Past exam of the mathematics course of the University of Cambridge / 2005 / iii / Paper 66 / 2 / i

Put $U=B_0-A_0$, $T=D_0-C_0$, $D=A_0-C_0$ and $v=V-W$. A point on each moving segment has the form $A_0+sU+tV$ and $C_0+rT+tW$, with $s,r\in [0,1]$. Contact is therefore equivalent toThe contact time of translating line segments is the minimum feasible $t$. This reduces the entire continuous-motion problem to three linear equations and linear inequalities.

In three-dimensional space form the matrix $H=[U,-T,v]$. If it is nonsingular, solveusing a stable linear solver. A candidate with $s,r$ in the unit interval and $t\ge 0$ is the only contact event and hence the earliest one; otherwise no contact occurs. Check initial intersection explicitly, so an already touching pair returns time zero.

If $H$ is singular, use a rank-revealing factorization to test consistency. Write every solution as $x=x_0+Nz$, with columns of $N$ spanning the null space, and minimize its time coordinate subject to the box and nonnegative-time constraints. This is a small linear programming problem. It handles parallel segments, collinear overlap, a continuum of contact times and segments degenerated to points. Infeasibility means no future contact. When $v=0$, the relative geometry is stationary: initially disjoint segments never meet. The first contact is the least feasible nonnegative time, not just an intersection of the two supporting lines.