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Box on an inclined plane

  1. Nov 2, 2013 #1
    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. jcsd
  3. Nov 3, 2013 #2
    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.
     
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