# Yuk physics

By Marcus Wilson 13/07/2010 2

What’s small, brown, damp, disguisting, goes round in circles, and is still alive?

Answer: A cockroach in a washing machine.  Finding one in amongst the sheets as you haul them into the laundry basket isn’t very pleasant, I can tell you. Unfortunately, in the northern North Island, the Gisborne cockroach (an Aussie import) is rather ubiquitous and you just have to get used to having them turn up just where you don’t want them.  Our useless ball of fur that masquerades as a cat isn’t much help – he’s never shown much interest in racing across the floor and intercepting one as it scuttles from under the cooker to under the fridge, though he is quite adept at racing across the floor and intercepting a box of cat biscuits before it reaches his food bowl.

Cockroaches are nigh-on indestructible. The rather soggy specimen I pulled from the washing machine had evidently not only survived the washing, but also a spin. That’s pretty impressive. A rough calculation of the centripetal acceleration in a washing machine spin is fairly simple:  At say about 400 rpm for a top load spin, we have an angular velocity of 2 pi  times 400 rpm / 60 s min-1  which gives us about 40 radians per second. Acceleration is then angular velocity squared times radius of the spinning object (maybe about 30 cm), giving 40 s-1 times 40 s-1 times 0.3 m or about 500 m s-2.  Compare that to the acceleration due to gravity of 10 m s-2. The cockroach is experiencing about 50 G of force.

Imagine if you weighed 50 times what you do now?  50 times 65 kg is 3.5 tonnes.  If that mass were placed on you I doubt you’d survive very long  (A large car is about 2 tonnes in weight). But the cockroach – that’s another story.  This one did loose a couple of legs in the process, but he was still kicking with the remaining ones at the end of it.  Yuk.

### 2 Responses to “Yuk physics”

• The thought of a cockroach surviving merely losing a couple of limbs after it was exposed to 50 G of force made me think of JBS Haldane’s very cool essay on the way the physics of biology changes with the size of animals. Which includes this memorable line:

You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft. A rat is killed, a man is broken, a horse splashes

• Marcus Wilson says:

I appear to have misrepresented the kittykat. A couple of days ago I heard a miaow (quite unusual for him) and turned round to see him sitting on top of the piano (where he shouldn’t be, and he knows it) looking intently at the ceiling. When my eyes went upward, there was a cockroach straight above him. I knocked it onto the floor and then cuddly little pussy cat turned into the terminator as he hunted it out and removed it from existence. Maybe he is useful after all.