# Homework Help: Acceleration on an Inclined Plane?

1. Mar 13, 2010

### haengbon

1. The problem statement, all variables and given/known data

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?

2. Relevant equations

Newton's Second law ( I think? )

g= 10 m/s2

3. The attempt at a solution

$$\sum$$y = 0
NF - Wysin30$$\circ$$ = 0
NF = Wysin30$$\circ$$
NF = [(20)(10)]sin30$$\circ$$
NF = 100 N

f = $$\mu$$NF
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

2. Mar 13, 2010

### ideasrule

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

3. Mar 13, 2010

### haengbon

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