Summer STEM Challenge: Whack-a-Rat and the Rat-O-Matic

The summer holidays are fast approaching. And while restrictions in the UK will be lifted in the weeks to come, there’s no reason why all those engineering families out there should stop engaging in interesting and fun DIY projects to do in the comfort of their own homes.

So, continuing the challenges we set over the lockdowns, we present the Summer STEM Challenge to further encourage creative and practical thought in the summer (and hopefully warmer) months.

This week, Lockdown Challenge’s Neil Downie turns physics into what looks like magic with just a tube and a brush, with googly eyes.

STEM Challenge #47: Whack-a-Rat and the Rat-O-Matic

A magician puts a rat-shaped brush into the bottom of a drainpipe, and then, holding the pipe near the top, hits the top industriously with a hammer. The rat tail disappears inside the pipe, and after a minute the brushy nose pops up at the top. How did that happen? Try it yourself to prove it isn’t magic.

You can make a Whack-a-Rat with a bottle brush that is just a little smaller than a pipe in diameter with something heavy on its tail. With each hammer blow, the pipe goes down suddenly, then back up rather less suddenly. Maybe this is what makes it work? When the pipe goes down suddenly the mass – the inertia – of the Rat makes it stay where it is while the pipe goes down. When the pipe goes up slowly, the Rat goes up with it.

Or maybe not. When you push it up the pipe, the bristles end up pointing slightly downwards. Maybe the magic is in the downward-pointing bristles? The friction might be greater downwards than upward, so as the pipe jiggles, the Whack-a-Rat will go up.

But which effect is it, the sudden down/slow up, or the bristle asymmetric friction? Try taking a video in slow-motion of the hammering. Or try hammering a pipe on the floor from one end and seeing what happens to something like a brush or wood block which fits loosely inside. Test asymmetric friction by tying a string to a bottle brush. Push it up the pipe and then measure how much weight you have to add to make it shift. Then test again, having pulled the brush down the pipe first. How different are the test results?

You can make another neat rat-up-drainpipe gadget which definitely uses the asymmetric friction effect: the ‘Rat-O-Matic’. You can make a Rat-O-Matic by connecting two bottle brushes with a motor with a crank wheel as shown in the picture. The result is a strange ratty creature which, once poked up a drainpipe, will crawl up it.

The motor needs to have a gearbox of about a 100:1 ratio. And the brushes must be a little larger than the drainpipe, of course. To complete its ‘rattiness’, add a tail and some eyes (maybe LEDs?) With a bit of luck, your ‘Rat-O-Matic’ should be able to move by about the radius of the crank with every turn of the motor. Once inside a tube all you see is its tail disappearing, and then scratchy and whirring sounds as it creeps creepily along, until its little brush-whiskers come poking out at the other end.

Brush friction has been studied scientifically, being important for some machines. The asymmetric friction of the bottle brush depends upon the angle the bristles are bent back. At right angles to the pipe wall, there is zero asymmetry. And as the bristles are increasingly bent over, they get more asymmetric, at least until they are squashed flat. With different diameter pipes, you can measure the friction asymmetry at different bristle angles.

How do you make a Rat-O-Matic faster? Higher speed motor? Use the bigger bottle brush to increase backward friction? Or pour in water or oil to make it slide forward more easily? And now you know how to make high-speed Rat-O-Matics, what about Rat-O-Matic racing?