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Why static friction is directely proposnal to normalreaction

  1. Feb 19, 2015 #1
    I want to know why
    Static friction∝normal reaction and
    Why not
    Static friction∝ area of surface
    Static friction∝weight
  2. jcsd
  3. Feb 19, 2015 #2

    Philip Wood

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    Gold Member

    (1) The 'laws' of friction are only approximately true. They are not fundamental laws of Physics.
    (2) The total area of contact is usually far smaller than the measured surface area. This is because, on a microscopic scale, the surfaces don't actually touch, except in certain places.
    (3) The total area of contact is not determined so much by the measured surface area as by the normal component of force between the surfaces, as this is what causes permanent or temporary squashing of 'high spots' in contact with the other surface, making the area of contact larger.
    (4) The frictional force itself arises usually, I believe, from adhesive forces between atoms in the two surfaces. In some cases there may be penetration of 'high spots' on one surface into the other surface.

    Hope this helps. I'm no expert, so let's hope someone else steps in...
  4. Feb 19, 2015 #3


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    Gold Member

    In addition to Phips Wood's comments,

    You wrote,
    A reference describing static friction is here,

    Static friction is applicable when the object is not moving against another surface. And no, it is not proportional to the normal force as static friction can be anything from 0 ( such as a book resting on a table ) to a maximum ( just before it begins to move if you are attempting to push the book sideways with your hand ). The MAXIMUM value of static friction would be proportional to the normal force multiplied by the coefficient of static friction, but below that, the static friction is just equal to your pushing force. ( we should say,the net sideways force acting on the book since all forces should be taken into consideration )

    The other type of friction when there is relative motion between surfaces is called kinetic friction ( just move down a bit on the site where it describes kinetic friction ).

    You wrote,
    The normal force can be a function of the object's weight, but not necessarily as a direct proportionality function. Also there may be other forces acting on the object other than just the weight of the object.

    See http://en.wikipedia.org/wiki/Proportionality_(mathematics [Broken])
    Friction ( ie the maximum static and the kinetic friction ) are thus considered instead to be a direct function of the normal force, through the proportionality coefficient, called the coefficient of static friction or the coefficient of kinetic friction, as the case may be.

    Thus, if one can find out the normal force acting on the body, one can thus use the direct proportionality to find the friction forces.
    Last edited by a moderator: May 7, 2017
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