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

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

f = $$\mu$$N_F
f = (0.3)(100)
f = 30

$$\sum$$x = 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