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Overcoming Friction

  1. Dec 12, 2007 #1
    There is a 730g object on a piece of wood measuring 91.5cm. If you slowly lift one end of the wood, predict high would you need to lift it in order for the object move.

    The attachment has a picture of what is happening
    physics.JPG


    I have figured that the force of gravity it 7.3N and I have drawn a force diagram. I know how to find all the forces, but i do not know how to tell when the object's force has overcome friction. If someone can tell me how to tell when this happenes, I can solve the rest of the problem by myself. Thank you
     
    Last edited: Dec 12, 2007
  2. jcsd
  3. Dec 12, 2007 #2

    Doc Al

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    The object begins to slide when there's a net force acting down the incline. What are the components of the weight? Are you given the coefficient of friction? How can you calculate the maximum value of static friction?
     
  4. Dec 12, 2007 #3
    All I was given was the mass of the object
     
  5. Dec 12, 2007 #4

    Doc Al

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    The mass is one piece of data that you don't need.
     
  6. Dec 12, 2007 #5
    Well, we had to predict at what height the object would move, and all we were given was the mass. We were not aloud to raise the board. Tho we were aloud to measure the lenght of the board, which was 91.5cm long.
     
    Last edited: Dec 12, 2007
  7. Dec 12, 2007 #6
    and wouldn't you need the mass to find the force of gravity, and from there you could find the force of friction and the natural force.
     
    Last edited: Dec 12, 2007
  8. Dec 12, 2007 #7

    Doc Al

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    If you knew the coefficient of friction, you could calculate the angle the board must have for the object to start sliding. I suggest you do that and then estimate (or guess) the coefficient of friction for this material against wood.
     
  9. Dec 12, 2007 #8

    Doc Al

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    The maximum friction force and gravity are both proportional to the mass, so it will cancel from your calculation.
     
  10. Dec 12, 2007 #9
    what is a coefficient of friction?
     
  11. Dec 12, 2007 #10

    stewartcs

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    The ratio of the maximum static frictional force between the surfaces in contact to the normal force.
     
  12. Dec 12, 2007 #11
    Here is my force diagram

    force d.JPG

    I'm trying to figure out how to tell when the friction force has decreased enough for the object to move.
     
  13. Dec 12, 2007 #12

    stewartcs

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    Like DocAl said, you'll have to estimate the coefficient of static friction since it was not given.

    Here are some typical values:

    http://www.engineershandbook.com/Tables/frictioncoefficients.htm

    Then you can use the length of the board to find the height required to get the object to slide. This experiment involves some assumptions so it's probably the concept that they want you to learn and not an exact value for height. That is to say that they want you to realize that the mass of the object is not related to the coefficient of friction.
     
  14. Dec 12, 2007 #13
    so how would i use the coefficient of friction to figure out the height?
     
  15. Dec 12, 2007 #14

    stewartcs

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    Hint: Coefficient of static friction = tan(x)

    where,

    x = the angle of the incline relative to the x axis.
     
  16. Dec 12, 2007 #15
    ok, i get that now. But is there a formula to determine when the static friction will become kinetic friciton? or is that the level at which the friction turns into kinetic?
     
    Last edited: Dec 12, 2007
  17. Dec 12, 2007 #16

    stewartcs

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    The frictional force becomes "kinetic" at the moment it overcomes the "static" frictional force (i.e. when it starts to slide).
     
  18. Dec 12, 2007 #17
    ok, i'll try to work it all out
     
  19. Dec 12, 2007 #18
    Is this where x is for Tan(x)?

    where is x.JPG
     
  20. Dec 12, 2007 #19
    now im all confused... Do I need to estimate a height, use that to get the angle from my attachment above, then take that angle put it into tan(x) to get the coefficent?
     
  21. Dec 12, 2007 #20
    I have guessed a height of 30 cm. That gives x an angle of 19.27 degress

    where is x.JPG

    And I have calculated that at 19.27 degrees, the force of friction would be 2.55N. Also, the natural force would be 6.86N. Now, I am stuck on how to figure what the coefficient of static friction would be to see if at this angle, the object would overcome the friction and move.
     
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