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Homework Help: Dynamic Friction between surfaces

  1. Jul 5, 2011 #1
    1. The problem statement, all variables and given/known data

    We have a body V1, which is in contact over body V2 over a flat (i.e. planar) surface S. (The planar approximation can be considered a good one). Body V1 is moving, V2 is stationary.

    A normal force F exists between the bodies at surface S, acting in point P. This force acts along the unit normal vector N.

    The body V1 is moving with a body velocity V.

    Q: What is the dynamic friction force between V1 and V2?

    I think I found the solution, but I'd like a double check. Or a rebuttal with correct answer...

    2. Relevant equations

    Ffriction = mu*F, acting opposed to the relative motion of the surfaces in contact.

    3. The attempt at a solution

    1. We find the relative speed in the plane S.

    As follows:
    N = unit normal surface vector in P.
    V = body velocity in point P.

    U = orthonormal projection of V on plane S. I think this is the speed the surfaces slide over each other. Is this correct?

    U = V - (V dot N)N

    2. The components of U have dimension [m/s], but as the friction force is independent of speed, we need to normalise vector U. I.e. we only need to know which direction U has.

    U_ = U/norm(U) = [U_x,U_y,U_z]

    3. The components of the friction force now become

    Ffriction,x = mu*|F|*U_x
    Ffriction,y = mu*|F|*U_y
    Ffriction,z = mu*|F|*U_z
  2. jcsd
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