Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Friction; what does not contribute?

  1. Jul 14, 2010 #1
    1. The problem statement, all variables and given/known data
    What two things do not contribute to friction (i.e. what is friction not dependent upon)?

    2. Relevant equations

    3. The attempt at a solution
    After googling I have read that it is speed and surface area - but they had no explanations.
    I don't understand how either of these would not contribute and I don't want to submit my assignment when I don't actually understand it.
  2. jcsd
  3. Jul 14, 2010 #2
    The formula that best approximates kinetic surface-to-surface friction is [tex]F_{friction}=\mu_k N[/tex] where [tex]\mu[/tex] is the coefficient of kinetic friction, and [tex]N[/tex] is the magnitude of the normal force between the two surfaces.
  4. Jul 14, 2010 #3
    That tells me what does contribute, but I need to know what does not contribute.
  5. Jul 14, 2010 #4
    This is a very vague question though. Look, there are 2 or 3 types of friction which you might encounter in a basic physics class. The first type its called "Static Friction" and the second is called "Kinetic Friction."

    In Static Friction the object is not yet moving and therefore it generates more friction.
    This can be modeled through the equation:

    [tex]\mu [/tex]static Fnormal [tex]\geq[/tex] FSfriction

    In Kinetic Friction the object is already moving and therefore there is less friction.
    This can be modeled through the equation:

    [tex]\mu [/tex]kinetic Fnormal [tex]\geq[/tex] FKfriction

    As you can see speed does matter if you are going from V0= 0 m/s to V= nonzero value m/s

    But once you start moving speed has nothing to do with the amount of friction

    As for surface area... Im not quite sure, but the equations seem to demonstrate no relation between the two.
    Last edited: Jul 14, 2010
  6. Jul 14, 2010 #5
    Thanks for that :) It is pretty vague, my friend suggested I put something silly like 'the colour of the object'

    I will put down surface area and speed only when calculating kinetic friction.
  7. Jul 14, 2010 #6
    Let me clarify the Static Friction and Kinetic Friction... Static Friction is the friction the object needs to overcome to start moving. Kinetic Friction is the friction the system or object encounters while moving.
    Last edited: Jul 14, 2010
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook