Maybe there are some problem with translation, the problem is in spanish, but I agree, if there is no compensation from S' the two starting guns fire simultaneously, so delta t must be zero. So
Δs^2 < 0
But how I show this?
Homework Statement
In a reference system S two athletes are aligned at a distance d relative to each other on the Y axis to make a run parallel to the x axis. Two starters, one next to each athlete, shoot their guns out at slightly different times, offsetting the advantage of better athlete...