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What direction does friction point in a diagram of a spring?

  1. May 20, 2015 #1
    1. The problem statement, all variables and given/known data
    A mass is at rest on a spring on an inclined plane. Which way would I draw the force of friction on a force diagram?

    2. Relevant equations
    Fnet=0
    F=-kx

    3. The attempt at a solution
    It could point either up the incline, or down the incline.
     
  2. jcsd
  3. May 20, 2015 #2

    TSny

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    Yes, that's correct.
     
  4. May 20, 2015 #3
    So, which is it?
     
  5. May 20, 2015 #4

    Dick

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    The force of friction always points opposite the direction the object is moving.
     
  6. May 20, 2015 #5

    SammyS

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    or in a direction to oppose motion. (It may be at rest.)
     
  7. May 21, 2015 #6

    haruspex

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  8. May 21, 2015 #7

    Dick

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  9. May 21, 2015 #8
    So in this case, would it point down the incline?
     
  10. May 21, 2015 #9

    haruspex

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    Suppose there were no friction. Which way would the block move? Is there enough information to decide?
     
  11. May 21, 2015 #10

    TSny

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    If a block, attached to a spring, is at rest on an inclined plane and that's all the information you have, then there is not enough information to deduce the direction of the friction force. It could be up the incline or down the incline, depending on the amount by which the spring is stretched or compressed. That's why I said your statement in the "attempt at a solution" section was correct.
     
  12. May 21, 2015 #11
    If friction was ignored, then I would assume the the net force would pull the block down the incline and strech the spring.
     
  13. May 21, 2015 #12

    haruspex

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    You wrote that it was "on" the spring. Is it above the spring or below it on the slope?
    Either way, as TSny keeps telling you, there is not enough information to decide. The spring may be under tension or under compression.
     
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