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Work on a Sliding Box

  1. Oct 12, 2013 #1
    1. The problem statement, all variables and given/known data

    A box of mass m is sliding along a horizontal surface.
    Part A) The box leaves position x=0 with a speed v0. The box is slowed by a constant frictional force until it comes to rest at a position x=x1. Find Ff, the magnitude of the average frictional force that acts on the box. (express in terms of m, v0, and x1.
    I found the correct answer to part A...(Ff=(mv02)/2x1)

    Part B) After the box comes to rest at a position x1, a person starts pushing the box giving it a speed v1. When the box reaches position x2 (where x2>x1), how much work Wp has the person done on the box? Assume that the box reaches x2 after a person has accelerated it from rest to speed v1. Express the work in terms of m,v0,x1,x2, and v1.

    2. Relevant equations

    W=ΔKE
    W=F*d


    3. The attempt at a solution

    So I figured that the total work would be the work done in part A (-(mv02)/2x1)*(x1)
    added to the work done between x1 and x2:
    -(mv02(x2-x1))/2x1

    however when I add these two solutions, it tells me that the answer needs to include v1. So then I thought to solve for v1 by setting the work done between x1 and x2 equal to the change in kinetic energy between that time frame which gave me:

    KE = (mv12)/2 = work between x1 and x2

    v02=(x1v12)/-(x2-x1)

    so that allowed me to have v1 in my final answer...but mastering physics is still telling me it's incorrect.

    Any advice/help would be great.
     
  2. jcsd
  3. Oct 12, 2013 #2

    Simon Bridge

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    You were nearly right the first time - what distance does the friction force act over?
     
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