Question

A car takes 9 m to stop after the driver applies the brakes.

Answer 1

Depends upon how soon you want to stop. You know the mass of the car, you know the initial velocity of 8.9 m/s. You know the final velocity which is zero. You can now calculate the deceleration of the car if you know how soon you want to stop.

Let's say you want to stop in one second.

a = (vf - vi) / t = - 8.9 m/s^2 (negative sign indicates deceleration).

F = ma = 800 x 8.9 = 7120 Newtons (kg*m/s^2)