Past exam of the short options course of the University of Oxford / 2014 / 7 / b / iii / Solution

Differentiate each segment and express the four-velocity using a continuous rapidity:At $T$ both rapidities are $\omega T$; at $3T$ both are $-\omega T$. Hence the four-velocity is continuous at both junctions. This expression also gives $g(U,U)=c^2$ everywhere, so the parameter remains proper time on the later segments.

Away from the junctions,with ${\theta }^{\prime }$ successively $+\omega ,-\omega ,+\omega$. The rapidity is continuous but its slope reverses. Thus the four-acceleration has a finite jump to its negative at each junction, while its magnitude stays $c\omega$ on either side. There is no velocity jump; the idealized path is continuously differentiable but not twice differentiable at the switching events.