Elelctrostatics doubt.....plz help?

two charges +q and -q are separated by distance r and m is same(m). We have to find the speed when they are separated by distance r/2. In my book(HC Verma) it finds the force then acceleration and find velocity by v = u+at but isnt it wrong? Because force will continuously change between the two as they come closer as F is inversely proportional to r^2 so acceleration is not constant......rite? Then how do we solvethis problem??