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

2 questions

  1. Feb 2, 2007 #1
    Why is the force of friction independent of the area of the 2 surfaces in contact? Name a subtance used for coating to reduce friction. Is it paint?

    What is meant by uniform magnetic field?
  2. jcsd
  3. Feb 2, 2007 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Please show some work, as we cannot help you until you do so. Do you have any thoughts about the questions?
  4. Feb 3, 2007 #3
    i think that surface area in contact should affect friction. As in the case of ball bearings, the surface in contact is quite less.

    I dont know about uniform magnetic field!
  5. Feb 4, 2007 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    The force of friction at any point is proportional to the local pressure at that point*, i.e, doubling the pressure there doubles the friction.

    The net "pressure force" F (or applied normal force) is gained by summing up the pressures at all points:
    where S is the surface we sum over, p is the local pressure, and dA the differential area element.

    Now, the proportionality factor between the force of friction and the local pressure will most likely only vary significantly if the contact surface properties themselves vary significantly**.
    So, if the contact surface can be regarded as relatively homogenous in material properties, the local force of friction can be written as:
    where [itex]dF_{fric}[/itex] is the local friction force
    and [itex]\mu[/itex] the constant proportionality factor.

    Thus, the net friction force is given by:

    Thus, the force of friction is strictly proportional to the applied normal force, which again equals the normal force N from the surface, since the objects don't slide into each other.
    Thus, the friction force is proportional to N.

    In particular, in so far as the applied force remains the same, whereas the contact surface is changed, no change will be seen in the net friction force.

    It is LOCALLY stronger, just as the local pressure is stronger, but the total surface is less, so the whole balances neatly.

    *This is our starting HYPOTHESIS, that has ample empirical verification.

    ** The friction coefficient happens to be EXTREMELY sensitive to a lot of factors: temperature, presence of material impurities and so on.
    This by no means reduce the validity of the argument below, but does, indeed, limit the usefulness of the model.
    Last edited: Feb 4, 2007
  6. Feb 4, 2007 #5
    Hint to one of the questions: Think about non-stick cookware
  7. Feb 5, 2007 #6
    A uniform magnetic field is one in which the magnetic flux density is constant throughout any area taken perpendicular to its direction.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook