Finding friction force for a braking car with known acceleration

[drinkingstraw]

The average rate of acceleration for a breaking car was -12.2m/s2. If the car had a mass of 925 kg, find the frictional force and direction.

a = -12.2m/s2
m = 925 kg
Fn = 9074.25 N

This is what I've figured out. I don't know how to find the coefficient of friction. If I get the coefficient I think I can multiply it by the normal force to get the frictional force.

 

[Nabeshin]

You're complicating the problem unnecessarily.

The question asks only for the frictional force and direction. Try drawing a free-body diagram for the car and then write down the F=ma equations.

 

[tnutty]

F_f = u*F_n

F_n = mgF_f = u *mg

find u

 

[3.211k]

The problem only asks for F_f
The free body diagram is the way to go. Fn and Fg cancel out. What other force or forces provide the acceleration?