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Coefficient of kinetic friction forces

  1. Apr 26, 2014 #1
    1. The problem statement, all variables and given/known data

    Find the applied force for a object if the coefficient of kinetic friction equals 0.30 and the acceleration equals 5m/s^2. The applied force makes a 40 degree angle with the horizontal. The mass of the object is 10kg.

    2. Relevant equations
    Fnet=ma


    3. The attempt at a solution
    Ff=(0.3)(10)(9.8)=29.4
    (coefficient)(normal force=mg)

    Fnet =Facos40-29.4
    Fnet=ma
    ma=Facos40-29.4
    Fa=103.6

    I don't know if I did this right. Can someone please explain to me how to do this correctly. Thanks!
     
  2. jcsd
  3. Apr 26, 2014 #2


    If I understand the question correctly, the above is not correct. That is, normal force does not equal mg in this case, rendering the rest of your calculations incorrect.

    I'd start by drawing a force diagram and clearly labeling ALL the forces acting upon the object.
     
  4. Apr 26, 2014 #3
    So how would you find out the friction force without knowing the normal force?
     
  5. Apr 26, 2014 #4
    Draw a force diagram. The problem states that the applied force makes a 40 degree angle with horizontal. Draw that force vector, along with friction force, normal force and gravitational force.

    After doing so, figure out the y-component and x-component of the net Force. Start with the y-component. What are all the upward and downward forces acting upon the object?
     
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