# Need help with simple mechanics problem

here's the problem.

a 1200kg vehicle is travelling at 20m/s along a level road. the force of friction opposing the vehicle's motion is 450 Newtons. the vehicle's motor is switched off and it continues to roll for 12 seconds......

we have to find the deceleration and the distance travelled..... now i am getting two different answers for the deceleration

a= F/m = 450 / 1200 = 0.375 m/s^2

and a = (v-u)/t = 0-20/12 = -1.67 m/s^2

there is also another point about finding distance..... since assuming that the vehicle is coming to rest, the work done by the friction must be equal to the kinetic energy of the vehicle at the instant when the motor is switched off.
so K = 1/2 m v^2 = 1/2 * 1200 * 20^2 = 240000 J

if the distance travelled is s then K = F * s
s = K/F = 240000/450 = 533.33 m

but this doesnt seem to be the right answer since this distance is more than the distance the vehicle would have travelled in 12 sec, in the absence of the friction force.........

so whats happening here ?