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Friction problem on inclined plane

  1. Jul 8, 2006 #1
    Alright, I have another question that I want to make sure I did correct. The question is "an objec of mass m is at rest on a rough inclined plane with height h, lenght 8 m, and which makes an angle of 30degrees with the horizontal. The object is allowed to move and it stops on a rough horizontal surface, at a distance of 4m from the botton of the inclined plane as shown. The coefficient of kinetic friction on the inclined plane is 0.4 and g=10m/s^2.

    What is the speed of the object ast the bottom of the inclined plane?

    Alright i used the equation KEfinal -PEfinal = KEinitial -PEinitial, put in the numbers and calculated the velocity to be 12.65 m/s.

    The second questin is...

    What is the coefficient of kinetic friction for the horizontal surface?

    I used the equation Normal force = Force of weight and got

    mass*gravity*distance *coefficient of kinetic friction = mass*gravity*distance*coefficient of friction on the incline.

    I cancelled the mass and the gravity and put in the numbers,

    (4m for distance on horizontal0*(uk)= distance of 4 m, (which i obtained from sin30=8/d)*cos30degrees*0.4(the planes coefficient of friction).

    I then solved for the coefficient of fricton from the horizontal and calculated 0.35.

    (Is this correct?
  2. jcsd
  3. Jul 8, 2006 #2


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    You do NOT have conservation of mechanical energy, since friction is present!
  4. Jul 8, 2006 #3
    So, for part A, would I first find the Wnc=-ukmgd, and then set that equal to change of KE + the change of PE?
  5. Jul 8, 2006 #4

    Andrew Mason

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    You have to factor in the angle. The force of friction is [itex]\mu_kNg[/itex] where N is the component of the block's weight perpendicular to the inclined surface. Apart from that, your approach is right.

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