Figure out how far up the incline of the hill the box slides.

[jumbogala]

Homework Statement

At the bottom of a hill, a box has a speed of 15 m/s. The hill forms an angle of 30 degrees with the horizontal. How far up the incline of the hill does the box slide?

Homework Equations

My answer:
d = (vf² + vi²) / 2a

The Attempt at a Solution

vi = 15 m/s
vf = 0 m/s

a = (-9.81 m/s²)*cos(30) = -8.49 m/s²

d = 13.25 m

However, the answer key says d = 23 m. This is the answer if you use a = (9.81 m/s^2)*sin(30) instead. Help?

 

[Doc Al]

What's the component of gravity acting down the incline? That's where sinθ comes in.

 

[jumbogala]

Oops, I had 30 degrees in the wrong corner of the triangle. It works now! Thanks.