Frictional force on an inclined plane

1. Oct 3, 2012

Butterfly30

A 2319kg car is moving down a road with a slope grade of 11% and slowing down at a rate of 3.8m/s^2.Find the direction and magnitude of the frictional force ( define positive in the forward direction ie down the slope)

So the equation I have is f= ma+ mgsinθ. For my angle I get arctan(.11)= 6.27

Plug in. f= 2319*3.8 +2319*9.81*sin6.27 Ive already drawn a FBD. since the friction is moving forward it should be a negative answer (according to the question).... BUT I'm confused because if its slowing down is my acceleration supposed to be negative when I plug it in. This would make my answer negative....but would the fact that the acceleration is moving in negative direction cancel it out and make it positive?? I know I'm confusing myself I just don't know if my (a) should be negative and how it will effect the sign of the overall answer....help please

2. Oct 3, 2012

CAF123

Your relevant equation of motion is: $$mg\sin\theta - F_f = ma$$

3. Oct 3, 2012

Butterfly30

Oh, but do u enter my (a) as negative since the car is slowing down?