A 600kg car is moving on a level road at 30m/s.how large is a retarding force is required to stop it in a distance of 70m

Answer 1

speed:

v = 30 + a t
what is a when v = 0
-30 = a t
a = -30/t

distance:
70 = 30 t + (1/2) a t^2
70 = 30 t -(15/t) t^2
70 = 15 t
t = 14/3 seconds to stop
a = -30/t = -90/14 m/s^2 = -45/7

then F = m a
F = -600 (45/7) Newtons

Answer 2

There is a quicker way:

vf^2=vi^2+2ad but a= forcefriction/mass

0=30^2+2*force/mass * 70
solve for force.