physics
posted by jay .
A freight train has a mass of 2.1 × 10^7 kg. If the locomotive can exert a constant pull of 6.2 × 10^5 N, how long would it take to increase the speed of the train from rest to 77.7 km/h? (Disregard friction.)

F=ma
solve for a (a=F/mass)
Vf=at conver km/hr to ms/s, solve for t.