Draw a freebody diagram showing the three forces acting on the box: weight straight down, normal force perpendicular to the incline up, and friction along the incline opposite to the direction of motion.
Call the direction parallel to the incline the xdirection and the direction perpendicular to the incline the ydirection. Find the sum of all forces in the x and ydirection:
∑F_{x}=mg sin(25)  F_{f}
∑F_{y}=F_{n}mg cos(25)
Use Newton's 2nd law for each: in the xdirection, ∑F_{x}=ma, while in the ydirection ∑F_{y}=0.
mg sin(25)  F_{f} = ma
F_{n} mg cos(25) =0
Get the normal force: F_{n} = mg cos(25)
Then the force of kinetic friction is: F_{f} = µ F_{n} = µ mg cos(25)
Therefore,
mg sin(25)  µ mg cos(25) = m a
Divide through by m, the answer is independent of the mass:
a = g sin(25)  µg cos(25)
a = 9.8 m/s² ( sin(25)  0.15 cos(25) )
a = 2.8 m/s²
10/21/2013

Andre W.