# Box on an inclined plane

1. Nov 2, 2013

### rumblef1sh

I am stuck on a problem from the 5 Steps to a 5: AP Physics B&C book and would really appreciate anyone's help.

1. The problem statement, all variables and given/known data
Determine (a) the acceleration of the block down the plane and (b) the time for the block to slide to the bottom of the plane.

http://02.edu-cdn.com/files/static/mcgrawhillprof/9780071623209/INCLINED_PLANES_08.GIF [Broken]

2. Relevant equations
F=ma
F, gravity = mg
F, friction = umg
Kinematic equation ???

3. The attempt at a solution

Fx, up the plane = mgsin25 + umgcos25 = 204 N
a = F/m = 204/30 = 6.8 m/s^2
Fx, down the plane = mgsin25 - umgcos25 = 44 N
a = F/m = 44/30 = 1.5 m/s^2

I tried using x = Vot + 1/2 at^2 (x = 20 m, Vo = 3m/s) and v = Vo + at (v = 0, Vo = 3 m/s) to solve for time, but my answers were way off on both attempts.

Last edited by a moderator: May 6, 2017
2. Nov 3, 2013

### Panphobia

If the question says the mass is going 3 m/s up the plane, and the a) part asks for the acceleration DOWN the plane this means that x != 20, but it is going to be 20 added to the distance travelled by the mass until it changes direction. I hope that helped.