The force of friction, Ff, that acts up the ramp (opposite to the direction in which
the block slides) has magnitude Ff = μFN. But the diagram shows that FN = Fwcosθ,
so Ff = μ(mg cosθ). Therefore, the net force down the ramp is
Fw sinθ − Ff = mg sinθ − μmg cosθ = mg(sin θ− μcos θ).Then, setting Fnet equal to ma, we solve for a: