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Homework Help: Friction and constant velocity

  1. Sep 22, 2009 #1
    1. The problem statement, all variables and given/known data
    A horizontal force of 20 N is required to move an object with a mass of 8 kg along a horizontal floor with constant velocity. Find the coefficient of kinetic friction.

    2. Relevant equations
    f = uN
    u = f/N

    3. The attempt at a solution
    u = 20 N/(8 kg X 9.8 m/s2)
    u = 20 N/78.4 N
    u = 0.255 or 0.26

    Is my answer correct? I'm confused with the mention of constant velocity. I know that one of the components of force is acceleration. Does it mean that acceleration is zero; therefore, there is no friction because velocity is constant?

    f = 0 X 78.4 N
    f = 0 ???
    Last edited: Sep 22, 2009
  2. jcsd
  3. Sep 22, 2009 #2
    No kinetic friction is there as long as the object is moving it doesn't disappear just because it is moving at constant velocity. Constant velocity meant the net force on the object is 0 that is friction=force applied. Your answer looks fine
  4. Sep 22, 2009 #3
    I think it just means that your net force is zero since box is moving at a constant velocity . Friction is related to the how rough two or more surfaces are when moving aganist each other. Their are 4 forces acting upon the object , two of which cancel each other out in the vertical direction; which leaves you with the frictional force and your applied force moving in the opposite direction; your solution looks correct to me;.
  5. Sep 22, 2009 #4
    Thanks, noblegas and semc.
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