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Homework Help: Work, Velocity, Friction, and Potential Engery with a sliding block

  1. Nov 20, 2008 #1
    (Green means correct)

    1. The problem statement, all variables and given/known data
    In the figure below, a small block is sent through point A with a speed of 8.1 m/s. Its path is without friction until it reaches the section of length L = 12 m, where the coefficient of kinetic friction is 0.70. The indicated heights are h1 = 8.0 m and h2 = 1.1 m.
    (Image Attached)

    (a) What is the speed of the block at point B?
    15 m/s
    (b) What is the speed of the block at point C?
    14.1 m/s
    (c) Does the block reach point D?
    ---Select--- yes no
    If so, what is its speed there and if not, how far through the section of friction does it travel?
    in units of m/s

    2. Relevant equations

    Kinematics equations
    Mu*N= Fkinetic

    3. The attempt at a solution

    Now, I've already finished a, b, c, I just need help with d. I first determined that all I had to do was set up a kinematics graph and use point C as my starting point. So, my inital velocity would be 14.1, my acceleration (which I got from using the second equation above, the normal force is equal to the weight for mg) is -6.86, and my delta-x is 12.

    I realize that the picture does not depict L to be 12 exactly, but I see no other way to do this. Then, using the Vf^2=Vo^2 +2ax equation I solved for Vf. My answer was 5.84 m/s, however, this is incorrect. Could someone lead me to the right direction???

    Thanks in advance!


    Attached Files:

  2. jcsd
  3. Nov 20, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Looks good to me.

    (Perhaps it's a fluke in the online system. Try redoing your calculations without rounding off until the last step.)
  4. Nov 21, 2008 #3
    Thanks for your response! At least I feel like I know what I'm doing.... I'll redo it right now.
  5. Nov 21, 2008 #4
    Hmm... turns out I didn't round anything off until the end. And still got the same answer. Perhaps I should merely confront my teacher tomorrow about the incident. Is there anything else I could possibly be doing wrong?
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