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Kinetic friction or static friction

  1. Oct 29, 2013 #1
    Can anyone explain this case ?
    If a moving box is pulling with a force F and the kinetic friction is 0.4F, of course, it will move in the direction of applied force with an acceleration and the net force is 0.6F
    But what if the force suddenly decreases to 0.3F ?
    The box is still moving so the friction is still kinetic friction?? But if it is the case, then the box will slow down! This seems quite strange to me@@
    Can anyone explain please?
     
  2. jcsd
  3. Oct 29, 2013 #2

    arildno

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    1. " the kinetic friction is 0.4F, of course,"
    Of course??
    Doesn't seem like an ofcoursity to me!
    Why should the frictional force be proportional to the pulling force?
    It might be, obviously, but generally, friction is proportional to the NORMAL force between the surface and te box.

    2. "But if it is the case, then the box will slow down!"
    So what?
    Kinetic friction occurs whenever there is a strict velocity difference between the surface and the box.
     
  4. Oct 29, 2013 #3
    Do you really find that surprising? If the driving force drops below the friction force, naturally the object will slowdown. If you suddenly ease on the accelerator, do you find it surprising that your car coasts down to a stop?
     
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