Large, heavy objects

By Marcus Wilson 13/01/2010

Our cat got the shock of his life a couple of days ago when the washing machine got up and chased him out of the room. It’s not often that inanimate objects start walking on their own accord. Poor cat is probably so traumatized he’d never set foot in the laundry again except for the fact that’s where his food bowl lives.

I got a bit of a shock too, but I could see it (or, more accurately, hear it) coming.  It was simply a very unbalanced load that the machine was beginning to spin. The effect is basically conservation of momentum and angular momentum. The machine on its own has no external forces on it (ignoring those  through its feet). Therefore its total momentum should remain zero.  Inside the machine there is a large mass of damp clothing that is spinning off centre, and the reaction this has on the machine makes it wobble – overall the momentum stays the same.

The forces on the machine due to the floor rather complicate things, and  exactly what happens depends on things such as the coefficients of friction between each foot and the floor. But all that wobbling may be enough for the machine to start walking, which is what it did in this case.

The easy fix is of course to stop it and redistribute the load more uniformly around the drum.

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