Summer STEM Challenge: Driving and Sweeping Under the Carpet

                        Can you fancy some engineering fun this weekend?  Neil Downey shows you how to make a machine that can be used to help clean floors without lifting the rug.
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                    <h2>STEM Challenge #57: The Submarine</h2>

What is the invisible car that is driven by a motor on the roof? A hovercraft, with propellers on the roof, perhaps? Nope, we are talking about a vehicle with wheels on its roof. He is invisible! Give up? It’s a whole new puzzle for a machine called ‘subrugmarine’.

You will need a gearbox unit with a low gear motor for the driven wheels. A good speed to start with is 2-5 cm/sec. Now we made the first marine sub with wheels driven under it, like a regular car or truck. But it probably won’t work as well as putting the sprockets on the roof. You’ll need a set of small floor wheels affixed to the chassis with the battery box, of course. The superugmarine should be small, but in particular, the wheels should be small enough so that the carpet at the top doesn’t droop forward or backward – although you can add small rollers in there to stop that.



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              <p class="image-caption">Equipment for "subrugmarine"</p>
                             <p class="image-credit-macro">Image credit: Neil Downey</p>
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Now find a rug or rug and remove all those things that you have vacuumed under. Place the superugmarin well under it, so that the mat falls behind it. With a little luck, it will move forward, and you will see a hump moving along the carpet until the superugmarin reaches the far edge of the carpet. How fast do you go? Does it matter if the subfloor is polished with little friction? Will you work under a rug on a larger rug? Does it matter what kind of rug he’s traveling under? How about a big blanket? And how fast can you make it go?

Subrugmarine, like any wheeled vehicle, is based on the laws of friction. They tell you that the friction pushing you along is proportional to the force – usually just the weight of the car – at right angles to the direction of motion, the “normal” force. But subrugmarine can be as light as a feather and still go well and pull a few things behind. This is because the weight of the bump in the carpet is pressing it down, and that force goes to the driving wheels on the surface. For the same reason, it does not matter whether the subfloor is too soft with a low coefficient of friction – due to the large vertical force.



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                             <p class="image-credit-macro">Image credit: Neil Downey</p>
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The camber made by the subrugmarine is roughly a “normal” curve, where the height is given by an exponential square of the mode – such as the distribution of people’s heights or exam scores and many other things. The steepness of the hump in a rug or rug is hard to predict. But rugs and rugs are more flexible than anything they’re usually thick. This is because they are made of separate tufts of colored wool and polyester tied to a fairly thin sheet of coarse woven “burlap” or similar backing. Sometimes there is a second thin sheet that is glued to the first, but both are still thin and therefore not rigid. This really is the secret to why subrugmarine works so well. It can lift a very stiff rug like a sheet of thin plywood, which wouldn’t be fun.



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                             <p class="image-credit-macro">Image credit: Neil Downey</p>
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Finally, how about a unicycle/subrugmarine five-wheeler, which has only one driven wheel on the roof? Worth a try – but don’t cheat by putting one wheel as wide as a roller! Or, in the other direction, how about a four-wheel drive subrugmarine—with the wheels still on top? And finally, for those who truly believe that cleanliness is next to piety, how about outfitting an advanced Superogmarin with a mini fan and filter bag? I think you can call this bot a subrughoverbot.

  

If you like this, you’ll find a lot of fun science stuff in Neil Downey’s books, like The Ultimate Book of Saturday Science from Princeton University, and for many more things (and a free copy of Exploding Disk Cannons), visit www.saturdayscience.org. In line with this experience, Neil’s current work includes developing a new ventilation system to support people with breathing difficulties – get more information about this great project at Exovent.org.

There is a back catalog of STEM challenges from last year to choose from if you’re looking for more options. The IET also has a range of resources that adults can use to engage children in the world of science, technology, engineering, and mathematics.

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