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Friction and dynamics

  1. Mar 15, 2012 #1
    1. The problem statement, all variables and given/known data

    1. a box has a mass of 100kg, and the second has a mass of 55kg. there is a rope between them that attaches them. theres a rope connecting to the first box, that is pulled with a force of 200N at 50 degrees above the horizontal. the box os moved 305cm for 5s
    a) what is the frictional force on the boxes
    b) what is the tension in the rope joining the two boxes

    2.
    a rope is hanging off the side of the table. the rope is 2m long. the coefficient of friction between the table and the rope is 0.62
    what is the maximum length of the rope that can hang over without slipping?

    2. Relevant equations

    1. i tried using d=vy1*t+0.5g*t^2 to find a. if i assumed vy1 is 0 because it starts from rest. then tried using Ef=Fappy-Ffs to find the mui in FFs...

    2. Have no idea how to start

    3. The attempt at a solution

    have no idea o-o'' what to do ><"
     
  2. jcsd
  3. Mar 15, 2012 #2

    vela

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    As you've written it, the first problem can't be solved because there isn't enough information. Please tell us the problem statement is exactly as it was given to you.

    With force problems, the first thing you should do is identify all the forces acting and draw a free-body diagram for each object.
     
  4. Mar 15, 2012 #3
    that's actually all the information given :s
    that's why i'm findin it hard to do. but i found out how to do a).
    to find friction:
    equate the two equations
    mass A: Ef=Fapp-FT --> Ft= Fapp-Ef
    mass B: Ef= Ft-Ffs --> Ft= Ffs + Ef
    therefore:
    Ffs+Ef = Fapp-Ef
    Ffs= Fapp-Ef-Ef(second one)

    acceleration found with d=v1y*t + 0.5g*t^2 = 0.24m/s^2
    ...but idk how to do b) :s

    & i'm finding it hard to imagine the rope as a free body diagram.. cause half of its on the table and the other half is off.
    so like half of it is lieing horizontally and the other half it lieing vertically.. i can't imagine what force is acting on what part..... :s
     
  5. Mar 15, 2012 #4

    vela

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    I think in relaying the question here, you've changed the wording and ended up inadvertently omitting some necessary info. That's why I asked you to tell us what the problem statement is exactly as written. Your solution doesn't make sense to me, and as you've written the problem, it can't be solved without making arbitrary assumptions.
     
  6. Mar 15, 2012 #5
    kkk....

    a person pulls a string of two boxes across level ground. the first box has a mass of 100kg and the second, 55kg. pulling the rope attached to the first box with a force of 200N at 50 degrees above the horizontal. the box os moved 305cm for 5s
    a) what is the frictional force acting on the boxes
    b) what is the tension in the rope joining the two boxes
     
  7. Mar 15, 2012 #6
    thats it^ the entire question.
     
  8. Mar 15, 2012 #7

    vela

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    You can't solve that problem. You don't know the initial velocity, if the boxes accelerate, etc.
     
  9. Mar 15, 2012 #8
    that's why we assumed that the inital velocity starts from rest, and found the acceleration to be 0.24m/s
    it might not be right, but that's what most of us had done.
     
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