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    Law of Friction: Surface Between Two Bodies Oppose Relative Motion

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    eltytan
    1) Friction is a force that acts between two surfaces in contact and opposes their relative motion. It has both static and kinetic components. 2) The maximum static frictional force that can occur without motion is called the limiting friction. It depends on the normal force of contact and the coefficient of friction between the surfaces. 3) The coefficient of friction is a property of the surfaces and varies with their material composition and conditions like wetness. It can be determined experimentally by finding the angle of inclination just before an object begins to slip.

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    Law of Friction: Surface Between Two Bodies Oppose Relative Motion

    Uploaded by

    eltytan
    58 views11 pages
    1) Friction is a force that acts between two surfaces in contact and opposes their relative motion. It has both static and kinetic components. 2) The maximum static frictional force that can occur without motion is called the limiting friction. It depends on the normal force of contact and the coefficient of friction between the surfaces. 3) The coefficient of friction is a property of the surfaces and varies with their material composition and conditions like wetness. It can be determined experimentally by finding the angle of inclination just before an object begins to slip.

    Original Description:

    Physics PPT

    Original Title

    1.2 Law of friction

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    1) Friction is a force that acts between two surfaces in contact and opposes their relative motion. It has both static and kinetic components. 2) The maximum static frictional force that can occur without motion is called the limiting friction. It depends on the normal force of contact and the coefficient of friction between the surfaces. 3) The coefficient of friction is a property of the surfaces and varies with their material composition and conditions like wetness. It can be determined experimentally by finding the angle of inclination just before an object begins to slip.

    Copyright:

    © All Rights Reserved
    Download as PPT, PDF, TXT or read online from Scribd
    Download as ppt, pdf, or txt
    58 views11 pages

    Law of Friction: Surface Between Two Bodies Oppose Relative Motion

    Uploaded by

    eltytan
    1) Friction is a force that acts between two surfaces in contact and opposes their relative motion. It has both static and kinetic components. 2) The maximum static frictional force that can occur without motion is called the limiting friction. It depends on the normal force of contact and the coefficient of friction between the surfaces. 3) The coefficient of friction is a property of the surfaces and varies with their material composition and conditions like wetness. It can be determined experimentally by finding the angle of inclination just before an object begins to slip.

    Copyright:

    © All Rights Reserved
    Download as PPT, PDF, TXT or read online from Scribd
    Download as ppt, pdf, or txt
    of 11

    Law of friction

    Properties of frictional force


    1. act along the surface between two bodies
    2. act in a direction so as to oppose relative
    motion of the surfaces.
    1
    Limiting friction

    Applied
     If the block remains at rest,
    force FA f = FA
     The frictional force to be
    overcome before it moves
    Frictional is called limiting friction.
    force f

    2
    Limiting friction
    FA / N f/N

    Applied
    0 0
    force FA 1 1
    2 2
    Frictional 3 3
    force f
    4 3

    Limiting friction = 3 N 5 3

    3
    Limiting friction

    Applied f/N
    force FA At rest Moving
    3
    2
    Frictional 1
    force f
    0 1 2 3 4 5 FA / N
    Limiting friction = 3 N
    Static Kinetic
    friction friction
    4
    Limiting friction
    f/N
    At rest Moving
    •Kinetic / Dynamic friction is
    3 the frictional force acting on
    2 an object when it is moving.
    1 •Static friction is the frictional
    force when the object is
    0 1 2 3 4 5 FA / N stationary.
    Static Kinetic
    friction friction

    5
    Friction f  From experiment, limiting
    friction f is slightly greater
    Static friction Kinetic friction than kinetic friction f’.

    f i.e. f > f’
    f’  Suppose f = 3.1 N, f’ = 3N
    A force of 3.1 N is required to
    make the block move.
    Applied force F But once the block is moving,
    a force of 3 N only is required
    to keep the block moving.

    For simplicity, take f = f’

    6
    Law of friction Normal
    reaction N

    Applied
    force FA
    Frictional force f

    1. The limiting frictional force f is directly proportional to


    the normal reaction N exerted by the surface.
    i.e. f ∝ N or f = mN where m is a constant called
    coefficient of limiting friction.
    2. The kinetic frictional force f is directly proportional to
    the normal reaction N exerted by the surface.
    i.e. f’ ∝ N or f’ = m’N where m is a constant called
    coefficient of kinetic friction.

    For simplicity, take f = f’ = mN and m = m’


    7
    For simplicity, take f = f’ = mN and m = m’

     If the surface is smooth, m = 0 ⇒ f = 0 N


     Coefficient of friction depends on the two
    contacting materials.

    Rubber Concrete (Dry) m = 0.6 – 0.85

    Rubber Concrete (Wet) m = 0.45 – 0.75

     Frictional force does not depend on the


    area of contact of the surfaces.

    8
    A block of mass 2 kg starts from rest, sliding down a
    rough inclined plane making an angle of 30o with the
    horizontal. Length of plane is 8 m. It takes 4 seconds to
    reach the bottom. Find the coefficient of kinetic friction.

     Solution:
     t = 4 s, s = 8 m, u = 0 ms-1 R
    f a
    By s = ut + ½ at2
    8 = 0(4) + ½ a(4)2 8m
    a = 1 ms-2
     mg sin 30o – f = ma
    mg 30o
    (2)(10)sin 30o – f = (2)(1)
    f=8N
     R = mg cos 30o = 17.32 N
     By f = mR
    8 = m(17.32)
    m = 0.462 9
    For the following situation, find the least coefficient of
    friction between the ground and the ladder.

     N = 250 Newton
     Take moment about A,
    S S x 4 sin 60o = 250 x 2 cos 60o
    smooth

    S = 72.17 Newton
    N  f = S = 72.17 Newton
     The ladder is about to slip,
    60o f = mN
    72.17 = m x 250
    f A
    m = 0.289
    250 N

    10
    How to find coefficient of limiting friction? p 22
     The coefficient of limiting friction, m, can be found by
    placing the block on a surface and tilting the latter to an
    angle q at which the block is just about to slip.
    N  Along the plane:
    f = mg sin q --- (1)
     Perpendicular to the plane:
    N = mg cos q --- (2)
    f  Since the block is just about to slip,
    f = mN --- (3)
    q  Sub (3) into (1):
    mN = mg sin q --- (4)
    mg
     (4)/(2):
    m = tan q
    Hence, the coefficient of limiting friction is just tan q.
    11

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