Once an object is moving, friction still exists.  This is called kinetic friction.

The equation is simple: Fkinetic friction = mN


A 800Kg truck skidding against a road surface has m = .6.  What frictional force is present?
USE:   mN
.6*1200Kg*9.8m/s/s = 7056N

What is the magnitude of the car's acceleration?

a = F/m = 7056N / 1200Kg = 5.9m/s/s NOTE: Since the question asks only for magnitude, we don't need to specify that our answer is negative.

If it were originally coasting at 15m/s, what would its stopping distance be?

Vf2 = Vo2 + 2aDx

0 = (15m/s)2 + (2)(5.9m/s/s)(x)

x = 225m2/s2 / 11.8m/s/s = 19.1m