1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Confusion regarding direction of kinetic friction on inclined plane.

  1. May 18, 2013 #1
    please help!

    The formula for kinetic friction acting on a sliding body is


    When the body is sliding with constant velocity down an inclined plane


    When the body is sliding up the inclined plane(say due to a force acting parallel to the inclined plane) with constant velocity

    even then


    The direction of kinetic friction is opposite in the 2 cases

    while the formula for fk is exactly the same.

    If friction is a vector shouldn't the formula for kinetic friction account for its direction too?
    Last edited: May 18, 2013
  2. jcsd
  3. May 18, 2013 #2


    User Avatar
    Science Advisor

    For an object that is moving over a surface, the direction of the frictional force is opposite to the direction of relative motion. The coefficient of dynamic friction dictates the magnitude of this frictional force.

    For an object that is stationary on a surface, the frictional force will act to oppose any other forces on the object so as to preserve its state of rest relative to the surface. The coefficient of static friction imposes a maximum magnitude on this frictional force.

    If the surface is accelerating, a term accounting for that acceleration may need to be added in the static case.
  4. May 18, 2013 #3
    i edited my post a little. could you elaborate please? i am not able to understand how it answers my question.
  5. May 18, 2013 #4

    Doc Al

    User Avatar

    Staff: Mentor

    The formula just tells you the magnitude of the friction force. You also have to know that the kinetic friction opposes the sliding between surfaces.
  6. May 18, 2013 #5

    So that means we would always have to know the direction of the body sliding against a surface to tell the direction of kinetic friction acting on it?
    Last edited: May 19, 2013
  7. May 19, 2013 #6


    User Avatar

    Staff: Mentor

  8. May 19, 2013 #7
    thank you!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook