(a) Repeat Prob. 12.2 using the (incorrect) definition p = mu, but with the (correct) Einstein velocity addition rule. Notice that if momentum (so defined) is conserved in S, it is not conserved in S. Assume all motion is along the x axis.
(b) Now do the same using the correct definition, p = mη. Notice that if momentum (so defined) is conserved in S it is automatically also conserved in S. What must you assume about relativistic energy?