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Pushing force vs. Friction force

  1. Sep 27, 2011 #1
    My teacher in mechanics says that when we have a force pushing an object (moving at constant velocity) forward we also have a friction force that equals to the force that is pushing the object. So if we call the friction force for Fr and the pushing force for F, F=Fr.

    But in my mind, when we have 2 forces in opposite directions the object is standing still. So if my teacher is correct, how am I thinking wrong? Or can any give some tips to solve this fault of thinking.

    Regards, Robin Andersson
     
  2. jcsd
  3. Sep 27, 2011 #2

    Doc Al

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    Staff: Mentor

    Your teacher is correct. Your error is in thinking that a net force of zero implies zero velocity. What it does imply is zero acceleration. If the net force is zero, then the velocity will be constant, not necessarily zero.
     
  4. Sep 28, 2011 #3
    In other words, once the pushing force exceeds the force of friction, the object accelerates from rest until the friction force and pushing force are equal....from then on constant velocity ensues.
     
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