Question: why does plastic get hot when it is bent repeatedly

Simon Langley-Evans answered on 11 Nov 2013:

When you bend a stiff plastic (a ruler) the molecules in the plastic will resist you. Bending the plastic requires you to apply a force which means you transfer mechanical energy. The resistance to bending means that some of the energy you apply is turned to heat and that is why the plastic will get hot. A soft plastic (cling film) doesn’t heat up because there is less resistance in the plastic, less energy is needed to bend it and less energy gets converted to heat.