How much force does it take to stop a 1,000 kg car traveling at 20 m/s in 10 seconds?

The initial momentum of the car is

                (mass) x (speed)  =  20,000 kg-m/s

"Impulse" is (force exerted) x (time the force lasts)
and it's equal to the change in momentum.

              (Force) x (10 sec)  =  20,000 kg-m/s

Divide each side by  (10 sec)  and you have

                      Force  =  (20,000 kg-m/s) / (10 sec)

                                 =          2,000 kg-m/s²  =  2,000 Newtons