A steel tube \(0.75 \mathrm{~m}\) long, \(25 \mathrm{~mm}\) external diameter and \(20 \mathrm{~mm}\) internal diamameter encloses a copper rod of the same length and \(15 \mathrm{~mm}\) in diameter. The tube is firmly joined to the rod at both the ends and its temperature is raised by \(120^{\circ} \mathrm{C}\). Calculate the stresses in tube and rod. Also calculate the increase in length of the composite system and the external force required to prevent this increase in length \(E_{s}=210 \mathrm{GPa}, E_{c}=100 \mathrm{GPa}, \alpha_{s}\) \(=10 \times 10^{-6} \mathrm{per}^{\circ} \mathrm{C}, \alpha_{\mathrm{c}}=15 \times 10^{-6} \mathrm{per}{ }^{\circ} \mathrm{C}\)