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Education and outreach

Education and outreach

Amazing science demo two: Newton’s Three Laws Cannon

08 Apr 2015
Taken from the April 2015 issue of Physics World

This is the second in a series of “five amazing physics demonstrations” presented by science-demo guru Neil Downie. In addition to his day job in industrial science, Downie has run Saturday science clubs for children for more than two decades, during which he creates fun and innovative science demonstrations that are all simple and quick to carry out.

In a special feature in the April issue of Physics World, Downie describes his five best demos of all time, all of which use everyday equipment to illustrate fundamental physics concepts. In the article, Downie describes how his fondness for the five experiments comes from the fact that, with a bit of creativity, each one can be easily adapted to explore physical concepts further. In the digital edition of the April issue, each demonstration is accompanied by a video in which Downie walks you through how you would present each demonstration to an audience. Full details of how to access the digital edition are available at the bottom of this article.

This second demo from the series is “Newton’s Three Laws Cannon”, in which Downie manages to show all three of Newton’s laws in action. Students of all ages are bound to love this one, as you’re creating a projectile launcher that creates a satisfying “Pop”. In addition to illustrating Newton’s laws in dramatic fashion, the demo can also be developed to discuss other physics concepts such as momentum conservation, air pressure and energy in compressed gas.

Newton’s three laws cannon

So what’s this all about? This project involves using compressed gas, stored in a plastic soda bottle, as the power source to accelerate two projectiles in opposite directions along a pipe and out through the two ends. The projectiles can be lumps of carrot or champagne corks weighted with a little modelling clay on their noses. This project is a beautiful way of illustrating all three of Newton’s laws of motion in one go.

What bits and pieces do I need? You need two lengths of 34 mm-diameter plastic plumbing tubes joined end-to-end with a T-piece in the middle. The soda bottle has to be adapted by fitting a one-way car-tyre valve into its base. The valve lets you pump air into the bottle without it coming out. You’ll then need to make a large hole in the bottle’s screw cap and stick a piece of tape over the top of the open bottle, before screwing the cap back on. You’ll also have to drill a small hole in the T-piece so that after you’ve fitted the bottle to it, you can get to the tape in order to pierce it with a pin, releasing the compressed air from the bottle into the pipe. Now arrange two large cardboard boxes – stuffed with screwed-up paper or packing material – facing the two free ends of the pipes. When the projectiles fire out of the cannon, they’ll therefore end up somewhere safe.

How do I get going? Using a bike- or car-pump, start pumping air into the soda bottle until it reaches a pressure of 4–5 bar. Then plug the bottle into the T-piece, fixing it into position with tape. Next, use a retort stand and clamp to hold the two pipes, T-piece and bottle in place. Finally, place your two projectiles in the opposite ends of the pipe before pushing each of them right down to the T-piece with a stick. Now don your safety goggles, pierce the tape with, say, a pin – and boom! With luck, the two projectiles will simultaneously whizz out and blast into the stuffed cardboard boxes, but the cannon itself will not move much. If you repeat the experiment with only a single projectile and the other end of the pipe blocked, the cannon and stand will recoil backwards.

And what physics will I learn? The fact that the cannon does not recoil when you fire two equal-mass projectiles shows that for every action there is an equal and opposite reaction, which is Newton’s third law. The projectiles get accelerated thanks to a force derived from gas pressure – Newton’s second law. Once they leave the muzzle, the projectiles both fly out at pretty much a constant speed, which is Newton’s first law.

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