<span>Since the lines are reflected, they will retain their identity that the original l and m had. This means that if the original lines were parallel, then l' and m' will be parallel as well. The same applies to if they were perpendicular, or any other type of line.</span>
The first thing you should remember is that the energy in this case will be given by the work done. The work is defined as: W = F * d Where F = Force d = displacement. Calculate it. 5 kg x 9.8 m / s ^ 2 x 21 m = 1029 J
