing quite what to expect. Four entries before 8:15am was an exciting start to the day though as I write at 4:30pm it is still four, which has brought me down to Earth somewhat!

Momentum Day

Today was “Momentum” day. After talking with Ben Rogers about cognitive Science (that is in the next episode) I changed the demo away from play (the students making the rockets) and in to a demo so they could concentrate on the key idea of impulse (Ft) being more for the longer rocket, and hence v being higher. I made four rockets myself before the lesson. Showing them the rockets was actually a good way to mention to them that a vernier can be used for internal diameters too. I tried to make all the tubes the same diameter, but it was pretty hard and I think this led to the inevitable inconsistencies in the heights.

There was much uncertainty! The pump was definitely pretty rough at the low pressures I needed to keep the long one below the ceiling, the diameters of the rockets and the release of pressure through the valve all affected each launch. The results were not quantitative at all (my main aim) but it was clear that the longer went higher (on average).  I think if I made better rockets on fixed tubes it would be more successful, it is certainly worth pursuing. More massive ones would allow more pressure too.

But as a learning demo it was very good. The idea of the force being constant as the rocket launches and the longer rocket being in contact for a greater time made it easy to imagine impulse and relate it to velocity. At least I thought so. Time will tell.

An Unexpected Misconception

What surprised me was that one person thought that the smallest rocket would go the highest. I’d said that all were made from one sheet of A4 so all were the same mass, but suspect that they were thinking “small is light”.

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Interact and Win a T-Shirt!

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Now we have listeners, and an Instagram page (@physics_teaching_podcast) we thought we would encourage you to share the podcast by having our first competition. Join us at the bottom of an exponential by winning a beautiful podcast T-shirt (in the colour of your choice) by interacting! To win, tell us why you like listening. There are many ways to do this:

Thomas and Robin will pick a winner in a couple of weeks.

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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.)

Momentum

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.

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Squeezing it all in + pondering momentum 😖

This week’s episode was a bumper edition, and we had to work hard to keep it down to just under 27 minutes. I really wanted to put in the Practicals In Memoriam (PIM) section and this led to cutting down the interview. Robin and I have talked a few times about the ideal length for the episodes, and hit on 20 minutes as short enough for a commute, and short enough to listen to a few on the bounce when you find the podcast.

As our production skills improve longer ones are easier to do and each podcast has been longer than the previous, but I think we will aim to keep them short. On reflection we felt that this Episode 5 was rather “busy” and maybe we should have kept back the PIM for another day. It deserved a whole podcast of its own maybe.  But when you are aiming to hit a deadline you have to make a decision and run with it. We were still tossing it back and forth on Wednesday night.

Pine nut shooters (straws)

In other news I asked Robin if he had any ideas for a class practical that could explore momentum. I wheel out the air track every year, but it is not very inspiring. He came up with pea shooters and impulse (Ft). I would really like to do something quantitative so my idea from this is you propel the pea (in my case pine nut) as hard as you can and see how far it goes as you reduce the length of the straw.  Assuming that the force is constant whilst the nut is in the straw, then the acceleration will be too and the time in the straw will be proportional to the square root of its length (suvat). The impulse is the change in momentum, so the velocity will also be proportional to the square root of the straws length and the distance the nut travels will be too. That’s my theory, but it seems over-complicated and could be done with suvat without any reference to impulse at all!  I am still humming and hahing about whether to use it.

An alternative would be Stuart’s practical, rolling balls down the slope in to cups. From suvat I can get the students to show that velocity is proportional to square root of the distance up the ruler. But where do I go from there? Pre-schoolers could tell you the cups will go further if the ball rolls faster. If I keep the velocity the same then heavier cups don’t go as far, but again, there is no obvious momentum related quantitative results I can gather. I did use Stuart’s experiment at Open Evening yesterday. IT worked well until the volunteers got excited about monkey-hunter!

I think I will be rolling out the air-track and doing the pea shooters qualitatively. 🙈