Acceleration on an Inclined Plane?

[haengbon]

Homework Statement

A box weighing 20 kg is on an inclined plane with an angle of 30\circ.The coefficient of friction is 0.3. What is the acceleration of the box?

Homework Equations

Newton's Second law ( I think? )

g= 10 m/s²

The Attempt at a Solution

\sumy = 0
N_F - W_ysin30\circ = 0
N_F = W_ysin30\circ
N_F = [(20)(10)]sin30\circ
N_F = 100 N

f = \muN_F
f = (0.3)(100)
f = 30

\sumx = ma

Wcos30\circ + f = m(-a)
20cos30\circ + 30 = -20a
47.32/-20 = -20a/-20
a = -2.36 m/s²

Is this correct? :D

 

[ideasrule]

You have your sines and cosines mixed up. The normal force is Wcos(theta); the parallel force is Wsin(theta).

 

[haengbon]

You have your sines and cosines mixed up. The normal force is Wcos(theta); the parallel force is Wsin(theta).

so that's the only problem ? :D so If I switched/corrected the two, then my answer would immediately be correct? :D