Inertial forces

A thick cord hangs below a block of wood, which itself hangs by a thin cord. If you quickly pull on the thick cord, the thick cord breaks and not the thin cord. This is because the inertial mass of the block of wood protects the thin cord. However, if you pull on the thick cord slowly, the thin cord breaks. This is because, in this case, the weight of the block of wood and the tensile force both act together on the thin cord.