A car has a mass of 1200 kg and is traveling perfectly horizontally on a road at 20 m/s. The car then applies the brakes and skids to a stop. If the coefficient of friction between the tires and road is .70, how much distance was required to get the car to stop?

Answer 1

F = m a = -mu m g (neg because braking)
so
a = - mu g = 0.70 * 9.81 m/s^2 = -6.87m/s^2

now the kinematics

v = Vi - 6.87 t = 20 - 6.87 t = 0 at stop

so t = 20/6.87 = 2.91 seconds to stop
distance = average speed * time
= 10 m/s * 2.91 s = 29.1 meters

notice, mass does not matter, on both sides of equation
in my world g = 9.81 m/s^2
Yours may be different.

Answer 2

thanks Damon