Making a Vacuum Cannon

The Tube

The tube does not need to be super strong (the tape will fail long before the tube), but it does need to be close in diameter to a table-tennis ball (40mm). The right tube is known as Imperial 1½ inch PVC, common in the USA but not so much in the UK. I collected it from Koi Logic for £5.05 a metre since delivery is quite expensive and it is not too far away. CLEAPSS suggest the plastic pipe shop where it is about the same cost but you get a longer length (so more expensive and a lot of waste).

1m of tube is fine, it is easy to transport and store. You can get tremendous speeds from a 1m tube. CLEAPSS recommend that you do not go longer than 1.5m. I chose 1m because above that, the delivery charges get silly.

The valve and its Hole

You need a way to connect the vacuum pump to the tube. Patrick Kaplo sent me a link to the hose elbow he used in the USA and CLEAPSS suggests a metal schrader valve that you just self tap (force in to the PVC) and glue in to the pipe. Their instructions here are a bit vague on the size of drill bit you need. A very unscientific poll of physics teacher friends suggested that they all had rubber hoses on their vacuum pumps so the elbow is much more sensible.

Being a bit of a perfectionist I thought that tapping it in to a tapered thread would allow for cheaper components, a better seal and for replacing components if needed. I found some cheap nylon fittings at Wreking Pneumatics, all they needed was a correctly threaded hole.

For pneumatics I learned you really should used tapered thread (BSPT) holes to get a good seal with no need for tape, glue or sealant. Tracy tools are helpful on the phone and sell reasonably priced tap, dies and drills. I wasn’t sure what would work best so bought a plug tap and the correct size drill (8.4mm for the ⅛” plug tap) for both ¼” and ⅛” tapered thread holes. The prototype has ¼” BSPT but I learned that the schrader valves suggested by CLEAPSS and potential vacuum gauges all use ⅛” BSPT so the ¼” tap I bought was redundant. (I actually bought another but it took so long to come in the post via amazon I gave up on it; Tracy Tools delivered within 24 hours of ordering.) Tapping PVC is a doddle compared to steel.

The Tape

*just* an overlap

The tape was the cheapest “three inch” (78mm wide) tape I could find on eBay, just bought in bulk. The prototype used “two inch” and worked, but barely covered the width of the tube (see photo).

The Balls

The balls are practise balls from eBay, bought in bulk. “Real” balls are much stiffer and about twice the mass. I bought 300. Softer balls expand a little in the tube I think, so possibly get more push, and being lighter can accelerate faster? This is worth investigating perhaps?

No Flanges 😥

Flanges make a great stand for the cannon and provide a good surface for the tape to adhere too. The best I could find was £8 a flange, but that would almost double the cost of the cannon. Robin and I tested the cannon with no flanges with very good results. My tech department whipped two up for free with no problem and you can make basic ones out of cardboard or 3D print them. There are many options. Our tube’s diameter was 48mm (Officially 48.26mm according to the Standard). This is a good opportunity for the school to personalise their cannon!

Adding a Pressure Gauge

I found pressure gauges on eBay for £7 including postage. They come with a ⅛” BSPT (taper) male and screw straight in to another ⅛” tapped hole. Just search for:
1/8″ BSPT pressure gauge.

If you make one, do let us know how you get on:

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Selling a Vacuum Cannon

We are selling Vacuum Cannons in the shop. I am setting it up with some trepidation: if nobody buys one I will be several tens of pounds out of pocket. If everyone buys one I will be unable to make them fast enough to keep up with demand. Of course, the most likely thing is somewhere in-between. Being part-time I can respond quite fast, and I can limit the number I have to make and dispatch by setting the stock levels in our shop software. And through the piss-poor planning you have come to expect, we are releasing them a day before I go away for a week over half-term. Oops.

The pay-off is that more people will get to play demonstrate with this fantastic toy piece of equipment. I had never heard of it, nor had Robin, but CLEAPSS are all over it so it must be pretty well known. When I tested the prototype in the kitchen I didn’t really think through the consequences of a ball travelling at several hundred miles an hour vs my kitchen wall. There was ball debris everywhere; I found one shard 5m away on a high window sill two weeks later.

I have set up stock of 2 in the shop but with unlimited back orders. Once I know what the demand is (or indeed if there is any!) I will start making and shipping them in batches. I intend to keep doing them at £20 until I lose the will to go through all the guff of making, packing and dispatching them. Then I will either give up or raise the price. So buy now to avoid disappointment.

Anyone with access to a vacuum pump can of course make their own. If that is your plan, you could use the CLEAPSS guidance I mentioned above. I also made my own “How To” notes which includes links to where I purchased things.

If you make one, please let us know how it goes.

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Using The Rope Model of Electricity

In the next podcast we inevitably talk about the rope model. I tried it a few times in the past and hated it 😝. It was only in making the podcast that I finally understood how to do it and how good it is. Not knowing how to do it is as much a function of my comfort with the donation model as it is my being alone in my school with no other physicist to talk to. But talking to Robin and Stuart about electricity really got me thinking. 

When I did the rope model it didn’t work because I had quite a few kids involved in the demo. I found it really hard to make the rope run smoothly through their hands (not least because the rope had a huge knot in it!) and it was a very static sort of demo. There was no fluidity at all, no flow. I had visions of attaching pulleys to the walls of the room to make it work (I like big scale). I chatted to Stuart about this and he was able to tell me a way to do it that worked. It works so well, and is so easy to do. I was bowled over.

my recipe for an effective rope model demo:

  1. Have a rope that is in a 3-4m loop where the join is as smooth as possible. (Cutting and melting together a rope is ideal).
  2. Choose one student only.
  3. Hold the loop and pass the other end to the student. Tell them to grip it lightly (they don’t want rope burns) in one hand with the rope passing vertically down through their fist (this is the detail I never understood – one student, one hand).
  4. Explain that the rope is the electrons and the grip is the resistance.
  5. Pull the rope hand over hand through their (one, stationary) hand.
    • Question: What do you feel in your hand? 
    • Answer you want: Warmer
  6. Tell them to add another hand, holding with the same light grip. The rope will get harder to pull, make this obvious, it will naturally run more slowly.
    • Question: why is the rope moving more slowly?
    • Answer you want: because there is more resistance.
    • Question: how can I increase the current to make it move as fast as before?
    • Answer you want: Pull harder.
    • Explain that this means more energy being delivered by the cell to the rope, or greater pd.
  7. Pull harder, to make the rope move at the original speed again.
  8. Tell them to grip tighter with one hand, but not to tell you which hand that is.
    1. Question: what has happened to the rope speed?
    2. Answer you want: slower.
    3. Question: does the rope/the pd know which hand is gripping harder?
    4. Answer you want: no.
    5. Question: what is happening at the tighter hand?
    6. Answer you want: warmer.
    7. Explain how this is energy transfer
    8. Question: are the electrons in any way different before and after the hands?
    9. Answer you want: no.
    10. Remind them that electrons just go where they are pushed/pulled by the pd, they aren’t changed, they don’t make a choice, they just go where they go.
  9. Get another rope, get them to hold one loop in each hand, but you pull them together. You can model parallel with this but I wouldn’t go too far as the model does tend to break down a bit.
  10. You can show AC nicely too. Remove one loop then tell them to hold their fist horizontally, not vertically. Now grip the loop in both your hands and pull it backwards and forwards. The hand gets warm just the same.

Good luck with this. Maybe report back in the forum?

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Somebody Out There Likes Us

Robin and I started with very limited ambitions, it wasn’t quite “more listeners than presenters” but not far off. We agreed to do episodes until the end of the academic year and then see where we stood. At that point we had not got much further than me telling Robin he and I should start a podcast and then months later him finding the statistic that about 50% of schools in the UK have one or zero physics teachers. We just wanted to try something, try to build a community and see what happened. I am a regular listener of The Cycling Podcast and modelled a lot of our approach on them.

As we started I had no idea about format, content, approach, social media, iTunes ratings, post production, how to talk to a microphone and had never considered a T-shirt or shop (or adding a forum) But I am top loaded with running web sites and general geekery, so I figured I would muddle through.

It was a revelation to me (physics teacher, but never an audiophile) to find that a microphone has preferred orientation. Listening back you can tell the episode where I worked out where to point my mouth when recording.  I found a great T-shirt supplier when looking for somewhere to buy a jokey shirt for Robin. They actually make the shirts on demand and provide a WordPress plugin, so one thing led to another and now… a shop! Just a bit of fun of course, as Robin says in Episode 9, this is nothing about money… But if it helps build a community then that’s brilliant.

As I went to sleep on the night before episode 1 I was not convinced I wouldn’t grab my phone at 4am and cancel it. Now, as we approach the recording of episode 10 I feel more comfortable hearing my own voice. I’ve also been dragged kicking and screaming in to social media and I feel a community is growing. (We have had a few emails thanking us for doing it and the interaction on Twitter has been fun). It has also been hugely enjoyable talking to different people and learning from them: I recorded another PIM this week which will most likely be in Episode 11. Please do continue to send us ideas. We love hearing them all. You can use the form below.

Finally, to get some 5* ratings in iTunes is pretty stunning to be honest, so thank you very much, listener(s!).

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Refutation Text for the Jelly Chair

I said in Episode 6 I would share what I came up with for the Jelly Chair lesson and refutation texts. I did put some thought in to this but think that there is room for improvement. I ended up writing a few sentences for the kids to do:

  • The ancient Greeks thought that arrows were pushed through the air by the god Apollo. We know this is wrong because…
  • Many people think that objects will slow down and come to rest if no resultant force acts upon them but really…
  • A typical GCSE level understanding of balanced forces allows a person to sit on a chair made of jelly. This is false because…
  • Galileo showed that two balls of different weights accelerate at the same rate when dropped.. Before this people thought heavier objects fell faster but this is wrong because…
  • Some people think the International Space Station has no forces acting upon it because it is in Space, the evidence that they are wrong is…
  • When a helicopter is hovering it is very common for people to think that the weight of the helicopter has an equal and opposite force that is the down-force from the rotors. This is wrong because…

I was particularly happy with the last one (though I have no idea if that is an actual misconception or not), and even happier when one of the students said something like:

The down-force is equal to the weight!

They had truly understood the situation, and was able to explain to their partner that the weight and down-force must be equal in magnitude and in the same direction as weight if the helicopter is hovering (not equal and opposite).

As I roamed the class and spoke to the students I was pretty confident the refutation sentences were having impact. The challenge is to use them again and again and embed them in my practice.

The podcast strikes again. Thank you Ben Rogers!

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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”.

Interact and Win a T-Shirt!

T-Shirt logo

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.)


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.

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. 🙈