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Friction equation confusing me

  1. Dec 20, 2015 #1
    1. The problem statement, all variables and given/known data


    Let's say I am pushing a couch with 30 N, the velocity is constant, therefore, the friction is also 30 N but in the opposite direction. But if a push it with 50 N then the friction force is going to change to 50 N as long as it is not accelerating. So how am I supposed to used this equation: Friction force=mu * Normal force. The normal force is the same whether I have a high speed or not.
    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. Dec 20, 2015 #2

    NascentOxygen

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    Normally, sliding friction would stay approximately constant at 30N here, (F = μN), and an increase in applied force would produce acceleration until opposing forces (such as increased air resistance or other resistance to motion) exactly cancelled the applied increase.
     
  4. Dec 20, 2015 #3

    CWatters

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    +1

    You have things the wrong way around. The friction isn't 30N _because_ the velocity is constant. The velocity is constant because you are pushing with a force equal to friction. If you push harder the couch will accelerate. Friction won't magically increase unless something else changes such as mu (μ) or the normal force.



    [Mod note: Fixed spelling checker travesty for CW - gneill]
     
    Last edited by a moderator: Dec 20, 2015
  5. Dec 20, 2015 #4

    CWatters

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    Sorry my spelling checker changed mu to my.
     
  6. Dec 20, 2015 #5
    but I can drive my car at a constant speed of 40km/h and 60km/h what about the friction there?
    The way I think of it is like this: I have a little box next to my computer and I am pushing it slowly, but I am pushing it with the same force. And then I pushed it again but with a higher speed. And the speed was constant too. So what happened with the friction? I know that the velocity was constant because I could see that the box didnt move slower of faster.
     
  7. Dec 20, 2015 #6

    NascentOxygen

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    It is sliding friction that 's approximately constant. When revolving, pneumatic tyres exhibit rolling resistance, and this increases with speed.

    When moving through a fluid (air or water), bodies encounter drag or fluid resistance, and this increases markedly as speed increases, it's at least proportional to (velocity)2.

    So your car needs extra power (more fuel) to push its way through air at a higher speed.
     
  8. Dec 20, 2015 #7
    This is an approximation. That the friction force is independent of the speed. At higher speeds air drag becomes significant and the approximation no longer works. That's the reason your car behaves the way it does. At the speeds you cited, air drag is the dominant force retarding the motion of your car. The friction force exerted on the tires by the road is the force that propels the motion of your car.
     
  9. Dec 21, 2015 #8

    CWatters

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    That is a very different situation. Air resistance isn't the same at both speeds, nor is engine power/force.

    Kinetic friction force is normally assumed and modelled to be constant independent of velocity. So its the same at both speeds. In this case air resistance is negligible.

    Once moving at constant speed the net force on the box is zero. Under those conditions the force you apply is equal and opposite to the friction force.
     
  10. Dec 21, 2015 #9
    Assuming ideal sliding friction, the friction force was the same in both cases. And the force you had to apply (equal to the friction force) to move the box at constant speed was the same in both cases. So, what's the problem?
     
  11. Dec 21, 2015 #10
    The 50N force on that same object will cause it to accelerate. Draw the force diagram. The 30N friction force will be the same as long as the object and surface remain the same. Fk= N×(Coef of kinetic friction). In order to have constant velocity your applied force must be exactly 30N no more no less. Any more will result in acceleration of the object.
     
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