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Education and outreach

Education and outreach

Amazing science demo one: The Vacuum Bazooka

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

This video is the first in a series of “five amazing physics demonstrations” presented by demo aficionado 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 to present each demonstration. Full details of how to access the print and digital issues are available at the bottom of this article.

The first demo is “The Vacuum Bazooka”, in which a vacuum cleaner is transformed into a missile launcher that can propel objects across a room. The video shows the front cover of Physics World being used for target practice, with Downie explaining how the experiment illustrates physics principles including air pressure, acceleration, ballistics, inertia and air flow down tubes.

The Vacuum Bazooka

So what’s this all about? This project redeploys the humble vacuum cleaner as a projectile launcher. Over the years I have made hundreds of Vacuum Bazookas, and thousands more have been made around the world by others. Some people have even used professional laboratory vacuum pumps and managed extraordinary feats such as firing ping-pong balls so fast that they will punch holes in beer cans.

What bits and pieces do I need? The Vacuum Bazooka can be swiftly plugged together from plumbing parts. In my favourite version, it consists of a long plastic tube, at one end of which you connect another long tube at right angles using a plastic T-piece. You’ll also need a suitable projectile, such as a cylinder of wood or a champagne cork, that can pass easily down the main tube, through the T-piece and out the end. Finally, you’ll need a reasonably powerful domestic vacuum cleaner (I use a Henry) that can reduce the pressure in the tube from atmospheric pressure (100 kPa) to 80 kPa or less.

How do I get going? Wearing safety goggles, set up your kit about 5–10 m from a target, such as a table placed on its side. Connect your vacuum cleaner to the end of the tube that’s at right angles to the first. Turn the cleaner on and then let a decent vacuum build up in the tube by placing a sheet of paper over the free end of the T-piece through which the projectile will leave. Reach over and insert your projectile into the far end of the tube, holding it tightly while you point the tube at your target. Wait for a moment and then let go of the projectile. Whoosh! It’ll zoom down the tube, fly out through the T-piece and whack into your target.

And what physics will I learn? The experiment reminds us that we live at the bottom of an “ocean” of air that creates an apparently preposterous pressure on our bodies. By removing the air from the tube, you can accelerate objects really fast because of the resulting pressure differential. Without the sheet of paper, however, the differential would be much smaller and the projectile would move only slowly – and probably jam the T-piece. In case you’re wondering, I’ve estimated that the cork will fly at a speed of (2P0L/zρ)1/2, where P0 is the pressure in the tube, L is its length, z is how far the projectile travels and ρ is the density of air. Why don’t you try checking the equation – perhaps by doing the experiment outside on a lawn? As for the sheet of paper, it rarely gets damaged – the puff of air just before the projectile exits the tube usually blows the paper harmlessly aside.

  • If you’re a member of the Institute of Physics (IOP), you can now enjoy immediate access to the April issue of Physics World with the digital edition of the magazine on your desktop via MyIOP.org or on any iOS or Android smartphone or tablet via the Physics World app, available from the App Store and Google Play. If you’re not yet in the IOP, you can join as an IOPimember for just £15, €20 or $25 a year to get full access to Physics World both online and through the apps
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