[المتفيزق]

yes;
the position vector is:
r = 5i + 10t j + 5/t k as i understood out of your explanation...
so now:
v = dr/dt = 10j -5/t2 k m/sec
and a = dv/dt = + 10/t3 k m/sec2


thus to find the time at which they are perpendicular we evaluate the dot product, ohhhhh nnooooooo there is something wrong we have:
-50/t5 =0 which means only at infinity we achieve this ... please chech the problem out thoroughly...
thanks