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Difference between Static and Kinetic coefficients of friction?

  1. Jan 2, 2009 #1

    Air

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    1. The problem statement, all variables and given/known data
    What is the difference between Static and Kinetic coefficients of friction?

    2. The attempt at a solution
    I think Static coefficient of friction is one which refers to friction which is considered when the object is stationary and Kinetic coefficient of friction is one which refers to the friction which is considered when the object is in motion.

    3. The assistance that I require
    Is this correct, is my wording correct too, how can my wording be improved?
     
  2. jcsd
  3. Jan 2, 2009 #2
    Re: Coefficients Of Friction

    Pretty much sounds good to me. I guess the only clarification you could give is static is when an objective is stationary relative to the other contact object and then kinetic is when the objective is rubbing/moving relative to the other contact object.
     
  4. Jan 2, 2009 #3

    radou

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    Re: Coefficients Of Friction

    Basically yes, when two bodies in contact are not moving relative to each other, we're talking about the static coefficient (which, multiplied by the normal reaction to the surface, produces the tangential frictional force which prevents motion), and when there exists motion, you're talking about the kinetic coefficient (which is usually less than the static coefficient). It's pretty much a fundamental concept, and can be found everywhere on the net. Try wiki, for example.
     
  5. Jan 2, 2009 #4

    tiny-tim

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    Hi Air! :smile:
    You could also say which coefficient is greater, and whether the friction force is equal to the normal force times the coefficient. :wink:
     
  6. Jan 2, 2009 #5

    Air

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    Re: Coefficients Of Friction

    You can never have both situations in one calculations, right? If given a question and we are given both values, when the object is stationary we use static coefficient and when in motion we use kinetic coefficient. In a calculation it's either one or the other? It's based on the question and we make judgements?
     
  7. Jan 2, 2009 #6
    Re: Coefficients Of Friction

    In introductory physics, that is correct; you'll have one or the other at each interface. You may have both in a problem (professors are fond of thinking up problems where one block is able to slide while another does not but just look at each individual interface).

    In mechanical engineering, there is a situation where both are present, but that is a unique problem and, to the best of my knowledge, never presented in physics.
     
  8. Jan 2, 2009 #7

    PhanthomJay

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    Re: Coefficients Of Friction

    I just want to emphasize what has already been pointed out: The static coefficient is used when the object is stationary with respect to the surface it is in contact with . An object might be in motion with respect to the ground, but not in motion with respect to the surface it is in contact with.
     
  9. Jan 2, 2009 #8

    radou

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    Re: Coefficients Of Friction

    Theoretically, you could have an incline with a growing angle A (imagine a box on the back of an unloading kipper truck). Since the angle is growing, the component of the weight of the box which opposes the force of static friction, G*sin(A), is growing too, and can overcome the force of friction. Once the box starts sliding down, the coefficient of kinetic friction is "activated".
     
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