You don't understand.
If you WANT the little car to accelerate at 7 m/s^2, then YOU have to push it with (0.005 kg)x(7m/s^2) = 0.035 Newton of force. If you ever stop pushing, the car will stop accelerating.
Does it hit a wall ? Well then, the average force it hits the wall with is
(the car's speed when it hits) times (0.005) divided by (the length of time the hit lasts) .