On Saturday night I was in a pub helping a gay tribute band to set up, and for a few tenths of a second I found myself in an A-level physics problem. I was trying to find the end of a loudspeaker cable, and pulled the cable quickly towards myself, going hand over hand until I found the plug at the end. We can assume that the plug is heavy compared with the cable, that its mass acts as a single point, and that the cable doesn't stretch. It is trivial to show that once the plug is off the floor it will fling itself around my hands faster and faster as the cable gets shorter and shorter. Hint: analyse the moment of inertia of the system in terms of the length of the cable, then use the constant angular momentum to calculate velocity. Most pupils should also be able to explain that by pulling on the cable I was adding energy to the system which manifested itself as the speed of the heavy plug. What I didn't see coming, and I doubt that the combined intellects of Sir Isaac Newton, Albert Einstein, and Dr Sheldon Cooper could have calculated, was that when I had about a foot of cable left, and the plug was going very quickly indeed, it would strike me squarely in the bollocks.
Richard "Rowley Birken QC rang me at the weekend" B