Fancy some engineering fun this weekend? Neil Downie shows you how to make a machine that can be used to help sweep your floors without lifting the carpet.

What invisible vehicle is driven by an engine on the roof? A hovercraft, with propellers on the roof, maybe? Nope, we’re talking about a vehicle driven by wheels on its roof. And it’s invisible! Give up? It’s a brand-new conundrum of a machine called the ‘subrugmarine’.

You’ll need a fairly low-geared motor-gearbox unit for the driven wheels. A good speed to aim for to start with is 2-5cm/sec. Now we made the first subrugmarine with driven wheels underneath, like a normal car or truck. But it probably won’t work quite as well as putting the driven wheels on the roof. You’ll need a set of small floor wheels glued onto the chassis with the battery box, of course. The subrugmarine needs to be small, but in particular, the wheels should be small enough so that the rug above won’t droop onto the front or back – although you could add little rollers there to stop that.

Equipment needed for the ‘subrugmarine’

Now find a rug or carpet and clear out all those things you have swept under it. Put the subrugmarine well underneath, so that the carpet behind it is flopping down. With a bit of luck, it will move forward, and you will see a hump moving along the carpet until the subrugmarine gets to the far edge of the carpet. How fast does it go? Does it matter if the floor underneath is polished with little friction? Will it work underneath a rug that’s on a bigger carpet? Does it matter what kind of rug it is travelling under? What about a big blanket? And how fast can you make it go?

The subrugmarine relies, like any wheeled vehicle, on the laws of friction. They tell you that the friction that drives you along is proportional to the force – usually just the vehicle’s weight – at right angles to the direction of travel, ‘normal’ force. But the subrugmarine could be light as a feather and still drive along nicely and pull a few things behind it. That’s because it has the weight of the hump in the rug pressing it down, and that force goes on the drive wheels on the roof. For the same reason, it doesn’t matter if the floor underneath is very smooth with a low coefficient of friction – because of the large perpendicular force.

The hump that the subrugmarine makes is roughly a ‘normal’ curve, where the height is given by the exponential of the position squared – like the distribution of people’s heights or exam scores and many other things. The steepness of the hump in the rug or carpet is something that is rather difficult to predict. But rugs and carpets are more flexible than a thing of their thickness would normally be. That’s because they are made from separate tufts of coloured wool and polyester tied into a fairly thin sheet of coarse woven ‘burlap’ or similar backing. Sometimes there is a second thin sheet glued to the first, but both together are still thin and hence not rigid. That’s really the secret of why the subrugmarine works so well. A very stiff carpet would just lift like a sheet of thin plywood, which wouldn’t be as interesting.

Finally, what about a unicycle/pentacycle subrugmarine, one with just a single driven wheel on the roof? Worth a shot – but no cheating by putting a wide roller-like single wheel! Or, going the other way, what about a four-wheel drive subrugmarine – still with the wheels on the top? And finally, for those who truly believe that cleanliness is next to godliness, what about equipping a sophisticated subrugmarine with a small fan and filter bag? I guess you could call that a subrughooverbot.

If you liked this, you will find lots more fun science stuff in Neil Downie’s books, like ‘The Ultimate Book of Saturday Science’ from Princeton University, and for lots of other things (and a free copy of the ‘Exploding Disk Cannons’ book), visit www.saturdayscience.org. In line with this experiment, Neil’s current work includes developing a new ventilator system to support people with breathing difficulties – get more information on this great project at Exovent.org.

There is a back catalogue of STEM-related challenges from the past year to choose from if you are looking for more options. The IET also has a host of resources that adults can use to engage children with the world of STEM.

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