An unknown weight falls throught \(10 \mathrm{~mm}\) on a collar rigidly attached to the lower end of a vertical bar, \(3 \mathrm{~m}\) long and \(6 \mathrm{~cm}^{2}\) in section. If the maximum instantaneous extension is known to be \(2 \mathrm{~mm}\), what is the corresponding stress and the value of unknown weight? take \(E=200 \mathrm{GPa}\).