Physics
posted by value .
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

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 
There is a quicker way:
vf^2=vi^2+2ad but a= forcefriction/mass
0=30^2+2*force/mass * 70
solve for force.
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