A car is parked on a hill, elevated at an angle . There is friction between the car and the hill (otherwise it would roll down). a) Draw a force...

A car is parked on a hill, elevated at an angle θ. There is friction between the car and the hill (otherwise it would roll down). 

a) Draw a force diagram for the car. Indicate your choice of coordinate axes. 

b) Show that the coefficient of friction required so that the car does not roll down the hill is μs = tan θ.