Ft = mv - mu
Choose the frame of reference where initially the spacecraft is at rest - I'm assuming it isn't accelerating.
Ft = 300ns
taking u=0,
First part of craft:
210 = 1300 * v
v= 0.25m/s
Second part:
210 = 1700 *v
v=0.1666m/s
These velocities will be in opposite directions so adding them together gives the relative speed of the 2 parts ie.
v rel = 0.16666+0.25 = 0.41666m/s
