We are asked for the force acting on the car and we know from Newton's second law that F = ma Therefore, we need to calculate the acceleration first. We use a = (vf - vo)/(t - to) where vf = final speed, vo = initial speed, t = final time and to = initial time. With all that, we have a = -20/0.05 = -400 m/(s^2)F = 1000 * (-400) = -400000 N