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Our cars (one of them in particular) are beginning to cause a few too many problems. Last night we were left stranded again, just outside Cambridge, and had to have those friendly AA guys rescue us. This time there was a sudden "Thwunk" sound and then a rapid "fwap-fwap-fwap" from underneath the bonnet. We pulled over to investigate. On opening the bonnet it didn’t take long even for a mechanical-phobe like me to spot the problem. We’d shed a drive belt (well, at least one), which seemed to have got itself mangled and around other belts. It was kind of like black rubber shredded spaghetti.

The garage are working on it as I write – they advise that belts usually don’t fly off of their own accord and quite probably its because one of the units being driven has failed in some way. Will find out shortly. Technically, I guess, we weren’t stranded, in that the engine would still go, but it was hard to ascertain what other systems had and hadn’t got power, so best not to risk it, especially down SH1.

What happens to a drive belt when it breaks suddenly (say) would make a nice problem to discuss in a first year physics class. There’s lots of physics principles buried in it. The belt is under tension, so there’s a force here. Then it’s rotating fast, so there is the angular momentum to think about. Possibly it’s been knocked sideways too, and there could be linear momentum to consider. And of course there is gravity. Which effects dominate the behaviour? It might make for an interesting discussion.

It will be interesting to find out, once the spaghetti has been untied, just how many belts (or parts of belts) we have in the car. That is, did we lose one onto the road (i.e. did gravity win) or are all belts accounted for?