What Causes Error in Calculating Acceleration on an Inclined Plane?

[mujadeo]

Homework Statement

A block of mass M = 3 kg is released from rest and slides down an incline that makes an angle q = 32° with the horizontal. The coefficient of kinetic friction between the block and the incline is µk = 0.15.

What is the acceleration of the block down the inclined plane?

Homework Equations

this is the easiste part of 4 part question and i can't get it.

The Attempt at a Solution

OK here's what i know
Normal = 3kg * (9.8sin32)
so friction is

fs = .15*15.58 = 2.34N
so I'm taking x-axis as the incline plane, (pos x = down the plane)
friction opposes the motion, so force runs along neg axis = -2.34N

forces in pos x direction are
3 * 9.8cos32 = 24.93N

24.93N - 2.34N = 22.59N (this is the net force)

Finally F/m = a
22.69/3 = 7.53m/s/s

= wrong.
??

 

[HallsofIvy]

Looks to me like you have sine and cosine reversed. You draw the inclined plane as a right triangle having angle 32 degrees at the bottom. The force of gravity is straight down, the two legs of that triangle are perpendicular and parallel to the inclined plane- the 32 degree angle is the bottom of that triangle. The normal force is given by cos(32), the force along the incline by sin(32).

 

[mujadeo]

yeah yer right
thanks