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Is friction an unbalanced force?

  1. Dec 25, 2014 #1
    If an object is stationary it has the force of friction acting on it. So since friction is acting on it then it should be an unbalanced force. But I have read that if there is unbalanced force then the object should accelerate.

    I would appreciate the help.
     
  2. jcsd
  3. Dec 25, 2014 #2

    Nugatory

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    If an object is stationary, that means that the force of friction on it is equal to and opposite to all the other forces acting on the object. The frictional force will be zero if the object is stationary and no other forces are acting on it.

    It's really no different than pushing on a brick wall: the wall doesn't accelerate and stubbornly remains at rest, so we know the net force on the wall is zero. The opposing force comes from the wall being solidy attached to the ground, and is equal in magnitude and opposite in direction to the force you're applying. Stop pushing, and this force will be zero.
     
  4. Dec 25, 2014 #3
    That means the force of friction comes into picture only when you move the object Right? Friction is not a property of matter.
     
  5. Dec 25, 2014 #4

    russ_watters

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    Only when you apply a force, regardless of if the object moves.
     
  6. Dec 25, 2014 #5

    rcgldr

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    As long as the object is stationary or moving at constant velocity, then the net force on the object is zero. For example, if someone is pushing a box so that it slides at constant speed, then that person is pushing with a force that exactly opposes friction, with a net force of zero.
     
    Last edited: Dec 25, 2014
  7. Dec 25, 2014 #6

    Nugatory

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    Not right, although with the addition that I've made above it's closer.

    There is static friction and dynamic friction, given by ##F_f=\mu_sN## and ##F_f=\mu_dN## respectively - generally the two coefficients are not the same. When the object is moving, the frictional force is the dynamic one. When the object is stationary the frictional force is whatever is needed to keep the object stationary by exactly balancing the other forces applied to the object, from zero up ##\mu_sN## - that's as strong as the static frictional force can be, and if more force is applies the object will start to move.

    BTW, the coefficient of dynamic friction is usually less than the coefficient of static friction. This is why it's generally easier to keep an object moving than to start it moving in the first place, and why things often start moving with a jerk.
     
  8. Dec 25, 2014 #7

    anorlunda

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    The previous replies to your question are correct. But you are also correct when you say that friction is not a "property of matter" like mass or charge.

    You should read the Wikipedia article about friction. It says:

    "Friction is not itself a fundamental force but arises from interatomic and intermolecular forces between the two contacting surfaces. The complexity of these interactions makes the calculation of friction from first principles impractical and necessitates the use of empirical methods for analysis and the development of theory."
     
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