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Static Friction

  1. Apr 8, 2016 #1
    1. The problem statement, all variables and given/known data
    A small textbook is resting on a larger textbook on a horizontal desktop. You apply a horizontal force to the bottom book and both books accelerate together. The top book does not slip on the lower book. What forces causes the top book to accelerate horizontally?

    2. Relevant equations
    --

    3. The attempt at a solution
    The answer in the solutions manual says that the top book's inertia makes it want to move backwards. There is a force of static friction acting to prevent this attempted motion. It acts opposite the direction of attempted motion, forwards. So, the top book accelerates with the bottom book and doesn't slip.

    But, I'm confused because, doesn't there need to be an applied force on an object for there to be attempted motion at all? There's no applied force acting on the top book that would explain it's attempted backwards motion. Do you always need an applied force to state that an object is attempting motion? Is there a fictitious force involved? This is a stupid question, but I'm just confused
     
  2. jcsd
  3. Apr 8, 2016 #2

    Charles Link

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    Any backwards motion of the top book is only relative to the bottom book. Inertia gives it a tendency to remain still in the frame of reference of the room that it is in.
     
    Last edited: Apr 8, 2016
  4. Apr 8, 2016 #3
    So there's no external force acting on the book causing it to move backwards relative to the bottom book?
     
  5. Apr 8, 2016 #4

    Charles Link

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    That is correct. In learning physics, I think most of the textbooks are first-rate, but it is important to make sure that your textbook has good explanations with sufficient attention to detail. I think you will find some books to be better than others.
     
  6. Apr 8, 2016 #5
    I just use the textbook that the class uses
     
  7. Apr 8, 2016 #6

    Charles Link

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    Most of the time, that is sufficient.
     
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