Moving at 20m/s a carr of mass 2000kg applies its brakes and comes to rest over a distance of 10m. Find the size of the net force that acted to stop the car.

Question

would I find t using t=dv. then would I use that for F=(m*v)/t ?

The speed doesn't stay constant, so that won't work. The simplest way to solve the problem is to use that the work done by the force must equal the kinetic energy. So:

F*d = 1/2 m v^2