Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Dynamics block problem

  1. Feb 13, 2007 #1

    srh

    User Avatar

    1. The problem statement, all variables and given/known data
    A 40-kg package is at rest on an inlcine when a force P is applied to it. Determine the magnitude of P if 4s is required for the package to travel 10 m up the incline. The static (us) and kinetic (uk) coefficients of friction between the package and the incline are 0.3 and 0.25 respectively.


    2. Relevant equations
    Newton's 2nd Law : sum of forces = mass x acceleration
    x=x0+v0t+0.5at^2

    3. The attempt at a solution
    First I changed my coordinate system to make the x plane parallel w/ the incline. I then summed the forces in the y direction in equilibrium to find the value of the N, normal force. I found this to be 368.73 N. (N-mgcos(20)=0)

    THen I summed the x forces and set equal to mass x acceleration:
    Pcos(50)-(uk)(N)=ma

    To find a I used the relation x=x0+v0t+0.5at^2:
    x-x0 = 10m
    v0=0
    t=4s
    I found a to be 1.25 m/s^2.

    Plugging in all the known values I got P = 355.35 N. I know that P is supposed to be 612 N. I was wondering if it was obvious where I was going wrong?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

    Last edited: Feb 13, 2007
  2. jcsd
  3. Feb 13, 2007 #2

    berkeman

    User Avatar

    Staff: Mentor

    Thread moved from Engineering to Intro Physics.

    Could you list all of the forces acting normal to the plane and parallel to it? I think you might be missing a term or two from the force pushing down and sideways on the block.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook